Diurnal variations of connective activity and rainfall in tropical Asia

The diurnal variations of convective activity and rainfall in tropical Asia are investigated using hourly equivalent black body temperature (T-BB) dat a from the Japanese Geostationary Meteorological Satellite (GMS-5), and hou rly (or 3-hourly) rainfall data from Bangladesh, Thailand, Vietnam and Mala ysia. As an index of convective activity we use the frequency of occurrence of the difference in T-BB between the infrared-1 and the water vapor chann el DeltaT(BB) (= T-BB(IR1) - T-BB(WV)) of less than 3K. By using this index , the diurnal variations of convective activity and rainfall have approxima tely the same phase. The time when convective activity reaches its maximum and minimum is examin ed in the domain of 80-120 degreesE and 0-30 degreesN. As a result, it is f ound that the largest number of grids exhibits the maximum at 17LT (local t ime) and 14LT, and the minimum at 11LT and 21LT over land and sea, respecti vely. Moreover, without using the harmonic analysis, which is a conventiona l method of analyzing the diurnal variation, areas with the maximum during the late night-early morning hours are clearly separated from those with th e afternoon-early evening maximum. This late night-early morning maximum is mostly found in the windward areas of mountains, in basins and valleys, an d in coastal areas. Therefore, this kind of maximum is most likely associat ed with terrain or its induced local circulations such as mountain and land breezes. From the analysis of rainfall data, it is also shown that the late night-ea rly morning maximum is found at stations with high rainfall. The number of stations with the late night-earl morning maximum is less than that with th e afternoon-early evening maximum. But, the mean daily rainfall at the form er stations is on average a few times greater than that at the latter stati ons. Thus, the diurnal variation averaged for all stations in the four coun tries has two nearly equal maxima at 05LT and 16LT. This result suggests a strong possibility that the late night-early morning maxima of convective a ctivity and rainfall have a great effect on energy and water cycles in trop ical Asia.