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