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)

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)

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

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

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

Histgram: JJAS rainfall amount There is a gap at about 1500-1600mm Less rainfall Heavy GAP A1 type A2 type 1100-1200 5 points 2200-2300 2 points

Histgram: Max. rainfall time B1 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

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

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

How about the Upper Wind? ？

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 Rawin-sonde 2001, 4-times daily 500-1000m 1250-1750m surface

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

Cause of Nocturnal Jet Geostrophic Balance Without friction

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

Cause of Nocturnal Jet nighttime friction

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

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

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?

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

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

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 Spatial distribution Early morning peak A1 A2 Peak time Less rain Heavy rain Afternoon peak Early morning peak

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

Rawin-sonde Observation

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

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

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

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

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

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

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

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.

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

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

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

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

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