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Toru Terao(Osaka Gakuin Univ. ) Md. N. Islam(BUET) T. Hayashi, F. Murata, T. Oka(Kyoto Univ.)

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.

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Toru Terao(Osaka Gakuin Univ. ) Md. N. Islam(BUET) T. Hayashi, F. Murata, T. Oka(Kyoto Univ.)

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  1. 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)

  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 (＋) • 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. C2 Early morning peak Cluster 2 Afternoon peak Cluster 1 C1 Early morning peak Results: Cluster analysis

  8. difference 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

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

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

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

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

  13. 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

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

  15. How about the Upper Wind? ？

  16. Rawin-sonde Observation

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

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

  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. 1250-1750m 500-1000m surface Vertical Profile of wind speed Rawin-sonde • Maximum diurnal variation: • Max. at 500-1000m • Low-Level Jet • midnight 2001, 4-times daily

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

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

  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

  25. Cause of Nocturnal Jet Geostrophic Balance Without friction

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

  27. friction Cause of Nocturnal Jet nighttime

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

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

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