diurnal variation of meteorological elements in bangladesh diurnal variations in precipitation and...
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Diurnal Variation of meteorological elements in BangladeshDiurnal Variations in Precipitation and Upper Wind over Bangladesh
Toru Terao(Osaka Gakuin Univ.)Md. N. Islam(BUET)
T. Hayashi, F. Murata, T. Oka(Kyoto Univ.)J. Matsumoto(Univ. of Tokyo)
Monsoon Rainfall Heavy rainfall in NE
part of the country Southern slope of
Meghalaya Mts. World record(Cherrapunji)
>20,000mm/yr
How about the diurnal variations in NE part ?
What is the mechanism responsible for ?
Matsumoto (1992)
This talk isfocust on
Structure of this talk Classify the diurnal rainfall variation
into several types With special attention to NE part
Diurnal lower tropospheric wind variation is examined Using rawin-sonde and pilot balloon data To discuss the possible cause of the
diurnal variation over NE part
Raingauges utilized BMD data ( + )
3-hourly Avail. only for 2000
JICA raingauge network (○) NE part Used only 2000 Special thanks to
Prof. Oka (Kyoto Univ.)/JICA
Rainfall Type Classification To classify type of diurnal
variation… A cluster analysis is conducted
Not shown today… Two histgrams are examined
Using monsoon rainfall (JJAS) Using time of maximum rainfall
Cluster analysis A statistical
method to partition a set of vectors into groups relatively similar to each other
x
y
z
Results: Cluster analysis
Early morningpeak
Early morningpeak
Cluster 1
C1
Cluster 2
C2
Afternoonpeak
Summary: Cluster analysis Cluster 1: C1
Central/Western part of country Less rainfall Afternoon(0-6PM) or Morning(3-6AM)
peak Cluster 2: C2
NE, SE part of country Heavy rainfall Early-morning(3-6AM) peak
difference
Histgram: JJAS rainfall amount
There is a gapat about
1500-1600mm
A1 A2
A1 typeA2 type
GAP
2200-23002 points
1100-12005 points
Lessrainfall
Heavyrainfall
A1
A2
Spatial distribution
A1
A2
Peak time
Histgram: JJAS rainfall amount
Early morningpeak
Early morningpeak
Heavy rain
Less rain Afternoonpeak
Histgram: Max. rainfall time
There are gapsAt 10-11am& 6pm-0am
B1 B2
B1 type B2 type
12:00-15:009 points
3:00-6:0014 points
Early-morningpeak
Afternoonpeak
GAPGAP
Scatter diagram
Type 1
Type 2
Type 3
Lessrainfall
HeavyrainfallA1 type A2 type
Early-morningpeak
Afternoonpeak
B1 type
B2 type
Sylhet3069.0mm/peak:3-6LST
Dhaka909.4mm/peak:3-6LST
Rajshahi1215.7mm/peak:15-18LST
Summary of Rainfall Types Type 1 (+)
Less / Afternoon Central & W part
Type 2 (*) Less / Early-Morning Band, NW-SE
Type 3 (o) Heavy / Early-
Morning NE & SE part
Comparison with prev. study
Our study Ohsawa et al. (2001)
Midnight
Noon
How about the Upper Wind?
?
Rawin-sonde Observation
Rawin-sonde obs. over Dhaka
time
Operational:Once a day
Diurnal variationCannot be resolved
Joint Intens. obs. (1999-2002)
4-times a day
time
Diurnal variationcan be resolved
Rawin sonde (4-times daily) Dhaka station, 4-times daily 2001 summer (18 days total)
June 28, 29 July 4, 9, 10-11, 17-18, 21-22, 26-27,
30-31 August 1, 5, 14-15
Wind variation over Dhaka Findings: night
time Wind acceleration Clockwise change
of wind direction
Vertical Profile of wind speed
Maximum diurnal variation: Max. at 500-1000m
Low-Level Jet midnight
2001, 4-times daily
Rawin-sonde
surface
500-1000m1250-1750m
Pilot balloon obs. Operational, 4-times a day
Dhaka and other places Wind accel. & clockwise change is
common?
?
At old observatory
Pilot Balloon / 5-places
Acceleration of wind evening-midnight
Clockwise changeSimilar to
Nocturnal Jet
Nocturnal jet ? Wind characteristics;
Evening acceleration of wind speed Clockwise change of wind direction Low-level jet structure
suggests that the diurnal wind variation over Dhaka has nocturnal jet like structure
Cause of Nocturnal Jet
Geostrophic BalanceWithout friction
Cause of Nocturnal Jet
friction
Descrepancy fromGeostrophic balance
With frictiondaytime
Cause of Nocturnal Jet
friction
nighttime
Cause of Nocturnal Jet
Accel.
Friction disappearsnighttime
Coriolis force
Effect of Nocturnal JetHeavy precipitation
On the slope ofMountains ?
nighttime