changes in the characteristics of typhoons ...philippines is the same for intense and less intense...
TRANSCRIPT
Approved OT public release; £NVPREDRSCHFAC
01st ibutiu iImited. Technical Paper No, 6-72
CHANGES IN THE CHARACTERISTICS OF TYPHOONS
CROSSING THE PHILIPPINES
SAMSON SIAND
ad
JACK W. BLELLOCH
MAY 1972
NATiON Nt TECHNICALINFORMAtON SERVICE
ENVIRONMENTAL PREDICTION RF•SEARCH FACItLITYN ¶AVAL PO4 ' AT" 7 " .
,MPONTERE BUIATfi SC99OO4MONHREYI •A !FORI•L 93940
S Approved for Public Release; ENVPRiEDRSCHFACDistribution Unlimited. Technical Paper No. 6-72
CHANGES IN THE CHARACTERISTICS OF TYPHOONS.CROSSING THE PHILIPPINES
by
I° Samson Brand
and
Jack W. Blelloch
MAY 1972
ENVIRONMENTAL PREDICTION RESEARCH FACILITYNAVAL POS-GRADUATE SCHOOL
MONTEREY, CALIFORNIA 93940
rf :ro
FABSTRACT
Thirty typhoons (1960-1970) are examined to determine
the effect of the Philippines on the intensity, speed of
movement and size characteristics of tropical cyclones
crossing the Philippines. The results show an average in-I •tensity (maximum surface wind) decrease of 33%, a northward
perturbation as the storms pass through the Islands, and a
decrease of circulation size for weak typhoons. The study
also showed an increase in speed of movement as storms
approach the Philippines.
Z
Preceding page blank
ii
It ~--
CONTENTS
P ageABSTRACT ............. ....................... .iiLIST OF ILLUSTRATIONS ........ ............... vii
1. INTRODUCTION ......... ..................... 1
2. DATA SOURCES ........... ...................... .. 5
3. DISCUSSION OF RESULTS ............. .............. 7
4. SUMMARY ......................... 21
REFERENCES .................................. 23
APPENDIX A -TRACKS OF TROPI( AL STORMS ANDTYPHOONS (1945-ý.ý9) . . . . . . . . . . . 25
Preceding page blank
LIST OF ILLUSTRATIONS
Figures Page
1. Map of the western North Pacific Ocean .... ....... 2
2. Topographical map of the Philippine Islands . . . . 3
3. Track segments for 30 typhoons crossing thePhilippines ................ .................... 9
4. Average intensity and speed profiles for crossingtyphoons and average 24-hour right-angle forecasterrors ................... ....................... 10
5. Schematic presentation of 24-hour right-anglefor'ecast error ......... ................... .i.11
6. Average intensity and speed profiles for intenseand weak typhoons ........ ................. ... 14
7. Average intensity of typhoons prior to crossingthe Philippines versus change in intensity oftyphoons crossing the Philippines . . . . . . . . . 15
8. Eye diameter and size variation of typhoonsScrossing the Philippines ..... .............. ... 17
9. Eye diameter and size variation of intense andweak typhoons crossing the Philippines ....... ... 19
Table
1. Typhoons crossing the Philippines in the period1960-1970 ............ ....................... 8
Preceding page blank vii
1. INTRODUCTION
In a previous publication the forecaster was given sta-
tistical information on the geographic and seasonal variations
of tropical cyclone intensity changes for 100 x 100 latitude/
longitude areas of WESTPAC (Brand and Gaya, 1971). Weakening
tropical cyclones at low latitudes were examined in a separate
publication (Brand, 1972). It became evident from these
general studies that a detailed examination of tropical
cyclones crossing the Philippines would provide a useful input
to the tropical cyclone forecaster, since in many situations
he has to forecast the movement and intensity of storms to the
west of the Philippines while they are still to the east of
the Islands.
The purpose of this report is to familiarize the fore-
caster with those characteristics of typhoons that are af-
fected by the Philippines and to quantitatively describe these
relationships. This information, in conjunction with conven-
tional prediction techniques, should be a useful forecast aid
to the tropical cyclone forecaster.
A map of the western North Pacific Ocean area and a
topographical map of the Philippine Islands are presented
as Figures 1 and 2,respectively.
lii
0 0 0 0 00 C2 3 CD C
0
C3-
0
0 04
C-
C1.
04
0
K ..ii Hf
7
1150 1200 1250 1300
20020-
I UZON
PHILIPPINESEA
- -~ 150 ~--
SOUTH CHINA
, ';
U Q 100-
S?7 MINDANAO
5 0-
0 1000 3000 R14XY.
I11 I11MR-- I t IMIIT480Y F4 Ipf IIFthi
Figure 2. Topographical map of the Philippine Islands
3
2. DATA SOURCES
The data used for this study were extracted from the
Annual Typhoon Reports published by FWC/JTWC, Guam and from
a history file of tropical storms and typhoons of the western
North Pacific from 1945-1969, compiled by the National Climatic
Center (NCC) for and with the Navy Weather Research Facility.
The history file for the period 1945-1969 contains 6-hourly
information on such storm characteristics as the location,
movement, size and intensity of tropical cyclonic circulations
which, during their life cycle, reached tropical storm or
typhoon intensity.
In this study the typhoons from 1960-1970 were examined.
During this 11-year period there were 34 typhoons 1which hit
the Philippines; of this number, two recurved and two dissipated
while in the Islands. The remaining 30 typhoons were studied
in the time from -48 hours prior to hitting the Philippines
to +24 hours after leaving the Philippines. The parameters
examined in this time period were position (from fix 2 and
ltshould be noted that in the context of this paper, a t~iphoonis defined as a tropical cyclonic circulation which reached
* typhoon intensity at some stage in its life cycle. This intensityis not necessarily achieved during that part of the circulationexistence considered in this paper.
2 The determination of the position of a tropical. cyclone at aprecise time, generally by reconnaissance aircraft penetrationof the center or by airborne, land or ship radar or satellitephotographs.
Preceding page blank 5
best track3 data), intensity (maximum surface wind), speed,
size of circulation (average diameter of outer closed surface
isobar) and eye diameter.
3A post analysis position incorporating all available data.
3. DISCUSSION OF RESULTS
* Table 1 provides a list of the 30 typhoons investigated
in this study, as separated by m',,nth of crossing the Islands.
Fourteen of the 30 typhoons crossed in the October-November
period. The later months of the typhoon season have a great
deal of tropical cyclone activity in the area of the Philippines
as can be seen in Appendix A, which shows the tracks for trop-
ical storms and typhoons for monthly and half-monthly periods
(1945-1969).
The track segments for the 30 typhoons investigated in
the time frame from -48 hours prior to hitting the Philippines
to +24 hours after leaving the-Islands (center position of
storms) can be seen in Figure 3. All typhoon movement _s
from east to west. If the 6-hourly values of intensity and
speed during this before and after period are averaged, an
average intensity and speed profile for the 30 typhoons
crossing the Philippines can be compiled. This is shown in
Figures 4(a) and 4(b). The shaded area in the middle represents
the Philippines; to the right is before crossing and to the
left, after crossing. 1 The intensity profile (Figure 4(a))
shows an average increase in intensity during tre period 48
to 24 hours prior to hitting the Philippines. The intensity
then levels off at about 92 knots and then decreases to about
62 knots while crossing the region of the Islands (a decrease
IThe average time the 30 typhoons existed in the Philippineswas 14½ hours.
7
P44
>C >i CN 00N 00 woc ()0 F14 w k kz w - Y 0 0% L'
1-3rH i.14 r- C-1 r- H P 4 H o
o- E- 4 1 o - , ' - E1r0 H
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w ~04
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... ...-..-.. -- - 100
(ktsl
(a) Z 80LLJ
z 70
S. .. ........ [~ ... . ... . 0
(b) u k
U, 11
-- - 10
PositiveferIcasts to the North
CM 114.1 nm"11081- -10
1 - ' m '°0-3.ný .O m in(c) a=.- 3,9nm -2.0nm 0
C.Z 11081 1821,,,r [ - 10
,-18.3nin Negative.Forecasts to the South
~I7)jINumber offorecasts
f24 012 0 0 -12 -24 -36 -48
AFTER CROSSING IHours) BEFORE CROSSING
Figure 4. Average intensity (a) and speed (b) profilesfor 30 tyjhoons crossing the Philippines from -48hours prior to hitting the Philippines to +24 hoursafter leaving the Islands. The shaded area rep -e-sents the Philippines. The average 24-hour right-angle forecast errors (from best track) are pre-sented in section (c) of the figure.
10
of approximately 33%) before starting to increase in intensity
in the South China and Sulu Seas.
The average speed of movement profile (Figure 4(b)) shows
a decrease in speed until about 20 hours before hitting the
Philippines, then a slight acceleration approaching the Islands
(approximately a 10% increase in speed). and finally a decreaseL__•- in bpeed again as thf- stornms crcoss i Ip.L'L" "ines an ^
into the South China and Sulu Seas.
"Another parameter that was examined was the movement
variations of storms crossing the Philippines. This was done
, indirectly by examining 24-hour right-angle forecast errors;
that is, the forecast error to the right or left of the best
track (see Figure 5). In general, a 24-hour forecast position
- - - 24-Hour Forecast Position
24-HourRight-Angle Forecast Error
__ Initial
Best Track Position
Figure 5. Schematic presentation of 24-hour right-angle forecast error
i1
indicates where persistence and/or steering is tending to move
the storm. Any deviation from this, such as an average right-
angle forecast error from the best track, would point to an
effect not considered in the normal forecast. For example,
Figure 4(c) shows that the average right-angle forecast error
for the 82 available forecasts in the 48 to 24 hours prior to
hitting the Philippines is 2 n mi to the south of the best tracJ
This is reasonably near zero, which is to be expected if no
bias is present. In the 24 hours prior to hitting the Islands
the forecasts are again slightly to the south but near zero. It
will be noted, however, that the average forecast error in
the Philippines is 18.3 n mi to the south of the best track.
This suggests storm movement to the north not considered by
the forecaster. As storms leave the Islands the average fore-
cast error is 14.1 n mi to the north of the best track, suggest-
ing that either the storm centers jumped south or that the
northerly movement in the Philippines was accepted by forecasters
and this had an affect on the persistence element of the forecast.
The question then arises as to whether the effect of the
Philippines is the same for intense and less intense typhoons.
If those typhoons having an initial average intensity of > 90
knots in the 24-hour period prior to hitting the Philippines
are separated from those typhoons having an average initial
intensity of < 90 knots in the 24-hour period prior to hitting
the Philippines, a dramatic difference is noted. For example,
12
as seen in Figure 6(a), intense typhoons are considerably affected
by the Philippines, showing an average decrease in maximum wind of
45-50% while crossing the Islands. However, the weaker typhoons
show a decrease in maximum wind of only 10-15% and return to their
initial intensity within 24 hours of leaving the Philippines.
The speed of movement profiles (Figure 6(b)) show an
acceleration towards the Islands within 18 hours of landfall¶ for both weak and intense systems. However, the weak typhoons
are generally faster moving systems and are more affected by
the Islands. Note the marked decrease in speed through the
Islands shown by the weak typhoons.
Another manner in which to examine the effect of the
Philippines on typhoon intensity is to plot the average inten-
* •sity of the storm approaching the Islands versus the change in
storm intensity which occurred while traversing the Islands.
This can be seen in Figure 7 which is a plot of the average
24-hour intensity prior to hitting the Philippines (I ), versus
the change in intensity (I, -I 2 ),where 12 is the average 24-hour2
intensity after leaving the Philippines The distribution
has a correlation coefficient of 0.83 and the computed re-
gression line based on the distribution could be of use to the
forecaster. For example, Figure 7 shows that if the maximum
2It should again be pointed out that we are examining tropicalcyclones which at some stage reached typhoon intensity (Ž 64knots) but that this level of development did not necessarilyfall within the period 48 hours before reaching the Philippinesto 24 hours after leaving the Philippines.
13
-- 130
-120
INTENSE TYPHOON- -110
100I-INTENSE TYPHOONS -These typhoons having IktsJ
Luprier to hitting the Philippines ofatil knots. 9I- W1 oEAK
WEA TYPHOONS -These typheens having
70
-60
/ ¾. ~WEAK TYPHOONNS ___ 5
-13
b -12
00,-11
10
+24 +12 0 0 -12 -24 *N- -48AFTER CROSSING (Hours] BEFORE CROSSING
Picgurc 6. Averacgo intensity(a) and spood (b) profilesfor intense and wo,-k typhoons crossinct thue Philip--":i !es.
14
h = AVERAGE 24-HOUR INTENSITY PRIOR TO HITTING THE PHILIPPINES
12 =AVERAGEU24-HOUR INTENSITY AFTER LEAVING THE PHILIPPINES
S- 11-12 CHANGE IN AVERAGE INTENSITY
r - 2
<'z
JC
100-______
~- 80.V .-
a-
"60Cm,
4 0- -
,r* OA-00203
0*
-20- -
0 20 40 60 80 100 120 140 160
INTENSITY (11 in knots)
Figure 7. Average intensity of typhoons 24-hours priorto hitting the Philippines (I,) versus the change inintensity (I 1 -I 2 ),where I is the average 24-hourintensity after leaving tde Philippines.
15
wind in a typhoon averages 120 knots in the 24-hour period
prior to hitting the Islands, then the average intensity in
the 24-hour period after leaving the Islands will be 50 knots
less. It will be noted that, based on the regression line,
tropical cyclones with average intensities of less than 60
knots in the 24-hour period prior to hitting the Philippines
exhibit an increase in average intensity in the 24-hour period
after leaving, the Islands. Thus, there is an indication that
the intensity of tropical cyclonic circulations of less than
typhoon intensity are affected very little by the Philippines.
During the course of analysis it was observed that, in
general, the tracks of the more intense typhoons were further
to the north than the tracks of less intense storms.
Two other parameters were examined for typhoons crossing
the Philippines. These were the eye diameter of the storms
and circulation size of the storms. The eye diameter data
were extracted from the Annual Typhoon Reports and the size
of 'circulation data were extracted from the history file of
tropical storms and typhoons discussed earlier. The size
used here is the diameter of the circulation given by the
average diameter of the outer closed surface isobar.
Figure 8 shows the variations of these parameters for the
30 typhoons considered. Eye diameter data were available for
21 of the typhoons and size data for 24. From Figure 8(a) it
can be seen that eye diameters increase throughout the period
16I;I - _ _______
21 IYPIIOOK CASES
S•-- - -- 35
Lij
mF5
124 TPHOO CASE; - .12S-"-11051
- 0
i 15
'24 TON 2ON 0.A2ES 22
S-- -10
AFTER CROSSING IHoursi BEFORE CROSSING
Pibure 8. 'he eye diameter (a) and size variation (b)8)' tyrphoorts crossing the Philippines. The "size"icrameter is a ieasuro of circulation size and re-lates to the averageo C&anieter of the outer closedsurface isobar. TAhe number of observations availableis prcsentet. in parentheses.
17
considered. 3 However, note the large values of average eye
diameter for the 24 hours after leaving the Philippines (33.2
n mi) compared with the average 24-hour eye diameter prior to
hitting the Philippines (22.4 n mi)ý Thus, there is an in-
dication that the Islands did have an effect on the eye dia-
meter. The circulation size parameter shows that storms
maintain their size throughout the 48-hour period prior to
hitting the Islands and then decrease in size (approximately
17% in areal extent) after leaving the Islands.4
These parameters were further examined for intense and
weak typhoons (same criteria as before) and the results cana be seen in Figure 9. The eye diameter variations appear to
be similar for each of the two intensity categories. Notice
that the eye diameter of the intense typhoons is consistently'
smaller (approximately 13%) than the weaker typhoons.
The effect on circulation size varies with typhoon in-
tensity. Intense typhoons decrease in size throughout the
period considered. Weaker typhoons appear to increase in
circulation size prior to hitting the Islands and then to
decrease during the remainder of the period. Notice that the
average circulation diameter of the intense typhoons is approx-
imately 1-2½O latitude greater then the weaker systems.
3 It should be noted that the average time the 21 typhoonsexisted in the Philippines was 13 hours.4 The average time the 24 typhoons existed in the Philippineswas 13½ hours.
18
¶• INTENSE TYPHOONS-Those typhoons having an average intensity inthe'24-hour period prior to hitting the Philippinesof >-90 knots.
{1 WEAK TYPHOONS-Those typhoons having an average intensity inthe 24-hour period prioi to hitting the Philippinesof <90 knots.
INTENSE TYPHOONS 110 cases)- "WIAK TYP'0tONS (11 cases4
-30
L• • 1371(a) •mInm "
a. -- - -- - - 20
LA 1461
-~ ----- 0L--- i20
M -- 5 1 4)15"- INTENSE TYPHOONS (10 cases)
WEAK TYPHOONS 114 casesl
* m .12
- 11
L1400
(b ) I, 1 Latl
.. . . - -. . -- -- - -
1701)
,24 ,12 0 0 .12 -24 -36 -48AFTER CROSSING 1H4u5sl BEFORE CROSSING
Figure 9. The eye diame::er (a) and size variation (b)of intense and weak typhoons crossing the Philippines.The number of obser-zitions available is presented inparentheses.
19
7'. W" M,
4. SUMMARY
An attempt has been made to document the effects of the
Philippines on tropical-cyclones. The results presented here
show -that' the intensityi speed, movement and size parameters
of typhoons are influenced by the Philippines. Primary causes
include the frictional effects of the land mass of the Philip-
pines and the reduction in heat and moisture supplied by the
ocean. However, especially in the later months of the typhoon
season, the ocean environment and mean synoptic conditions of
the South China Sea are different from those of the Philip-
pine Sea, and these characteristics may well affect the behav-1
ior of storms. The degree of land influence is a function
of the area and topography of the terrain over which the storm
is passing. The terrain of the Philippine Islands varies a
great deal (see Figure 2), ranging from extensive mountainous
regions on Luzon and Mindanao to a sea-island mix in the central
Philippines. The forecaster should be aware of these topographi-
cal variations and keep in mind that the central region should
have less effect on the storms.
The synoptics of the situation should also be taken into
consideration. For example, if a tropical cyclone is crossing
1 Some of the effects of the summer and fall season on tropicalcyclones in the South China Sea have been discussed in previouspublications (Adler et al.; 1970; and Brody and Jarrell, 1969).
Preceding page blank 21
the Philippines in the late typhoon season and this crossing
coincides with an early surge of the northeast monsoon, cool
air may enter the circulation of the storm with a resulting
decrease in intensity. However, a surge may only increase the
low-level cyclonic shear in the region of the South China Sea
trough, if present, and this wil] present an excellent path
for the storm.
In the later months of the typhoon season 'the ocean be-
comes a factor since the sea6.surface temperature in the South
China Sea decreases more rapidly than the Philippine Sea. The
thermal structure parameters, other than sea-surface temperature,
are also important in that they determine how much warm water is
available for the energy of the storm (Brand, 1971). For ex-
ample, in the tater months of the typhoon season (October
through Decezý.ber) there is less available warm water in thL
South China Sea than in the Philippine Sea.
It i2 suggested that the forecaster keep the above com-
ments in mind bnd subjectively modify the forecast aids pre-
sented in this report based on the above considerations.
22
REFERENCES
Adler, R. F., L. R. Brody and W. L. Somervell, Jr., 1970:A preliminary survey of SASI'A fall transformationseason weather. NAVWE2'1jSCHFAC Tech. Paper No. 10-70.
Brand, S., 1971: The effects on a tropical cyclone of coolersurface waters due to upwel.ling and mixing produced bya prior tropical cyclone. Jc.rn.al of Applied Meteoro7logy, 10,No. 5, 865-874.
Brand, S., 1972: Geographic and monthly, variation of rapidintensification and low-latitude Weakening of tropýicalcyclones of the western North Pacific Ocean.ENVPREDRSCHFAC Tech. Paper No. 5-72.
Brand, S. and R. F. Gaya, 1971: Intensity changes of tropi-cal storms and typhoons of the western North .'acificOcean. NAVWEARSCHFAC Tech. Paper No. 5-71.
Brody, L. R., and J. D. Jarrell, 1969: A technique for pre-dicting South China Sea tropical cyclones.' NAVWEARSCHFAC
TVch. Paper No. 8-69.
U.S. FLEET WEATHER CENTRAL/JOINT TYPHOON WARNING CENTEP,GUAM, Annual Typhoon Reports, 1960-1970.
23
APPENDIX A
TRACKS OF TROPICAL STORMS
AND TYPHiOONS (1945-1969)
BY MONTHLY AND HALF-MONTHLY PERIODS
FOR THE WESTERN NORTH PACIFIC OCEAN
Preceding page blank 25
A _____ ____________25
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