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) data from the Japanese Geostationary Meteorological Satellite (GMS-5), and hourly (or 3-hourly) rainfall data from Bangladesh, Thailand, Vietnam and Malaysia. 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 channel 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 approximately the same phase. The time when convective activity reaches its maximum and minimum is examined in the domain of 80-120 degreesE and 0-30 degreesN. As a result, it is found that the largest number of grids exhibits the maximum at 17LT (local time) and 14LT, and the minimum at 11LT and 21LT over land and sea, respectively. Moreover, without using the harmonic analysis, which is a conventional method of analyzing the diurnal variation, areas with the maximum during the late night-early morning hours are clearly separated from those with the afternoon-early evening maximum. This late night-early morning maximum is mostly found in the windward areas of mountains, in basins and valleys, and in coastal areas. Therefore, this kind of maximum is most likely associated 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-early 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 the afternoon-early evening maximum. But, the mean daily rainfall at the former stations is on average a few times greater than that at the latter stations. Thus, the diurnal variation averaged for all stations in the four countries has two nearly equal maxima at 05LT and 16LT. This result suggests a strong possibility that the late night-early morning maxima of convective activity and rainfall have a great effect on energy and water cycles in tropical Asia.