Diurnal Cycle of Monsoon Convection

The diurnal variability of monsoon precipitation and associated circulation features, e.g. land/sea breezes, mountain/valley breezes, and low-level jets, in the monsoon regions of the world are reviewed, with a particular focus on the Asian monsoon. The dominant pattern of diurnal variability is a maximum in rainfall over land during the afternoon/evening in response to solar heating of the surface and a morning maximum over the oceans; however, there are important exceptions to this pattern over both land and ocean. Nocturnal maxima in rainfall are found over land in proximity to major mountain barriers, e.g., at the foot of the Himalayas; downstream of the Tibetan Plateau, the Rocky Mountains, and the Andes; and within interior basins such as the Sichuan Basin. Daytime maxima in rainfall are observed over partially enclosed seas such as the Bay of Bengal, the South China Sea, and the Gulf of Guinea. These latter regions are characterized by seaward propagation of convection from adjacent coastlines. The mechanisms for the diurnal cycle of convection over land, particularly the afternoon maximum in precipitation, are generally well understood, but the timing of precipitation is often not well handled in models. Several theories for the diurnal cycle over the ocean have been proposed, involving surface and free-tropospheric radiative heating or their horizontal gradients; however, the precise mechanisms remain unclear and are not well represented in global models. Mechanisms for propagation of convective systems over both land and ocean are not completely understood and require further research.