1. Diurnal Variation of meteorological elements in Bangladesh Diurnal 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)

2. 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 ?
This talk is
focust on
Matsumoto (1992)

3. 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

4. Raingauges utilized BMD data (＋) JICA raingauge network (○) 3-hourly
Avail. only for 2000
JICA raingauge network (○)
NE part
Used only 2000
Special thanks to
Prof. Oka (Kyoto Univ.)/JICA

5. 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

6. Cluster analysis
A statistical method to partition a set of vectors into groups relatively similar to each other z y x

7. Results: Cluster analysis
Early morning
peak
Cluster 1 C1
Afternoon
peak
Early morning
peak

8. 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

9. Histgram: JJAS rainfall amount
There is a gap
at about mm
Less
rainfall
Heavy
GAP
A1 type
A2 type
5 points
2 points

10. Histgram: JJAS rainfall amount
Spatial distribution
Early morning
peak A1 A2
Peak time
Less rain
Heavy rain
Afternoon
peak
Early morning
peak

11. Histgram: Max. rainfall timeB1 B2
Early-morning
peak
Afternoon
There are gaps
At 10-11am
& 6pm-0am
GAP
B1 type
B2 type
3:00-6:00
14 points
12:00-15:00
9 points

12. Scatter diagram Rajshahi 1215.7mm/peak:15-18LST Early-morning peak
Afternoon
B1 type
B2 type
Type 1
Type 3
Type 2
Sylhet
3069.0mm/peak:3-6LST
Dhaka
909.4mm/peak:3-6LST
Less
rainfall
Heavy
A1 type
A2 type

13. Summary of Rainfall Types
Less / Afternoon
Central & W part
Type 2 (*)
Less / Early-Morning
Band, NW-SE
Type 3 (o)
Heavy / Early-Morning
NE & SE part

14. Comparison with prev. study
Our study
Ohsawa et al. (2001)
Midnight
Noon

15. How about the Upper Wind?？

16. Rawin-sonde Observation

17. Rawin-sonde obs. over Dhaka
time
Operational:
Once a day
Diurnal variation
Cannot be resolved

18. Joint Intens. obs. (1999-2002) Diurnal variation 4-times a day
can be resolved

19. 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

20. Wind variation over Dhaka
Findings: night time
Wind acceleration
Clockwise change of wind direction

21. Vertical Profile of wind speed
Maximum diurnal variation:
Max. at m
Low-Level Jet
midnight
Rawin-sonde
2001, 4-times daily m m
surface

22. Pilot balloon obs. Operational, 4-times a day Dhaka and other places？
Operational, 4-times a day
Dhaka and other places
Wind accel. & clockwise change is common?
At old observatory

23. Pilot Balloon / 5-places
Acceleration of wind
evening-midnight
Clockwise change
Similar to
Nocturnal Jet

24. 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

26. Cause of Nocturnal Jet
Geostrophic Balance
Without friction

27. Cause of Nocturnal Jet Descrepancy from daytime Geostrophic balance
With friction
daytime
friction

28. Cause of Nocturnal Jet
nighttime
friction

29. Cause of Nocturnal Jet nighttime Friction disappears Coriolis force
Accel.

30. Effect of Nocturnal Jet
Heavy precipitation
On the slope of
Mountains ?
nighttime