Diurnal variation of meso-scale cloud clusters which formed in August of 1998 in Changjiang Basin and were associated with the large flood in 1998 in China was investigated based on hourly GMS TBB data.

The convective-cloud activity and 17 meso-scale cloud clusters occurred in August were analyzed. It was found that obvious regular diurnal variations exist in the evolution of all the meso-scale cloud clusters which can be classified into 3 types: the diurnal variation with midnight to early morning peak of low TBB area (around 23 to 2 LST) , with late afternoon to evening peak (around 17 to 20 LST) and with both the midnight to early morning and the late afternoon to evening two kinds of peaks. The deep convection peak at midnight to early morning peak time are more intense than that at evening peak time so that the former is proposed to mainly contribute to the heavy rainfall. Some characters of the meso-scale cloud clusters such as trajectory, appearance time and life span are found to related to the peak times of their development.

The midnight to early morning peak of low TBB area was proposed to be resulted from the difference of nocturnal radiational cooling between the meso-scale cloud cluster and its surrounding cloud-free region and the increased relative humidity at night due to the atmospheric temperature cycle in the moisture environment. The evening peaks were associated with the general boundary layer heating and the enhancement of the daytime middle-level convergence to the cloud clusters caused by the daytime solar heating of the upper layer of cloud cluster.