1. Diurnal variation of the convective activity over Bangladesh in the summer monsoon season and its relation with the nocturnal jet
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 over Bangla.
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 of NE heavy rainfall?
This talk is
focust on
NE part
Matsumoto (1992)

3. Outline 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 data
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 total monsoon rainfall (JJAS)
Using time of maximum rainfall

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

7. 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

8. Scatter diagram Early-morning peak Afternoon B1 type B2 type Type 1
Less
rainfall
Heavy
A1 type
A2 type

9. 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

10. How about the Upper Wind?？

11. 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

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

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

14. 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 mechanism

15. Cause of Nocturnal Jet
Geostrophic Balance
Without friction

16. Caused by small convections over plain area
Cause of Nocturnal Jet
Descrepancy from
Geostrophic balance
With friction
daytime
Caused by small convections over plain area
friction

17. Cause of Nocturnal Jet
nighttime
friction

18. Clockwise change of wind dir.
Cause of Nocturnal Jet
Clockwise change of wind dir.
Friction disappears
nighttime
Coriolis force
Accel.

19. Effect of Nocturnal Jet
Heavy precipitation
On the slope of
Mountains ?
Emphasizes upslope wind
nighttime

20. Summary Patterns of diurnal variation of rainfall
NE, SE: midnight-early morning peak
C, W: afternoon peak
Nocturnal jet like diurnal wind variation
Evening acceleration of low level wind speed
Clockwise change of wind direction
Amplifies midnight rainfall over NE part?

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

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

23. 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

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

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

26. Rawin-sonde Observation

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

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

29. 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

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

32. バングラデシュの夏季降水量 6-8月降水量 Meghalaya山脈 北東部と南東部に多雨域が広がる インド領 記録的な多雨域
年間20000mmの記録
標高1500mくらい

33. Diurnal Variation of Echo area
Echo Embedded Area
Freq. of Occurrence
Afternoon:
small / many

34. A typical pattern of the diurnal variation will be shown
事例にあたる
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
17 July
18 July
8:30, 11:30, 14:30, 17:30, 20:30, and 23:30

35. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
8:30
17 July
18 July
No conv.

36. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
11:30
17 July
18 July SW
scattering

37. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
14:30
17 July
18 July SW
developing

38. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
17:30
17 July
18 July E
forming

39. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
20:30
17 July
18 July
E -> NE
Squall line

40. A typical pattern of the diurnal variation will be shown
Convection: 17/18 July 2001
A typical pattern of the diurnal variation will be shown
using Dhaka RADAR data.
23:30
17 July
18 July NE
spread