Climatological characteristics of monsoon seasonal transitions over Bangladesh

Climatological characteristics of monsoon seasonal transitions in Bangladesh, focusing on onset and withdrawal phases, were examined using pentad and 20-day mean wind, moisture flux, and precipitable water distributions obtained from Japanese 25-year re-analysis data from 1979 to 2003 and rainfall data from the Bangladesh Meteorological Department (BMD) from 1948 to 2008. Analysis of the pentad and 20-day mean horizontal wind at 850 hPa, the total column water vapor flux, and the precipitable water distribution around Bangladesh showed that the onset and withdrawal of the summer monsoon season in Bangladesh occurred between pentad 31 (May 31-June 4) and pentad 32 (June 5-9), and between pentad 56 (October 3-7) and pentad 57 (October 8-12), respectively. Changes in wind at 850 hPa, total column water vapor flux, precipitable water, and rainfall distribution associated with monsoon onset and withdrawal phases are described by consecutive 20-day mean maps. The annual cycle of 5-day mean rainfall is also presented for each BMD station. High rainfall occurred in the northeast region (Sylhet) during the pre-monsoon season and in the southeast region (Teknaf) during the monsoon season. The central west region (Ishurdi) received relatively less rainfall in all seasons. In the post-monsoon season, rainfall amounts did not differ much among the stations