A remarkable decrease in water vapor was observed during daytime on calm days over coastal Thailand. Precipitable water vapor (PWV) and surface specific humidity (SSH) decreased by about 5 mm and 3 g kg(-1), respectively, in Bangkok from the late morning to the late afternoon on sunny days with weak low-level winds between March 1998 and December 1999. A daytime moisture decrease occurs in the lower atmosphere over coastal areas near the Gulf of Thailand, where the sea breeze often prevails. The sea breeze is relatively strong near the northern gulf. Wind profile observed over Bangkok revealed strong descending flow in the lower atmosphere during daytime, suggesting divergence of the sea-breeze circulation. This descending flow explains the decrease in daytime moisture over coastal areas.
A numerical experiment assuming calm conditions successfully simulated the daytime decrease in water vapor around Bangkok. The most remarkable drying associated with descending flow was simulated around the northern edge of the Bight of Bangkok, where the coastline is sheltered by inland areas. The shape of the coastline and the topography enhanced the sea-breeze circulation around Bangkok. In another model run assuming wet soil conditions, the diurnal range of water vapor was smaller than that simulated by the control run. Clear diurnal variation was observed on calm days, especially under dry soil moisture conditions. A large diurnal range in PWV tended to appear before the monsoon onset when the low-level wind was weak and the soil moisture was dry.