- 5 shy -l J4 ltJ ~szligH4A4

Contents ~ s - t~Page

1 Introduction 83 2 Calculation of the Vertical Veloc1ty 86 3 Discussion of the Results 91

References 99

H FLOHN

ELEMENTS OF A SYNOPTIC CLlMATOLOGY OF THE

INDO-PAKISTAN SUBCONTINENT

- 6 shy

Abstract

From synoptic and aerologie data - together with many pictures from meteorological satellites - tbe seasonal varying largeshy

scale flowpatterns above the lndo-Pakistan subcontinent are outlined together with the rain frequency distribution Special consideration is given to the development of summer monsoon winds and rainfall patterns and the differences

between tbem and to tbe role of tbe Tibetan highlands in these processes

Zusammenfassung

Auf der Grundlage synoptischer und meteorologischer Daten

sowie einer groszligen Zahl Von Wolkenbildern meteorologischer Satelliten werden die jahreszeitlich wechselnden groszligshy

raumlumigen Stroumlmungsverteilungen uumlber dem indo-pakistanischen Subkontinent dargestellt und gleichzeitig damit die Vershyteilung der Niederschlagshaumlufigkeit Besonderes Gewicht

wird auf die Untersuchung der Entwicklung der Sommermonsunshy

Winde und -Niederschlagsverteilung die dabei auftretenden Differenzen und die Rolle des Tibetanischen Hochlandes in diesen Prozessen gelegt

- 7 shy

Elements of a Synoptic Climatology of the

lndo-Pakistan Subcontinent

The availability of aerological soundings for the lndo-Pakistan

Subcontinent - especially since the introduction of RAWIN Techniques and since the establishment of an aerological

network in the Chinese Peoples Republic including Tibet (1956) - has greatly improved our knowledge of the synoptic climatology of that area For a complete understanding we

have to include tbe whole area from the Persian Gulf and Pamir Mts to the western provinces of China from northern Tibet to the oceanic areas south of the equator

Our attempt to outline the large-scale synoptic features

responsible for tbe weather and climate of this vast area is partly based on extensive but hitherto unpublished investigations dealing with the summer season from my collaborators Dipl Met F ALFUTH Dr M WAGNER using aerological data of 1953 andor

1956 Mr HARBHAJAN ANAND M Sc (Tibet) and Dipl Met E RUPRECHT (Tharr Desert) Many studies by Indian meteorologistics

have also been published which are far too numerous to be quoted here the reader may refer to tbe Indian Journal of Meteorology and Geophysics and to three proceedings of symposia+)

Since 1949 the author has given several reviews on the Indian

Summer Monsoon (FLOHN 1950 1956 1960 1965 1964) and on the peculiar summer climate of Tibet (FLOHN 1958 1959 1968) in substance this report is a largely revised and extended

version of FLOHN (1965) An inspection of many tbousands of satellite pictures from tbis area (FLOHN 1968 1970) has much enlarged the limited personal experience of the author

+) 1) Monsoons of the World India Meteor Dept New Delhi

1960 2) Proceedings of the Symposium on Tropical Meteorology

Rotorua N Z 1963 3) Proceedlngs of the Symposium on Meteorological Results

of the Indian Ocean Expedition Bombay 1965

- 8 shy

A) During winter - especially from mid-December to February shy

upper westerlies prevail at the maximum wind level near 200 mb

above the whole area north of latitude ISoN and in all layers

above the Punjab and northernmost India They may even reach

the southern tip of the peninsula Below the westerlies the

well-known north-east monsoon - whieh mayaiso correct1y be

defined as the north-east trade - is quite shallow (1 - 15 km)

in the latitudinal belt between 220 and 300 N its vertical

depth increases towards the south above the peninsula Oue to

the Foumlhn effeet of the surrounding high mountains this

north-east monsoon eonsists of dry eontinental air diverging

and subsiding as in most trade wind areas

Within the upper westerlies the subtropical jet ean be obshy

served nearly every day between latitude 250 and 30o

N with

frequent troughs and ridges (Rossby waves) moving fair1y

rapidly in an easterly direetion from North Africa to southern

China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee

seetion of the troughs where vertical stretching prevails

together with upper divergenee and lower eonvergenee

(RAMASWAMY 1956) we observe not_ infrequently strong eonshy

veetive activity whieh is indieated at first by Ae cast or

Ae floe then by large towering eumulonimbus (with strong

shear in the forerunning anvil) Due to the extremely dry

air with eumulus bases frequently situated at 4 or 5 km

a large part of the falling rain evaporates before reaching

the ground but heavy gusts thunder and lightning and in

arid regions dust or sand storms are not rare events On

the warm southern side of the subtropieal jet preeipitation

eonneeted with these systems is in most eases only weak In

the north-west of the subeontinent espeeially in northern

Punjab and Baluchistan and in the Himalayas and their footshy

hills these Christmas rains (equivalent to the Mediterranean

winter rains) form adefinite rainy season varying in intensity

largely from year to year These rains are also responsible

- 9 shy

for the high snow cover of the Hindukush of the Karakorum

and of the north-western Himalayas (FLOHN 1969) These high

precipitations are by no means reflected in the measurements

of valley stations like Leh or Gilgit here the regular

occurence of daytime upslope breezes diverging from the

valley bottom prevents substantial precipitation (SCHWEINFURTH

1956) Only on rare oeeasions do these winter rains extend

into the Indian peninsula but then may reach as far as lSoN

The weak orographie eurvature of the Himalayas seems to be

responsible for the frequent occurrenee of troughs near 900 E

(Gulf of Benga1) where the average wind direction shifts

from 2S0-2900 in the west to about 260-2700 in the east The

ro1e of the Himalayas during this season can be interpreted

as a persistent cold source due to the eooling power of the

elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+

Together with the warm subtropieal low-lands of northern India

this cold source forms a strong baroelinic zone probably

together with a weak direct meridional ce1l with rising warm

air and subsiding cool air In the northern Punjab - as weIl

+ According to satellite pictures (ESSA 3 and 6 January 1967

and FebruaryjMareh 1965) the eoverage of permanent snow

in the Himalayas east of 780 E is usually only 30-50 percent

in contrast to Karakorum and Hindukush mountains with more

than 70 percent Large areas of southern Tibet - especially

between 7SoE and 920 E and south of 300 N - are snow-free on

about SO percent of all days The other 20 of the days

travelling disturbances produce clouds and snow which

disappears rapidly due to strong daytime radiation and

evaporation Even centra1 Tibet appears to be snow-free in

about 50 percent of all cases (FLOHN 1968) In contrast to

the marginal mountains the highlands of southern and cen~ral

Tibet ean act as an elevated heat source even during winter

- 10 shy

as in adjacent Afghanistan and Baluchistan _ the winter rains

increase in intensity (and sometimes also in frequency six to

ten days a month) during March and April but decrease during May

The above-mentioned direct meridional cell is not reflected

in the precipitation distribution In fact th~ average preshy

cipitation in the mountains is much greater due to orography

than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the

winter-spring snowfall in the mountains is much greater than

hitherto estimated (FLOHN 1969) This confirrns other studies

on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc

B) During spring the distribution of the upper westerlies

is nearly the same as during winter including the existence

of the quasi-permanent Bengal trough But the increasing solar

radiation leads to the development of shallow but persistent

heat lews above the southern part of the peninsula which

gradually move to the north and eventually reach (in early

June) the Punjab On their southern side shallow and weak

westerlies - which during the last century were falsely

called sea-breezes - occur with little or no effect on

weather Over the Bay of Bengal and across its northern coast

weak southerly (SW-SE) winds with moist maritime air gradushy

ally replace the dry north-east monsoon Now the divergence

aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal

Assam East Pakistan and perhaps also Upper Burma) with a

shallow layer of unstable moist air a situation which

typically produces severe storms Therefore from mid-March

to May the squalls of the Bengal Nor-westers increase

greatly in frequency intensity and rainfall With this

- 11 shy

synoptic mechanism the rainy season of north-eastern India

starts during spring much earlier than the marked reversal

in the upper winds At many places the nurnber of rainy days

(with 25 rnrn and more) increases to 12-19 per month due to

the orographical trapping of the low-level southerly flow in

the Assam area which is surrounded on three sides by high

mountains During April and May (FtoHN 1968) 10 typical synshy

optic stations in the Assam plains receive as much as 24 rnrn

per rainy day the whole area receives as much as 88 rnrn per

day regardless of the nurnber of stations with rain and in

only 7 percent of all cases is the area rainless (The same

nurnbers for northern Germany during summer are 50 rnrn 30 rnrn

and 10 percent respectively)

Over the Indian Ocean some tropical cyclones are observed

most of which can be followed from the South China Sea or

the Gulf of Thailand as they progress to the eoast Occasioshy

nally they develop into full-size tropical hurricanes with

warm eores and with anticyclonic flows above 200 rnb They are

then steered toward the north-west or even north in the

advance section of a westerly trough and are accompanied by

torrential rains and floods Despite their rareness they

may even re-oceur after a week or so and are sometimes

accompanied by disastrous flood effects in the densely popushy

lated low-level area in the delta of the Ganges and Brahrnashy

putra

Cl During May and early June the seasollal warming of the air

above NE-India - produeed by the release of latent heat from

precipitation processes - and above southern Tibet - here

caused by the elevation of the heated surface near 4500 m

(equivalent to about 580 rnb) - weakens and finally reverses

the meridional gradient of temperature between subtropical

latitudes and the equator (FLOHN 1964) According to

- 12 shy

aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The

reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete

reversal of the upper winds ( between about 450 and 100 mb)

from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy

south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days

of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known

in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall

distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event

at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds

and monsoon rains is responsible for frequent misinterpreshy

tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic

features

This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average

position of the subtropical jet shifts to about 40oN with a

- 13 shy

weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)

Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather

marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or

Szechwan) trough (at lOO-10SoB)

We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears

together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of

Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe

unfortunately misnamed the European summer monaoon Furthershy

more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the

sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern

China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are

certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect

to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation

The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In

both areas westerly winds of themiddot lower troposphere - mostly

250-2900 - converge along a slowiy displacing convergenoe

- 6 shy

Abstract

From synoptic and aerologie data - together with many pictures from meteorological satellites - tbe seasonal varying largeshy

scale flowpatterns above the lndo-Pakistan subcontinent are outlined together with the rain frequency distribution Special consideration is given to the development of summer monsoon winds and rainfall patterns and the differences

between tbem and to tbe role of tbe Tibetan highlands in these processes

Zusammenfassung

Auf der Grundlage synoptischer und meteorologischer Daten

sowie einer groszligen Zahl Von Wolkenbildern meteorologischer Satelliten werden die jahreszeitlich wechselnden groszligshy

raumlumigen Stroumlmungsverteilungen uumlber dem indo-pakistanischen Subkontinent dargestellt und gleichzeitig damit die Vershyteilung der Niederschlagshaumlufigkeit Besonderes Gewicht

wird auf die Untersuchung der Entwicklung der Sommermonsunshy

Winde und -Niederschlagsverteilung die dabei auftretenden Differenzen und die Rolle des Tibetanischen Hochlandes in diesen Prozessen gelegt

- 7 shy

Elements of a Synoptic Climatology of the

lndo-Pakistan Subcontinent

The availability of aerological soundings for the lndo-Pakistan

Subcontinent - especially since the introduction of RAWIN Techniques and since the establishment of an aerological

network in the Chinese Peoples Republic including Tibet (1956) - has greatly improved our knowledge of the synoptic climatology of that area For a complete understanding we

have to include tbe whole area from the Persian Gulf and Pamir Mts to the western provinces of China from northern Tibet to the oceanic areas south of the equator

Our attempt to outline the large-scale synoptic features

responsible for tbe weather and climate of this vast area is partly based on extensive but hitherto unpublished investigations dealing with the summer season from my collaborators Dipl Met F ALFUTH Dr M WAGNER using aerological data of 1953 andor

1956 Mr HARBHAJAN ANAND M Sc (Tibet) and Dipl Met E RUPRECHT (Tharr Desert) Many studies by Indian meteorologistics

have also been published which are far too numerous to be quoted here the reader may refer to tbe Indian Journal of Meteorology and Geophysics and to three proceedings of symposia+)

Since 1949 the author has given several reviews on the Indian

Summer Monsoon (FLOHN 1950 1956 1960 1965 1964) and on the peculiar summer climate of Tibet (FLOHN 1958 1959 1968) in substance this report is a largely revised and extended

version of FLOHN (1965) An inspection of many tbousands of satellite pictures from tbis area (FLOHN 1968 1970) has much enlarged the limited personal experience of the author

+) 1) Monsoons of the World India Meteor Dept New Delhi

1960 2) Proceedings of the Symposium on Tropical Meteorology

Rotorua N Z 1963 3) Proceedlngs of the Symposium on Meteorological Results

of the Indian Ocean Expedition Bombay 1965

- 8 shy

A) During winter - especially from mid-December to February shy

upper westerlies prevail at the maximum wind level near 200 mb

above the whole area north of latitude ISoN and in all layers

above the Punjab and northernmost India They may even reach

the southern tip of the peninsula Below the westerlies the

well-known north-east monsoon - whieh mayaiso correct1y be

defined as the north-east trade - is quite shallow (1 - 15 km)

in the latitudinal belt between 220 and 300 N its vertical

depth increases towards the south above the peninsula Oue to

the Foumlhn effeet of the surrounding high mountains this

north-east monsoon eonsists of dry eontinental air diverging

and subsiding as in most trade wind areas

Within the upper westerlies the subtropical jet ean be obshy

served nearly every day between latitude 250 and 30o

N with

frequent troughs and ridges (Rossby waves) moving fair1y

rapidly in an easterly direetion from North Africa to southern

China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee

seetion of the troughs where vertical stretching prevails

together with upper divergenee and lower eonvergenee

(RAMASWAMY 1956) we observe not_ infrequently strong eonshy

veetive activity whieh is indieated at first by Ae cast or

Ae floe then by large towering eumulonimbus (with strong

shear in the forerunning anvil) Due to the extremely dry

air with eumulus bases frequently situated at 4 or 5 km

a large part of the falling rain evaporates before reaching

the ground but heavy gusts thunder and lightning and in

arid regions dust or sand storms are not rare events On

the warm southern side of the subtropieal jet preeipitation

eonneeted with these systems is in most eases only weak In

the north-west of the subeontinent espeeially in northern

Punjab and Baluchistan and in the Himalayas and their footshy

hills these Christmas rains (equivalent to the Mediterranean

winter rains) form adefinite rainy season varying in intensity

largely from year to year These rains are also responsible

- 9 shy

for the high snow cover of the Hindukush of the Karakorum

and of the north-western Himalayas (FLOHN 1969) These high

precipitations are by no means reflected in the measurements

of valley stations like Leh or Gilgit here the regular

occurence of daytime upslope breezes diverging from the

valley bottom prevents substantial precipitation (SCHWEINFURTH

1956) Only on rare oeeasions do these winter rains extend

into the Indian peninsula but then may reach as far as lSoN

The weak orographie eurvature of the Himalayas seems to be

responsible for the frequent occurrenee of troughs near 900 E

(Gulf of Benga1) where the average wind direction shifts

from 2S0-2900 in the west to about 260-2700 in the east The

ro1e of the Himalayas during this season can be interpreted

as a persistent cold source due to the eooling power of the

elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+

Together with the warm subtropieal low-lands of northern India

this cold source forms a strong baroelinic zone probably

together with a weak direct meridional ce1l with rising warm

air and subsiding cool air In the northern Punjab - as weIl

+ According to satellite pictures (ESSA 3 and 6 January 1967

and FebruaryjMareh 1965) the eoverage of permanent snow

in the Himalayas east of 780 E is usually only 30-50 percent

in contrast to Karakorum and Hindukush mountains with more

than 70 percent Large areas of southern Tibet - especially

between 7SoE and 920 E and south of 300 N - are snow-free on

about SO percent of all days The other 20 of the days

travelling disturbances produce clouds and snow which

disappears rapidly due to strong daytime radiation and

evaporation Even centra1 Tibet appears to be snow-free in

about 50 percent of all cases (FLOHN 1968) In contrast to

the marginal mountains the highlands of southern and cen~ral

Tibet ean act as an elevated heat source even during winter

- 10 shy

as in adjacent Afghanistan and Baluchistan _ the winter rains

increase in intensity (and sometimes also in frequency six to

ten days a month) during March and April but decrease during May

The above-mentioned direct meridional cell is not reflected

in the precipitation distribution In fact th~ average preshy

cipitation in the mountains is much greater due to orography

than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the

winter-spring snowfall in the mountains is much greater than

hitherto estimated (FLOHN 1969) This confirrns other studies

on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc

B) During spring the distribution of the upper westerlies

is nearly the same as during winter including the existence

of the quasi-permanent Bengal trough But the increasing solar

radiation leads to the development of shallow but persistent

heat lews above the southern part of the peninsula which

gradually move to the north and eventually reach (in early

June) the Punjab On their southern side shallow and weak

westerlies - which during the last century were falsely

called sea-breezes - occur with little or no effect on

weather Over the Bay of Bengal and across its northern coast

weak southerly (SW-SE) winds with moist maritime air gradushy

ally replace the dry north-east monsoon Now the divergence

aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal

Assam East Pakistan and perhaps also Upper Burma) with a

shallow layer of unstable moist air a situation which

typically produces severe storms Therefore from mid-March

to May the squalls of the Bengal Nor-westers increase

greatly in frequency intensity and rainfall With this

- 11 shy

synoptic mechanism the rainy season of north-eastern India

starts during spring much earlier than the marked reversal

in the upper winds At many places the nurnber of rainy days

(with 25 rnrn and more) increases to 12-19 per month due to

the orographical trapping of the low-level southerly flow in

the Assam area which is surrounded on three sides by high

mountains During April and May (FtoHN 1968) 10 typical synshy

optic stations in the Assam plains receive as much as 24 rnrn

per rainy day the whole area receives as much as 88 rnrn per

day regardless of the nurnber of stations with rain and in

only 7 percent of all cases is the area rainless (The same

nurnbers for northern Germany during summer are 50 rnrn 30 rnrn

and 10 percent respectively)

Over the Indian Ocean some tropical cyclones are observed

most of which can be followed from the South China Sea or

the Gulf of Thailand as they progress to the eoast Occasioshy

nally they develop into full-size tropical hurricanes with

warm eores and with anticyclonic flows above 200 rnb They are

then steered toward the north-west or even north in the

advance section of a westerly trough and are accompanied by

torrential rains and floods Despite their rareness they

may even re-oceur after a week or so and are sometimes

accompanied by disastrous flood effects in the densely popushy

lated low-level area in the delta of the Ganges and Brahrnashy

putra

Cl During May and early June the seasollal warming of the air

above NE-India - produeed by the release of latent heat from

precipitation processes - and above southern Tibet - here

caused by the elevation of the heated surface near 4500 m

(equivalent to about 580 rnb) - weakens and finally reverses

the meridional gradient of temperature between subtropical

latitudes and the equator (FLOHN 1964) According to

- 12 shy

aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The

reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete

reversal of the upper winds ( between about 450 and 100 mb)

from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy

south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days

of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known

in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall

distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event

at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds

and monsoon rains is responsible for frequent misinterpreshy

tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic

features

This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average

position of the subtropical jet shifts to about 40oN with a

- 13 shy

weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)

Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather

marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or

Szechwan) trough (at lOO-10SoB)

We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears

together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of

Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe

unfortunately misnamed the European summer monaoon Furthershy

more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the

sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern

China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are

certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect

to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation

The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In

both areas westerly winds of themiddot lower troposphere - mostly

250-2900 - converge along a slowiy displacing convergenoe

- 8 shy

A) During winter - especially from mid-December to February shy

upper westerlies prevail at the maximum wind level near 200 mb

above the whole area north of latitude ISoN and in all layers

above the Punjab and northernmost India They may even reach

the southern tip of the peninsula Below the westerlies the

well-known north-east monsoon - whieh mayaiso correct1y be

defined as the north-east trade - is quite shallow (1 - 15 km)

in the latitudinal belt between 220 and 300 N its vertical

depth increases towards the south above the peninsula Oue to

the Foumlhn effeet of the surrounding high mountains this

north-east monsoon eonsists of dry eontinental air diverging

and subsiding as in most trade wind areas

Within the upper westerlies the subtropical jet ean be obshy

served nearly every day between latitude 250 and 30o

N with

frequent troughs and ridges (Rossby waves) moving fair1y

rapidly in an easterly direetion from North Africa to southern

China (KRISHNAMURTI 1961 RAMAGE 1952) In the advanee

seetion of the troughs where vertical stretching prevails

together with upper divergenee and lower eonvergenee

(RAMASWAMY 1956) we observe not_ infrequently strong eonshy

veetive activity whieh is indieated at first by Ae cast or

Ae floe then by large towering eumulonimbus (with strong

shear in the forerunning anvil) Due to the extremely dry

air with eumulus bases frequently situated at 4 or 5 km

a large part of the falling rain evaporates before reaching

the ground but heavy gusts thunder and lightning and in

arid regions dust or sand storms are not rare events On

the warm southern side of the subtropieal jet preeipitation

eonneeted with these systems is in most eases only weak In

the north-west of the subeontinent espeeially in northern

Punjab and Baluchistan and in the Himalayas and their footshy

hills these Christmas rains (equivalent to the Mediterranean

winter rains) form adefinite rainy season varying in intensity

largely from year to year These rains are also responsible

- 9 shy

for the high snow cover of the Hindukush of the Karakorum

and of the north-western Himalayas (FLOHN 1969) These high

precipitations are by no means reflected in the measurements

of valley stations like Leh or Gilgit here the regular

occurence of daytime upslope breezes diverging from the

valley bottom prevents substantial precipitation (SCHWEINFURTH

1956) Only on rare oeeasions do these winter rains extend

into the Indian peninsula but then may reach as far as lSoN

The weak orographie eurvature of the Himalayas seems to be

responsible for the frequent occurrenee of troughs near 900 E

(Gulf of Benga1) where the average wind direction shifts

from 2S0-2900 in the west to about 260-2700 in the east The

ro1e of the Himalayas during this season can be interpreted

as a persistent cold source due to the eooling power of the

elevated snow-cover in the layer 3 - 7 km (or 700 - 400 mbs)+

Together with the warm subtropieal low-lands of northern India

this cold source forms a strong baroelinic zone probably

together with a weak direct meridional ce1l with rising warm

air and subsiding cool air In the northern Punjab - as weIl

+ According to satellite pictures (ESSA 3 and 6 January 1967

and FebruaryjMareh 1965) the eoverage of permanent snow

in the Himalayas east of 780 E is usually only 30-50 percent

in contrast to Karakorum and Hindukush mountains with more

than 70 percent Large areas of southern Tibet - especially

between 7SoE and 920 E and south of 300 N - are snow-free on

about SO percent of all days The other 20 of the days

travelling disturbances produce clouds and snow which

disappears rapidly due to strong daytime radiation and

evaporation Even centra1 Tibet appears to be snow-free in

about 50 percent of all cases (FLOHN 1968) In contrast to

the marginal mountains the highlands of southern and cen~ral

Tibet ean act as an elevated heat source even during winter

- 10 shy

as in adjacent Afghanistan and Baluchistan _ the winter rains

increase in intensity (and sometimes also in frequency six to

ten days a month) during March and April but decrease during May

The above-mentioned direct meridional cell is not reflected

in the precipitation distribution In fact th~ average preshy

cipitation in the mountains is much greater due to orography

than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the

winter-spring snowfall in the mountains is much greater than

hitherto estimated (FLOHN 1969) This confirrns other studies

on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc

B) During spring the distribution of the upper westerlies

is nearly the same as during winter including the existence

of the quasi-permanent Bengal trough But the increasing solar

radiation leads to the development of shallow but persistent

heat lews above the southern part of the peninsula which

gradually move to the north and eventually reach (in early

June) the Punjab On their southern side shallow and weak

westerlies - which during the last century were falsely

called sea-breezes - occur with little or no effect on

weather Over the Bay of Bengal and across its northern coast

weak southerly (SW-SE) winds with moist maritime air gradushy

ally replace the dry north-east monsoon Now the divergence

aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal

Assam East Pakistan and perhaps also Upper Burma) with a

shallow layer of unstable moist air a situation which

typically produces severe storms Therefore from mid-March

to May the squalls of the Bengal Nor-westers increase

greatly in frequency intensity and rainfall With this

- 11 shy

synoptic mechanism the rainy season of north-eastern India

starts during spring much earlier than the marked reversal

in the upper winds At many places the nurnber of rainy days

(with 25 rnrn and more) increases to 12-19 per month due to

the orographical trapping of the low-level southerly flow in

the Assam area which is surrounded on three sides by high

mountains During April and May (FtoHN 1968) 10 typical synshy

optic stations in the Assam plains receive as much as 24 rnrn

per rainy day the whole area receives as much as 88 rnrn per

day regardless of the nurnber of stations with rain and in

only 7 percent of all cases is the area rainless (The same

nurnbers for northern Germany during summer are 50 rnrn 30 rnrn

and 10 percent respectively)

Over the Indian Ocean some tropical cyclones are observed

most of which can be followed from the South China Sea or

the Gulf of Thailand as they progress to the eoast Occasioshy

nally they develop into full-size tropical hurricanes with

warm eores and with anticyclonic flows above 200 rnb They are

then steered toward the north-west or even north in the

advance section of a westerly trough and are accompanied by

torrential rains and floods Despite their rareness they

may even re-oceur after a week or so and are sometimes

accompanied by disastrous flood effects in the densely popushy

lated low-level area in the delta of the Ganges and Brahrnashy

putra

Cl During May and early June the seasollal warming of the air

above NE-India - produeed by the release of latent heat from

precipitation processes - and above southern Tibet - here

caused by the elevation of the heated surface near 4500 m

(equivalent to about 580 rnb) - weakens and finally reverses

the meridional gradient of temperature between subtropical

latitudes and the equator (FLOHN 1964) According to

- 12 shy

aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The

reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete

reversal of the upper winds ( between about 450 and 100 mb)

from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy

south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days

of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known

in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall

distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event

at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds

and monsoon rains is responsible for frequent misinterpreshy

tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic

features

This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average

position of the subtropical jet shifts to about 40oN with a

- 13 shy

weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)

Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather

marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or

Szechwan) trough (at lOO-10SoB)

We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears

together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of

Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe

unfortunately misnamed the European summer monaoon Furthershy

more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the

sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern

China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are

certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect

to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation

The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In

both areas westerly winds of themiddot lower troposphere - mostly

250-2900 - converge along a slowiy displacing convergenoe

- 10 shy

as in adjacent Afghanistan and Baluchistan _ the winter rains

increase in intensity (and sometimes also in frequency six to

ten days a month) during March and April but decrease during May

The above-mentioned direct meridional cell is not reflected

in the precipitation distribution In fact th~ average preshy

cipitation in the mountains is much greater due to orography

than in the plains Recent investigations on the water balance of the Hindukush and adjacent mountains have shown that the

winter-spring snowfall in the mountains is much greater than

hitherto estimated (FLOHN 1969) This confirrns other studies

on the mass balance of the enormous glaciers of the Karakorurn the Pamirs etc

B) During spring the distribution of the upper westerlies

is nearly the same as during winter including the existence

of the quasi-permanent Bengal trough But the increasing solar

radiation leads to the development of shallow but persistent

heat lews above the southern part of the peninsula which

gradually move to the north and eventually reach (in early

June) the Punjab On their southern side shallow and weak

westerlies - which during the last century were falsely

called sea-breezes - occur with little or no effect on

weather Over the Bay of Bengal and across its northern coast

weak southerly (SW-SE) winds with moist maritime air gradushy

ally replace the dry north-east monsoon Now the divergence

aloft in front of a travelling upper trough of the subtropical jet frequently coincides above north-east India (Bengal

Assam East Pakistan and perhaps also Upper Burma) with a

shallow layer of unstable moist air a situation which

typically produces severe storms Therefore from mid-March

to May the squalls of the Bengal Nor-westers increase

greatly in frequency intensity and rainfall With this

- 11 shy

synoptic mechanism the rainy season of north-eastern India

starts during spring much earlier than the marked reversal

in the upper winds At many places the nurnber of rainy days

(with 25 rnrn and more) increases to 12-19 per month due to

the orographical trapping of the low-level southerly flow in

the Assam area which is surrounded on three sides by high

mountains During April and May (FtoHN 1968) 10 typical synshy

optic stations in the Assam plains receive as much as 24 rnrn

per rainy day the whole area receives as much as 88 rnrn per

day regardless of the nurnber of stations with rain and in

only 7 percent of all cases is the area rainless (The same

nurnbers for northern Germany during summer are 50 rnrn 30 rnrn

and 10 percent respectively)

Over the Indian Ocean some tropical cyclones are observed

most of which can be followed from the South China Sea or

the Gulf of Thailand as they progress to the eoast Occasioshy

nally they develop into full-size tropical hurricanes with

warm eores and with anticyclonic flows above 200 rnb They are

then steered toward the north-west or even north in the

advance section of a westerly trough and are accompanied by

torrential rains and floods Despite their rareness they

may even re-oceur after a week or so and are sometimes

accompanied by disastrous flood effects in the densely popushy

lated low-level area in the delta of the Ganges and Brahrnashy

putra

Cl During May and early June the seasollal warming of the air

above NE-India - produeed by the release of latent heat from

precipitation processes - and above southern Tibet - here

caused by the elevation of the heated surface near 4500 m

(equivalent to about 580 rnb) - weakens and finally reverses

the meridional gradient of temperature between subtropical

latitudes and the equator (FLOHN 1964) According to

- 12 shy

aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The

reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete

reversal of the upper winds ( between about 450 and 100 mb)

from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy

south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days

of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known

in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall

distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event

at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds

and monsoon rains is responsible for frequent misinterpreshy

tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic

features

This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average

position of the subtropical jet shifts to about 40oN with a

- 13 shy

weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)

Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather

marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or

Szechwan) trough (at lOO-10SoB)

We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears

together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of

Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe

unfortunately misnamed the European summer monaoon Furthershy

more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the

sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern

China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are

certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect

to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation

The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In

both areas westerly winds of themiddot lower troposphere - mostly

250-2900 - converge along a slowiy displacing convergenoe

- 12 shy

aero1ogieal data this proeess starts from the uppermost troposphere (150-200 mb) and proeeeds slow1y downwards The

reversal of this meridional temperature gradient in the 1ayer 800-500 mb oecurs almost simu1taneous1y with the onset of monaoon rains along the western eoast of the peninsula This results - almost simultaneously with the northward displaeement of the low-level heat low - in a complete

reversal of the upper winds ( between about 450 and 100 mb)

from west to east a rapid inerease of the vertical depth of the equatorial westerlies now known as the south-west monsoon to 6-7 km and their intrusion into the low-pressure area extending from the Punjab to Assam and further to the eastshy

south-east This syndrome of nearly simu1taneous events is observed under normal conditions during the first ten days

of June in some years it oeeurs already during late May in others it may be postponed unti1 late June It is wel1-known

in classical textbooks as the burst of the monsoon But after recognizing the lack of coincidence between rainfall

distribution thunder-storm frequency and three-dimensiona1 wind patterns we see that it is actually quite a diffieult job to define exaet1y one (and only one) date for this event

at a given point The literal translation of the Arabic term maus im is quite generally season therefore this term has originally no specifie meaning like monsoon winds or monsoon rains The lack of coincidenee between monsoon winds

and monsoon rains is responsible for frequent misinterpreshy

tations and confusion the onset of the rainymiddot season is eontrolled in different areas by quite different synoptic

features

This reversal of the three-dimensional wind field 1a aceompanied or better triggered by the occurrence of anticyclonic cells in the divergence zone between the high-tropospheric eastershylies and the westerlies near latitude 30oN The average

position of the subtropical jet shifts to about 40oN with a

- 13 shy

weak anticyelonic curvature on the northern fringe of the Tibetan highlands This rapid displacement coincides with a re-arrangement of the quasi-permanent troughs (Figure 1)

Instead of the weak cyclonie eurvature of the subtropical jet near 900 E (Bengal trough) we now observe two rather

marked troughs just upstream and downstream of the Tibetan block the Pamir trough (near 650 E) and the west Chinese (or

Szechwan) trough (at lOO-10SoB)

We might remark at this point without further explanations that nearly simultaneously the quasi-permanent secondary trough over eastern Europe (centred around 300 E) disappears

together with a rapid decrease of the number of b10cking anticyclones above north-western Europe a consequence of

Rossbys well-known stationary wave formula This causes a marked singularity of weather in large parts of Europe

unfortunately misnamed the European summer monaoon Furthershy

more at this time the occurrence of a quasi-stationary antishycyclonic cell above the cool Okhotsk Sea together with the

sharpening of the Szechwan trough is responsible for the occurrence of Bai-U - rains above central and northern

China and Japan (ASAKURA 1968) which are mainly produced by weak cyclones running along the marked baroclinic frontal zone (Pacific Polar Front) Wh1le these two events are

certainly related in a dynamical sense to the events between Pamir and Szechwan this 1s probably not the case with respect

to a third event the onset of Arizona summer rains which apparently have many parallels to the simultaneous developshyments in the Punjab These large scale teleconnections deserve a comparative investigation

The usual distinction between an_Arabian branch and a Bengal branch of the south-west monsoonis somewhat misleading In

both areas westerly winds of themiddot lower troposphere - mostly

250-2900 - converge along a slowiy displacing convergenoe

_____ _

- 14 shy

4~ m1c-1

~_--15

10 0-000 SIIt1f11lf bullbullbullbull Co~1 S a~orl I bull

bull I Sbtntpual _1 bullbull is lJlff tnhl9h AXIS

Fig 1 Position of the Fig 3 Meridional crossshysubtropical and of the section of zonal winds semi-permanent troughs along Long 780 E (arrows = during summer and during upper air stations) Julyshythe cool seasons August 1961-62 (October-May) bull

Fig 2 Position of the monsoon pressure trough (= northern

branch of the ITC) and streamlines of the resultant

wind at 09 kms during summer

- l c shy

line with a deep current from east-south-east (Figure 2)

which extends right up to the easterlies of the upper troposhy

sphere At the southern edge of this convergence - which forms

part of the main ITCZ (northern branch) and the equatorial

pressure trough - isolated cyclonic disturbances without

distinct fronts and air-masses frequently travel from eastshy

south-east to west-north-west producing much rain in their

southern and western sections These monsoon depressions

are responsible for the bulk of the summer rains in the

interior of the lndo-Pakistan subcontinent Their forecast

depends mainly on sufficient knowledge of the rather comshy

plicated three-dimensional wind field in that area up to at

least 200 mb Not infrequently these disturbances can be

followed during their earlier track from the east Apart

from these low-level cyclones frequent strong convective

rains are observed in two regions in Bengal and Assam where

the low-tropospheric south-west flow permanently converges

with the easterlies (including orographically foreed lifting

at the Khasia Hills) and at the steep escarpments of the

western eoasts of both South-Asian peninsulae But even along

these eoastal escarpments large variations from day to day

oeeur probably related to variations of the divergenee of

low-level winds The strong contribution of eonvective

activity- interrupted by short rainless subsidence periods shy

to the summer rain in Nepal has been demonstrated by DITTMANN

(1970)

On the average the boundary between the extra-tropical

westerlies and the easterlies above the south-west monsoon

(Figure 3) is situated between 400-100 mb (7-16 km) near

300 N ie just above a large part of the Himalaya mountains

At the same time the subtropical jet is situated at latitude

40-420 N ie above the Tarim Basin as revealed by Chinese

aerological data with a rernarkable constaney The boundary

between these currents is marked by well-defined anticyclonic

- 16 shy

cells frequently centered in the south-eastern portion of

the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy

tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy

east it can be intensified and steered into a northerly

direction and eventually merge with the trough Then torrential

rains floods and landslides are observed in the Himalayas

while the divergence of the west-south-west flow above the

peninsula produces widespread subsidence and suppression of

convective activity This is at least one of the mechanisms

responsible for the well-known monsoon breaks (RAHMATULLAH

1952 RIEHL 1954) In fact if we disregard the relatively

small areas of orographically forced convective rains and of

permanent convergence in the north-eastern portion of the

subcontinent we may safely say that ove~ the largest part

of the Indo-Pakistan area the summer rains are produced by

well-differentiated synoptic events and cannot be conceived

as a more or less steady climatic feature Even the quasishy

permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough

with a cold air outbreak across Tibet it recovers however

after two to three days sometimes being reinforced by warm

air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the

well-known maps of precipitation amount

The track of these monsoon depressions is controlled by the

upper flow or bette~ by integration of the wind field

horizontally and vertically with respect to pressure and

taking into account the well-known meridional variation of

the Coriolis parameter szlig = dfdy Under normal conditions

they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall

productivity mostly as fairly symmetrie vortices near the

- 17 shy

o0 I I Irltt I 60 0 110deg

Flow Patterns 800-900mbs-+

100 - 300 IIE-

Cold Warm Co Id Warm100 I mbs INDIRECT

500 CELL GEClCELL

Conv900 Div Div Conv I

A A B 8

Fig 4 Large-scale features of the Indian Summer Monsoon

Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)

and A - A (exit region of the Tropical Easterly Jet)

- 18 shy

S50 roh level Apparently lifting and consequently rainfall

reach a maximum in their southern section there is hardly

sufficient evidence of definite air-mass differences in their

area Generally speaking air-mass differences during the

monsoon season above the subcontinent are much smaller than

in middle latitudes and hardly detectable in the lower

troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air

above SON is about 7-10oC cooler than the subtropical air

above the heat eentre at 28-30oN This reversed temperature

gradient is eorrelated with an extremely persistent tropiealo

easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is

of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh

have a duration of three to six days and varying between

about 40 and more than 100 knots in the centre According to

recent studies by RAMAN (1964) these pulses are eorrelated

with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which

intensifies the anticyelonie eells above the Himalayas

On the average the speed of the Tropical Easterly Jet inshy

creases (together with a northerly component of the winds)

east of 850 E and decreases (together with a weak southerly

eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign

near 8SoE (Figure 4) Above SE Asia ascending motion with

strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions

are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN

1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence

Zone at the surface) in contrast to lifting motions along

~

- 19 shy

the southern edge (the belt of highest rainfall along Lat

7-14oN)

Nevertheless climatic features near surfaee are by no means

unimportant in the large-scale meehanism of the summer-monsoon

rains in southern Asia We list only a few of them

a) The reversal of the temperature pressure and wind field

of the middle and upper troposphere is preceded by the gradual

warming of northern India the Tibetan highlands and the

adjaeent highlands Bere we have to distinguish two main

sources of internal and potential energy

1 The flux of sensible heat into the air from the elevated

heat souree of the arid (western and central) seetion of

the Tibetan highlands Here all terms of the heat balance

can be assumed to be nearly equal to those in the adjacent

arid lowlands of centra1 Asia Suffieient evidence for

this assumption has been presented by AIZENSHTAT (1966)

from many heat balance investigations in the arid high

mountains of the USSR at altitudes of 3100-4000 m The

average input of sensible heat into the atmosphere amounts

to 180-250 Lyday (1 Ly = 1 calcm2)

2 The release of latent heat from the enormous quantities

of rain falling on the superhumid mountain areas of Bengal

Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more

than the global radiation at the surfaee can yield Its

effect has been suggested generally (FLOHN 1953) as an

addition to the anticyclonic curvature of the upper westershy

lies along high mountain ridges Oue to the rapid decrease

of moisture with height it will increase the thiekness of

the lower troposphere however the aseending motion in the

eb towers carries much heat upward to the 150-300 mb layer

- 20 shy - 21 shy

Most of this latent heat is used for heating up the upper

troposphere to a moist-adiabatic lapse-rate Since in SEshy

Tibet and Assam the average vertical lapse-rate is slightly

super-moist-adiabatie any ascending parcel of air in the

eondensation layer between 775 and 205 mb is about 16oC

warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in

the arid areas

The effect of the direct warming of air above southernmost

and western Tibet and north-eastern India has been qualitashy

tively stressed by the author (1950 1960 1968) Strong

evidenee has been contributed in a model eomputation by

MURAKAMI (1958) who started from the average 300 mb-flow

during May introduced a heat source above the whole

Tibetan plateau and obtained after a model time of 72

hours an antieyelonic eell in southeastern Tibet In a

more recent model of a two-dimensional meridional cross-

section along BOoE the role of the mountains and of the

hydrologie cycle has been investigated numerically by the

same author (1969) Using correlation analysis ASAKURA

(196B) recently demonstrated the strong persistence and

the large-seale influence of the Tibetan anticyelone at

the 500 mb-level

Careful analysis of all available aerological data shy

taking Into account the diurnal variation of upper air

temperatures as well as thermaI winds (FLOHN 196B) shy

revealed beyond any doubt the existenee of an elongated

heat zone in the upper troposphere (300-500 mb) above the

Himalayas and southern Tibet centered at 300 N and 850S

(Figure 5)

r e

N

~ I

2 I

~

abull

+ lt lJl middotri

t 0

Q) ~ III U Ul

Ul Cl U t 0

-rl ~

-1 Q) III gtlt

III ~ ~ Q)

c + 0

Q)Cl t nl l +

nl (l) (l)

gtltIII ~

-1 Q) +

~ t rtI Q)0

0 S If)

I 0 0 + 0 c M -rl

Ul t Ul Q) Q) t + 10( + U lt1l - p c

+ Il Ul Ul Q) + t 104

10( Ql Q ~ c 0

u If)

lJl r

- 22 shy

b) In the arid north-western part of the subcontinent (Sind

Rajasthan) the aridity is maintained even during the monsoon

season when moist air with an almost constant dewpoint near 0

25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days

when monsoon depressions travelling from the east or south

penetrate this area On all other days - ie during more

than 90 per cent of the season - the permanent and steady

monsoon flow is divergent turning from south-west above

Jodhpur and Gwalior to south-south-east above Jacobabad and

therefore subject to large-seale subsidence (FLOHN et al

196B) which reduces the vertical extension of the moist

layer to about 15-2 km and its eloud cover to shallow

stratocumuli (sc) This divergence can be understood if we

take into aceount the average position of the pressure trough

with its anticyelonic curvature from eastern Arabia to the

Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1

Similarly the SW-monsoon above the Arabian Sea is - at least

west of Long 65-68o

E - essentially rainless with a shallow

layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by

directional diffluence but by the increasing velocity along

the stream lines About 500-BOO kms off the Indian west coast

the wind speed decreases accompanied by a remarkable increase

of shower activity These facts strongly support our present

view of the dynamic effects of the wind vergences in contrast

to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)

cl The remarkable Foumlhn effect on the eastern side of the

West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a

frequency of 29-30 rain-days a month from June to September)

we observe only 20-40 km east of the crest not more than

- 23 shy

600-800 mm mostly produced by a few travelling synoptic

disturbances In some of the deepest valleys of the Himalaya

Mountains - even in the rainiest areas north of Assam - the

valley bottom is almost arid (as demonstrated by the vegeshy

tation) (SCHWEINFURTH 1956) since the local circulations

always control the pattern of wind components clouds and

rain (FLOHN 1970 DITTMANN 1970) This is also true in the

deep meridional gorges of Upper Burma - in spite of the high

cloudiness and moisture content of the air during late spring

and summer below the quasi-permanent anticyclonic cell in the

upper troposphere The slight anticyclonic curvature of the

lower westerlies crossing the Ghats (Figure 2) might also be

related to the permanent release of latent heat

d) The distribution of land- and sea-breezes controls a large

part of the convective activity The frequency and extend of

these diurnal circulations along all the coasts of the subshy

continent deserves much more attention together with their

effect on the weather During the summer monsoon these

diurnal circulations are to a large extend weakened or even

suppressed During the remaining seasons the quasi-geostrophic

flow near the surface is much weaker the diurnal radiation

cycle is more effective due to low cloudiness and the diurnal

circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)

middote) Careful analysis of the diurnal wind circulation at the

Himalayas led to the unexpected conclusion that the usual

reversal from the ascending day-time circulation to the

descending nighttime circulation is to a large extend

suppressed by the permanence of the above-mentioned heat

center in the upper troposphere during night (FLOHN 196B

1970) Thus in many valleys in the Himalayas (including the

meridional gorges) the weak nighttime mountain winds are

either superimposed with upstream winds or are replaced by calms

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 16 shy

cells frequently centered in the south-eastern portion of

the Tibetan highlands Sometimes the travelling upper troughs of the westerlies may penetrate near 200 mb to about latishy

tude 26-27oN but hardly ever farther south If~ in such a case a low-level monsoon depression approaches from southshy

east it can be intensified and steered into a northerly

direction and eventually merge with the trough Then torrential

rains floods and landslides are observed in the Himalayas

while the divergence of the west-south-west flow above the

peninsula produces widespread subsidence and suppression of

convective activity This is at least one of the mechanisms

responsible for the well-known monsoon breaks (RAHMATULLAH

1952 RIEHL 1954) In fact if we disregard the relatively

small areas of orographically forced convective rains and of

permanent convergence in the north-eastern portion of the

subcontinent we may safely say that ove~ the largest part

of the Indo-Pakistan area the summer rains are produced by

well-differentiated synoptic events and cannot be conceived

as a more or less steady climatic feature Even the quasishy

permanent anticyclonic cell above south-eastern Tibet (FLOHN 1958 1968) can be swept away by a travelling westerly trough

with a cold air outbreak across Tibet it recovers however

after two to three days sometimes being reinforced by warm

air advection Thererore maps of rainfall frequency (SCHWEINshyFURTH et al 1970) are a necessary implementation of the

well-known maps of precipitation amount

The track of these monsoon depressions is controlled by the

upper flow or bette~ by integration of the wind field

horizontally and vertically with respect to pressure and

taking into account the well-known meridional variation of

the Coriolis parameter szlig = dfdy Under normal conditions

they are therefore steered from east-south-east to west-northshywest only gradually diminishing in intensity and rainfall

productivity mostly as fairly symmetrie vortices near the

- 17 shy

o0 I I Irltt I 60 0 110deg

Flow Patterns 800-900mbs-+

100 - 300 IIE-

Cold Warm Co Id Warm100 I mbs INDIRECT

500 CELL GEClCELL

Conv900 Div Div Conv I

A A B 8

Fig 4 Large-scale features of the Indian Summer Monsoon

Above flow patterns in lower and upper troposphere Below cross-circulation along B - B (entrance)

and A - A (exit region of the Tropical Easterly Jet)

- 18 shy

S50 roh level Apparently lifting and consequently rainfall

reach a maximum in their southern section there is hardly

sufficient evidence of definite air-mass differences in their

area Generally speaking air-mass differences during the

monsoon season above the subcontinent are much smaller than

in middle latitudes and hardly detectable in the lower

troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air

above SON is about 7-10oC cooler than the subtropical air

above the heat eentre at 28-30oN This reversed temperature

gradient is eorrelated with an extremely persistent tropiealo

easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is

of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh

have a duration of three to six days and varying between

about 40 and more than 100 knots in the centre According to

recent studies by RAMAN (1964) these pulses are eorrelated

with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which

intensifies the anticyelonie eells above the Himalayas

On the average the speed of the Tropical Easterly Jet inshy

creases (together with a northerly component of the winds)

east of 850 E and decreases (together with a weak southerly

eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign

near 8SoE (Figure 4) Above SE Asia ascending motion with

strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions

are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN

1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence

Zone at the surface) in contrast to lifting motions along

~

- 19 shy

the southern edge (the belt of highest rainfall along Lat

7-14oN)

Nevertheless climatic features near surfaee are by no means

unimportant in the large-scale meehanism of the summer-monsoon

rains in southern Asia We list only a few of them

a) The reversal of the temperature pressure and wind field

of the middle and upper troposphere is preceded by the gradual

warming of northern India the Tibetan highlands and the

adjaeent highlands Bere we have to distinguish two main

sources of internal and potential energy

1 The flux of sensible heat into the air from the elevated

heat souree of the arid (western and central) seetion of

the Tibetan highlands Here all terms of the heat balance

can be assumed to be nearly equal to those in the adjacent

arid lowlands of centra1 Asia Suffieient evidence for

this assumption has been presented by AIZENSHTAT (1966)

from many heat balance investigations in the arid high

mountains of the USSR at altitudes of 3100-4000 m The

average input of sensible heat into the atmosphere amounts

to 180-250 Lyday (1 Ly = 1 calcm2)

2 The release of latent heat from the enormous quantities

of rain falling on the superhumid mountain areas of Bengal

Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more

than the global radiation at the surfaee can yield Its

effect has been suggested generally (FLOHN 1953) as an

addition to the anticyclonic curvature of the upper westershy

lies along high mountain ridges Oue to the rapid decrease

of moisture with height it will increase the thiekness of

the lower troposphere however the aseending motion in the

eb towers carries much heat upward to the 150-300 mb layer

- 20 shy - 21 shy

Most of this latent heat is used for heating up the upper

troposphere to a moist-adiabatic lapse-rate Since in SEshy

Tibet and Assam the average vertical lapse-rate is slightly

super-moist-adiabatie any ascending parcel of air in the

eondensation layer between 775 and 205 mb is about 16oC

warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in

the arid areas

The effect of the direct warming of air above southernmost

and western Tibet and north-eastern India has been qualitashy

tively stressed by the author (1950 1960 1968) Strong

evidenee has been contributed in a model eomputation by

MURAKAMI (1958) who started from the average 300 mb-flow

during May introduced a heat source above the whole

Tibetan plateau and obtained after a model time of 72

hours an antieyelonic eell in southeastern Tibet In a

more recent model of a two-dimensional meridional cross-

section along BOoE the role of the mountains and of the

hydrologie cycle has been investigated numerically by the

same author (1969) Using correlation analysis ASAKURA

(196B) recently demonstrated the strong persistence and

the large-seale influence of the Tibetan anticyelone at

the 500 mb-level

Careful analysis of all available aerological data shy

taking Into account the diurnal variation of upper air

temperatures as well as thermaI winds (FLOHN 196B) shy

revealed beyond any doubt the existenee of an elongated

heat zone in the upper troposphere (300-500 mb) above the

Himalayas and southern Tibet centered at 300 N and 850S

(Figure 5)

r e

N

~ I

2 I

~

abull

+ lt lJl middotri

t 0

Q) ~ III U Ul

Ul Cl U t 0

-rl ~

-1 Q) III gtlt

III ~ ~ Q)

c + 0

Q)Cl t nl l +

nl (l) (l)

gtltIII ~

-1 Q) +

~ t rtI Q)0

0 S If)

I 0 0 + 0 c M -rl

Ul t Ul Q) Q) t + 10( + U lt1l - p c

+ Il Ul Ul Q) + t 104

10( Ql Q ~ c 0

u If)

lJl r

- 22 shy

b) In the arid north-western part of the subcontinent (Sind

Rajasthan) the aridity is maintained even during the monsoon

season when moist air with an almost constant dewpoint near 0

25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days

when monsoon depressions travelling from the east or south

penetrate this area On all other days - ie during more

than 90 per cent of the season - the permanent and steady

monsoon flow is divergent turning from south-west above

Jodhpur and Gwalior to south-south-east above Jacobabad and

therefore subject to large-seale subsidence (FLOHN et al

196B) which reduces the vertical extension of the moist

layer to about 15-2 km and its eloud cover to shallow

stratocumuli (sc) This divergence can be understood if we

take into aceount the average position of the pressure trough

with its anticyelonic curvature from eastern Arabia to the

Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1

Similarly the SW-monsoon above the Arabian Sea is - at least

west of Long 65-68o

E - essentially rainless with a shallow

layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by

directional diffluence but by the increasing velocity along

the stream lines About 500-BOO kms off the Indian west coast

the wind speed decreases accompanied by a remarkable increase

of shower activity These facts strongly support our present

view of the dynamic effects of the wind vergences in contrast

to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)

cl The remarkable Foumlhn effect on the eastern side of the

West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a

frequency of 29-30 rain-days a month from June to September)

we observe only 20-40 km east of the crest not more than

- 23 shy

600-800 mm mostly produced by a few travelling synoptic

disturbances In some of the deepest valleys of the Himalaya

Mountains - even in the rainiest areas north of Assam - the

valley bottom is almost arid (as demonstrated by the vegeshy

tation) (SCHWEINFURTH 1956) since the local circulations

always control the pattern of wind components clouds and

rain (FLOHN 1970 DITTMANN 1970) This is also true in the

deep meridional gorges of Upper Burma - in spite of the high

cloudiness and moisture content of the air during late spring

and summer below the quasi-permanent anticyclonic cell in the

upper troposphere The slight anticyclonic curvature of the

lower westerlies crossing the Ghats (Figure 2) might also be

related to the permanent release of latent heat

d) The distribution of land- and sea-breezes controls a large

part of the convective activity The frequency and extend of

these diurnal circulations along all the coasts of the subshy

continent deserves much more attention together with their

effect on the weather During the summer monsoon these

diurnal circulations are to a large extend weakened or even

suppressed During the remaining seasons the quasi-geostrophic

flow near the surface is much weaker the diurnal radiation

cycle is more effective due to low cloudiness and the diurnal

circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)

middote) Careful analysis of the diurnal wind circulation at the

Himalayas led to the unexpected conclusion that the usual

reversal from the ascending day-time circulation to the

descending nighttime circulation is to a large extend

suppressed by the permanence of the above-mentioned heat

center in the upper troposphere during night (FLOHN 196B

1970) Thus in many valleys in the Himalayas (including the

meridional gorges) the weak nighttime mountain winds are

either superimposed with upstream winds or are replaced by calms

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 18 shy

S50 roh level Apparently lifting and consequently rainfall

reach a maximum in their southern section there is hardly

sufficient evidence of definite air-mass differences in their

area Generally speaking air-mass differences during the

monsoon season above the subcontinent are much smaller than

in middle latitudes and hardly detectable in the lower

troposphere However in the upper troposphere we observe a remarkable baroclinie structure where the equatorial air

above SON is about 7-10oC cooler than the subtropical air

above the heat eentre at 28-30oN This reversed temperature

gradient is eorrelated with an extremely persistent tropiealo

easterly jet (FLOHN 1964) centred near latitude IS N one or two km below the tropieal tropopause This easterly jet is

of remarkable persistence from day to day with only oecasional fluetuations of the wind velocity (pulses) whieh

have a duration of three to six days and varying between

about 40 and more than 100 knots in the centre According to

recent studies by RAMAN (1964) these pulses are eorrelated

with fluctuations of the rainfall above the subcontinent and thus with the varying release of latent heat which

intensifies the anticyelonie eells above the Himalayas

On the average the speed of the Tropical Easterly Jet inshy

creases (together with a northerly component of the winds)

east of 850 E and decreases (together with a weak southerly

eomponent) towards the west This causes (FLOHN 1964) a eross-isobaric meridional circulation reverting its sign

near 8SoE (Figure 4) Above SE Asia ascending motion with

strong cloudiness and rainfall prevails along the northern edge of the system (Lat 20-32oN) while subsiding motions

are frequently found along th~ southern edge near the equator Above SW-Asia and Africathe reverse is true (FLOHN

1964) large-scale subsidence tendency along the northern flane (including the area of the Intertropical Convergence

Zone at the surface) in contrast to lifting motions along

~

- 19 shy

the southern edge (the belt of highest rainfall along Lat

7-14oN)

Nevertheless climatic features near surfaee are by no means

unimportant in the large-scale meehanism of the summer-monsoon

rains in southern Asia We list only a few of them

a) The reversal of the temperature pressure and wind field

of the middle and upper troposphere is preceded by the gradual

warming of northern India the Tibetan highlands and the

adjaeent highlands Bere we have to distinguish two main

sources of internal and potential energy

1 The flux of sensible heat into the air from the elevated

heat souree of the arid (western and central) seetion of

the Tibetan highlands Here all terms of the heat balance

can be assumed to be nearly equal to those in the adjacent

arid lowlands of centra1 Asia Suffieient evidence for

this assumption has been presented by AIZENSHTAT (1966)

from many heat balance investigations in the arid high

mountains of the USSR at altitudes of 3100-4000 m The

average input of sensible heat into the atmosphere amounts

to 180-250 Lyday (1 Ly = 1 calcm2)

2 The release of latent heat from the enormous quantities

of rain falling on the superhumid mountain areas of Bengal

Assam and Upper Burma This has an estimated area-averaged value for the cloud layer of 900-1200 Lyd ie more

than the global radiation at the surfaee can yield Its

effect has been suggested generally (FLOHN 1953) as an

addition to the anticyclonic curvature of the upper westershy

lies along high mountain ridges Oue to the rapid decrease

of moisture with height it will increase the thiekness of

the lower troposphere however the aseending motion in the

eb towers carries much heat upward to the 150-300 mb layer

- 20 shy - 21 shy

Most of this latent heat is used for heating up the upper

troposphere to a moist-adiabatic lapse-rate Since in SEshy

Tibet and Assam the average vertical lapse-rate is slightly

super-moist-adiabatie any ascending parcel of air in the

eondensation layer between 775 and 205 mb is about 16oC

warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in

the arid areas

The effect of the direct warming of air above southernmost

and western Tibet and north-eastern India has been qualitashy

tively stressed by the author (1950 1960 1968) Strong

evidenee has been contributed in a model eomputation by

MURAKAMI (1958) who started from the average 300 mb-flow

during May introduced a heat source above the whole

Tibetan plateau and obtained after a model time of 72

hours an antieyelonic eell in southeastern Tibet In a

more recent model of a two-dimensional meridional cross-

section along BOoE the role of the mountains and of the

hydrologie cycle has been investigated numerically by the

same author (1969) Using correlation analysis ASAKURA

(196B) recently demonstrated the strong persistence and

the large-seale influence of the Tibetan anticyelone at

the 500 mb-level

Careful analysis of all available aerological data shy

taking Into account the diurnal variation of upper air

temperatures as well as thermaI winds (FLOHN 196B) shy

revealed beyond any doubt the existenee of an elongated

heat zone in the upper troposphere (300-500 mb) above the

Himalayas and southern Tibet centered at 300 N and 850S

(Figure 5)

r e

N

~ I

2 I

~

abull

+ lt lJl middotri

t 0

Q) ~ III U Ul

Ul Cl U t 0

-rl ~

-1 Q) III gtlt

III ~ ~ Q)

c + 0

Q)Cl t nl l +

nl (l) (l)

gtltIII ~

-1 Q) +

~ t rtI Q)0

0 S If)

I 0 0 + 0 c M -rl

Ul t Ul Q) Q) t + 10( + U lt1l - p c

+ Il Ul Ul Q) + t 104

10( Ql Q ~ c 0

u If)

lJl r

- 22 shy

b) In the arid north-western part of the subcontinent (Sind

Rajasthan) the aridity is maintained even during the monsoon

season when moist air with an almost constant dewpoint near 0

25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days

when monsoon depressions travelling from the east or south

penetrate this area On all other days - ie during more

than 90 per cent of the season - the permanent and steady

monsoon flow is divergent turning from south-west above

Jodhpur and Gwalior to south-south-east above Jacobabad and

therefore subject to large-seale subsidence (FLOHN et al

196B) which reduces the vertical extension of the moist

layer to about 15-2 km and its eloud cover to shallow

stratocumuli (sc) This divergence can be understood if we

take into aceount the average position of the pressure trough

with its anticyelonic curvature from eastern Arabia to the

Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1

Similarly the SW-monsoon above the Arabian Sea is - at least

west of Long 65-68o

E - essentially rainless with a shallow

layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by

directional diffluence but by the increasing velocity along

the stream lines About 500-BOO kms off the Indian west coast

the wind speed decreases accompanied by a remarkable increase

of shower activity These facts strongly support our present

view of the dynamic effects of the wind vergences in contrast

to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)

cl The remarkable Foumlhn effect on the eastern side of the

West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a

frequency of 29-30 rain-days a month from June to September)

we observe only 20-40 km east of the crest not more than

- 23 shy

600-800 mm mostly produced by a few travelling synoptic

disturbances In some of the deepest valleys of the Himalaya

Mountains - even in the rainiest areas north of Assam - the

valley bottom is almost arid (as demonstrated by the vegeshy

tation) (SCHWEINFURTH 1956) since the local circulations

always control the pattern of wind components clouds and

rain (FLOHN 1970 DITTMANN 1970) This is also true in the

deep meridional gorges of Upper Burma - in spite of the high

cloudiness and moisture content of the air during late spring

and summer below the quasi-permanent anticyclonic cell in the

upper troposphere The slight anticyclonic curvature of the

lower westerlies crossing the Ghats (Figure 2) might also be

related to the permanent release of latent heat

d) The distribution of land- and sea-breezes controls a large

part of the convective activity The frequency and extend of

these diurnal circulations along all the coasts of the subshy

continent deserves much more attention together with their

effect on the weather During the summer monsoon these

diurnal circulations are to a large extend weakened or even

suppressed During the remaining seasons the quasi-geostrophic

flow near the surface is much weaker the diurnal radiation

cycle is more effective due to low cloudiness and the diurnal

circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)

middote) Careful analysis of the diurnal wind circulation at the

Himalayas led to the unexpected conclusion that the usual

reversal from the ascending day-time circulation to the

descending nighttime circulation is to a large extend

suppressed by the permanence of the above-mentioned heat

center in the upper troposphere during night (FLOHN 196B

1970) Thus in many valleys in the Himalayas (including the

meridional gorges) the weak nighttime mountain winds are

either superimposed with upstream winds or are replaced by calms

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 20 shy - 21 shy

Most of this latent heat is used for heating up the upper

troposphere to a moist-adiabatic lapse-rate Since in SEshy

Tibet and Assam the average vertical lapse-rate is slightly

super-moist-adiabatie any ascending parcel of air in the

eondensation layer between 775 and 205 mb is about 16oC

warmer than its surrounding and releases thus furthermore about 220 Lyd~y similar to the input of sensible heat in

the arid areas

The effect of the direct warming of air above southernmost

and western Tibet and north-eastern India has been qualitashy

tively stressed by the author (1950 1960 1968) Strong

evidenee has been contributed in a model eomputation by

MURAKAMI (1958) who started from the average 300 mb-flow

during May introduced a heat source above the whole

Tibetan plateau and obtained after a model time of 72

hours an antieyelonic eell in southeastern Tibet In a

more recent model of a two-dimensional meridional cross-

section along BOoE the role of the mountains and of the

hydrologie cycle has been investigated numerically by the

same author (1969) Using correlation analysis ASAKURA

(196B) recently demonstrated the strong persistence and

the large-seale influence of the Tibetan anticyelone at

the 500 mb-level

Careful analysis of all available aerological data shy

taking Into account the diurnal variation of upper air

temperatures as well as thermaI winds (FLOHN 196B) shy

revealed beyond any doubt the existenee of an elongated

heat zone in the upper troposphere (300-500 mb) above the

Himalayas and southern Tibet centered at 300 N and 850S

(Figure 5)

r e

N

~ I

2 I

~

abull

+ lt lJl middotri

t 0

Q) ~ III U Ul

Ul Cl U t 0

-rl ~

-1 Q) III gtlt

III ~ ~ Q)

c + 0

Q)Cl t nl l +

nl (l) (l)

gtltIII ~

-1 Q) +

~ t rtI Q)0

0 S If)

I 0 0 + 0 c M -rl

Ul t Ul Q) Q) t + 10( + U lt1l - p c

+ Il Ul Ul Q) + t 104

10( Ql Q ~ c 0

u If)

lJl r

- 22 shy

b) In the arid north-western part of the subcontinent (Sind

Rajasthan) the aridity is maintained even during the monsoon

season when moist air with an almost constant dewpoint near 0

25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days

when monsoon depressions travelling from the east or south

penetrate this area On all other days - ie during more

than 90 per cent of the season - the permanent and steady

monsoon flow is divergent turning from south-west above

Jodhpur and Gwalior to south-south-east above Jacobabad and

therefore subject to large-seale subsidence (FLOHN et al

196B) which reduces the vertical extension of the moist

layer to about 15-2 km and its eloud cover to shallow

stratocumuli (sc) This divergence can be understood if we

take into aceount the average position of the pressure trough

with its anticyelonic curvature from eastern Arabia to the

Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1

Similarly the SW-monsoon above the Arabian Sea is - at least

west of Long 65-68o

E - essentially rainless with a shallow

layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by

directional diffluence but by the increasing velocity along

the stream lines About 500-BOO kms off the Indian west coast

the wind speed decreases accompanied by a remarkable increase

of shower activity These facts strongly support our present

view of the dynamic effects of the wind vergences in contrast

to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)

cl The remarkable Foumlhn effect on the eastern side of the

West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a

frequency of 29-30 rain-days a month from June to September)

we observe only 20-40 km east of the crest not more than

- 23 shy

600-800 mm mostly produced by a few travelling synoptic

disturbances In some of the deepest valleys of the Himalaya

Mountains - even in the rainiest areas north of Assam - the

valley bottom is almost arid (as demonstrated by the vegeshy

tation) (SCHWEINFURTH 1956) since the local circulations

always control the pattern of wind components clouds and

rain (FLOHN 1970 DITTMANN 1970) This is also true in the

deep meridional gorges of Upper Burma - in spite of the high

cloudiness and moisture content of the air during late spring

and summer below the quasi-permanent anticyclonic cell in the

upper troposphere The slight anticyclonic curvature of the

lower westerlies crossing the Ghats (Figure 2) might also be

related to the permanent release of latent heat

d) The distribution of land- and sea-breezes controls a large

part of the convective activity The frequency and extend of

these diurnal circulations along all the coasts of the subshy

continent deserves much more attention together with their

effect on the weather During the summer monsoon these

diurnal circulations are to a large extend weakened or even

suppressed During the remaining seasons the quasi-geostrophic

flow near the surface is much weaker the diurnal radiation

cycle is more effective due to low cloudiness and the diurnal

circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)

middote) Careful analysis of the diurnal wind circulation at the

Himalayas led to the unexpected conclusion that the usual

reversal from the ascending day-time circulation to the

descending nighttime circulation is to a large extend

suppressed by the permanence of the above-mentioned heat

center in the upper troposphere during night (FLOHN 196B

1970) Thus in many valleys in the Himalayas (including the

meridional gorges) the weak nighttime mountain winds are

either superimposed with upstream winds or are replaced by calms

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 22 shy

b) In the arid north-western part of the subcontinent (Sind

Rajasthan) the aridity is maintained even during the monsoon

season when moist air with an almost constant dewpoint near 0

25 C sweeps aeross with southerly winds from the eoast to the foothilis of the Himalayas Rain oceurs only on a few days

when monsoon depressions travelling from the east or south

penetrate this area On all other days - ie during more

than 90 per cent of the season - the permanent and steady

monsoon flow is divergent turning from south-west above

Jodhpur and Gwalior to south-south-east above Jacobabad and

therefore subject to large-seale subsidence (FLOHN et al

196B) which reduces the vertical extension of the moist

layer to about 15-2 km and its eloud cover to shallow

stratocumuli (sc) This divergence can be understood if we

take into aceount the average position of the pressure trough

with its anticyelonic curvature from eastern Arabia to the

Punjab and the Indo-Gangetic plains which forces the frictional low-level winds to diverge (Figure 2) 1

Similarly the SW-monsoon above the Arabian Sea is - at least

west of Long 65-68o

E - essentially rainless with a shallow

layer of broken sc Here also subsidence is produced by divergence but in eontrast to the Indus plains not by

directional diffluence but by the increasing velocity along

the stream lines About 500-BOO kms off the Indian west coast

the wind speed decreases accompanied by a remarkable increase

of shower activity These facts strongly support our present

view of the dynamic effects of the wind vergences in contrast

to the (formerly much overestimated) concept of conservative air-masses (FLOHN et al 1968)

cl The remarkable Foumlhn effect on the eastern side of the

West Ghats is well-known while near the crest of the hills the seasonal rainfall reaches 6000 mm (Mahabaleshwar has a

frequency of 29-30 rain-days a month from June to September)

we observe only 20-40 km east of the crest not more than

- 23 shy

600-800 mm mostly produced by a few travelling synoptic

disturbances In some of the deepest valleys of the Himalaya

Mountains - even in the rainiest areas north of Assam - the

valley bottom is almost arid (as demonstrated by the vegeshy

tation) (SCHWEINFURTH 1956) since the local circulations

always control the pattern of wind components clouds and

rain (FLOHN 1970 DITTMANN 1970) This is also true in the

deep meridional gorges of Upper Burma - in spite of the high

cloudiness and moisture content of the air during late spring

and summer below the quasi-permanent anticyclonic cell in the

upper troposphere The slight anticyclonic curvature of the

lower westerlies crossing the Ghats (Figure 2) might also be

related to the permanent release of latent heat

d) The distribution of land- and sea-breezes controls a large

part of the convective activity The frequency and extend of

these diurnal circulations along all the coasts of the subshy

continent deserves much more attention together with their

effect on the weather During the summer monsoon these

diurnal circulations are to a large extend weakened or even

suppressed During the remaining seasons the quasi-geostrophic

flow near the surface is much weaker the diurnal radiation

cycle is more effective due to low cloudiness and the diurnal

circulations are much more regular and effective than in summer (ANANTHAKRISHNAN amp RAO 1964)

middote) Careful analysis of the diurnal wind circulation at the

Himalayas led to the unexpected conclusion that the usual

reversal from the ascending day-time circulation to the

descending nighttime circulation is to a large extend

suppressed by the permanence of the above-mentioned heat

center in the upper troposphere during night (FLOHN 196B

1970) Thus in many valleys in the Himalayas (including the

meridional gorges) the weak nighttime mountain winds are

either superimposed with upstream winds or are replaced by calms

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 24 shy

D The retreat of the monsoon during September and October

(REITER amp HEUBERGER 1960) seems not to be merely a mirror

of the events during the onset in early June The large-scale

reversamiddotl of the wind field is somewhat more gradual and

relatively slow but low-Ievel cyclones still cross the

peninsula and may be steered in a northward direction causing

veritable cloudbursts and severe floods Due to the gradual

~~ifting of their tracks they often have the opportunity to

ltevelop over ocean areas into full-size warm-core hurricanes

notably over the Gulf of Bengal From October to December

tropical hurricanes and other cyclones of minor intensity

are fairly frequent here they can almost certainly be tracked

along their path from the east ie from the Gulf of Thailand

They are responsible for the late fall rains on the east coast

(and over the southcentral part) of the peninsula as weIl as

for the rainy season on all eastfacing coasts of south-east

Asia In contrast to spring they are in this season relatively

rare (but not absent) over the Arabian Sea Since the northshy

east flow itself is shallow and mostly stable at least north

of about latitude 12oN its effect on rainfall is small The

fall rains along the east coasts are synoptic events quite irregular (6-12 days a month) and subject to large variations

from year to year Since the cyclonic disturbances usually

travel towards west the shearing intensity reaches its maxishy

mum at their western side Together with the increase of

surface stress at the east-facing coasts this leads to a

marked rainfall maximum along these coasts

E In order to avoid further misunderstanding it should be

stressed that the monsoonal reversal of the surface winds is

by no means identical with begin and end of the rainy season

In fact the rainy season starts in the northeastern area of

the subcontinent about two months earlier than the reversal

of (upper) winds while in the heart of the peninsula the

- 25 shy

low-level wind shift precedes the onset of the monsoon rains

by about the same time

The early onset of rains in NE-India contributes - together

with the seasonal warming of the elevated Tibetan highlands shy

to the development of the high-tropospheric summer anticyclones

above the Himalayas This event produces the reversal of the

winds of the mlddle and upper troposphere which now triggers

the onset of the monsoon rains above the largest portion of

the subcontinent Such an interaction between thermodynamic

and dynamic processes is by no means uncommon in the largeshy

scale atmospheric circulation In this area it is not merely

a seasonal shift but an orographically fixed reversal of

temperature and pressure gradients which leads to such dramatic

(and far-reaching) consequences Before the existence of the

Tibetan highlands - ie before the late-Tertiary (and recent)

uplift of the mountains - the seasonal sequence of weather was much more gradual similar to that above northern Africa

or Australia This is also evident from the recent model

computation of MURAKAMI (1969)

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 26 shy

References

AIZENSHTAT BA 1966 Investigations on the heat budget of

Central Asia

In MI BUDYKO Actual problems of climatology

(russ) Leningrad 94-129

ANANTHAKRISHNAN Rand KV RAO 1964 Diurnal variation of

~ow level circulation over India

Proc WMO SymposTropMeteor Rotorua NZ 5-13

Nov 1963 New Zealand Meteor Service Wellington 89-95

ASAKURA T 1968 Dynamic climatology of atmospheric circushy

lation over East Asia centered in Japan

Papers in Meteorology and Geoohysics 19 1-68

DITTMANN E 1970 Statistische Untersuchungen zur Struktur

der Niederschlaumlge in Nepal

In W HELLMICH (Hrsg) Khumbu Himal 7 Lieferung 2 47-60

FLOHN H 1950 Studien zur allgemeinen Zirkulation der Erdshyoberflaumlche

Berichte des Deutschen Wetterdienstes in der US-Zone Nr 18 52 S

FLOHN H 1953 Hochgebirge und allgemeine Zirkulat~on II Die

Gebirge als Waumlrmequellen

Archiv fuumlr Meteorologie Geophysik und Bioklimatoloshy

gie Sero A 5 265-279

FLOHN H 1956 Der indische Sommermonsun als Glied der planeshy

tarischen Zirkulation der Atmosphaumlre

Berichte des Deutschen Wetterdienstes Nr 22 134-139

FLOHN H 1958 Beitraumlge zur Klimakunde von Hochasien

Erdkunde 12 294-308

- 27 shy

FLOHN H 1959 Bemerkungen zur Klimatologie von Hochasien

Aktuelle Schneegrenze und Sommerklima

Akademie der Wissenschaften und der Literatur

Abhandlungen der mathematisch-naturwissenschaftlichen

Klasse Nr 14 Mainz 1409-1431

FLOHN H 1960 Monsoon winds and general circulation

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1960 Recent investigations on the mechanism of the

summer monsoon of southern and eastern Asia

In Monsoons of the world Sympos Meteor Office

New Delhi 19-21 Febr 1958 New Delhi 65-74

FLOHN H 1964 Investigations on the tropical easterly jet

Bonner Meteorologische Abhandlungen Heft 4 83 pp

Flohn H 1965 Comments on a synoptic climatology of southern

Asia

WMO Techn Note No 69 245-252

FLOHN H 1968 Contributions to a meteorology of the Tibetan

Highlands

Colorado State Univ Fort Collins Atmosph Sei

Papers No 130 120 pp

FLOHN H 1969 Zum Klima und Wasserhaushalt des Hindukuschs

und der benachbarten Hochgebirge

Erdkunde 23 205-215

FLOHN H 1970 Beitraumlge zur Meteorologie des Himalaya

In W HELLMICH (Hrsg) Khumbu Himal 7 Lief2 25-47

FLOHN H M HANTEL ana E RUPRECHT 1968 Air-mass dynamics

or subsidence processes in the Arabian Sea summer

monsoon

J Atmosph Sci 25 527-529

KRISHNAMURTI TS 1961 1he subtropical jet stream of winter

J Meteor 18 172-191

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9

- 28 shy

MURAKAMI T 1958 The sudden change of upper westerlies

near the Tibetan plateau at the beginning of summer

season

J of the Meteorological Society of Japan 36 239-247

MURAKAMI T and Coll 1969 unpubl report Univ Hawaii

Dept of Geosciences

RAHMATULLAH M 1952 Synoptic aspects of the monsoon circushy

lation and rainfall over Indo-Pakistan

J Meteor 9 176-179

RAMAGE CS 1952 Relationship of general circulation to

normal weather over southern Asia and the western

Pacific during the cool season

J Meteor 9 403-408

RAMAN CR and Y RAMANATHAN 1964 Interaction between lower

and upper tropical tropospheres

Nature 204 Nr 4953 31-35

RAMASWAMY C 1956 On the sub-tropical jet stream and its role in the development of large-scale cenvection~

Tellus 8 26-60

REITER ER and H HEUBERGER 1960 A synoptic exampe of the

retreat of the Indian summer monsoon

Geogr Ann 42 Nr 1 Stockholm 17-35

RIEHL H 1954 Tropical meteorology New York Toronto London McGraw-Hill 392 pp

SCHWEINFURTH U 1956 Uber klimatische Trockentaumller im

Himalaya

Erdkunde 10 297-302

SCHWEINFURTH U H Flohn and M DOMROumlS 1970 Studies in the

climatology of South Asia

Wiesbaden Steiner 16 pp 15 maps

• K216_8und9
• K216_ohne_8und9