Observation study during the pre-monsoon season over northeast Bangladesh

The moisture variability during pre-monsoon season over Bangladesh was investigated using the upper balloon observation, OLR, and reanalysis data sets. As synoptic phenomena, the passage of trough on middle troposphere and inflow of warm and moisture air from Bay of Bengal was suggested from these aspects; high potential temperature on lower troposphere during active convection, increase of moisture quantity and relative humidity, prevailing of southwesterly, and decrease of potential temperature on middle troposphere by intensive observation of upper balloon on Dhaka, Bogra, and Sylhet. According to the OLR spatial pattern in each year, the northern India has no rainfall during pre-monsoon, even though in same latitude of Bangladesh. On the other hand, the northern Myanmar, and the southern China as same latitude with Bangladesh have also rainfall activity during pre-monsoon. It is suggested that the active phase of convection during pre-monsoon is the synoptic phenomena. A large lower troposphere integrated moisture flux flows into Bangladesh from the northern India during active phase of convection. In contrast, this moisture flux is weak during break phase of convection. It is interesting that the northern India where has no pre-monsoon rainfall, is located the large moisture flux. The existence of upper trough is confirmed around Bangladesh during active phase of convection from composite analysis of wind fields on 500 hPa and 300 hPa. From the viewpoint of local circulation, it was observed that a strong subsidence in 1–5 km over Dhaka and Bogra and in 4–5 km over Sylhet, whereas topographic uplifting of southwesterly airflow in 1–3 km was observed over Sylhet. Temperature profiles of Sylhet up to 3 km were lower than that of other stations, and easterly wind was observed at the surface.