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)
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)
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 (＋) 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
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 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 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: JJAS rainfall amount Spatial distribution Early morning peak A1 A2 Peak time Less rain Heavy rain Afternoon peak Early morning peak
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 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
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
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 variation Cannot be resolved
Joint Intens. obs. (1999-2002) Diurnal variation 4-times a day can 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 Rawin-sonde 2001, 4-times daily 500-1000m 1250-1750m surface
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
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 Balance Without friction
Cause of Nocturnal Jet Descrepancy from daytime Geostrophic balance With friction daytime friction
Cause of Nocturnal Jet nighttime friction
Cause of Nocturnal Jet nighttime Friction disappears Coriolis force Accel.
Effect of Nocturnal Jet Heavy precipitation On the slope of Mountains ? nighttime