Diurnal variations of summer precipitation over the Asian monsoon region as revealed by TRMM satellite data

Climatological characteristics of diurnal variations in summer precipitation over the Asian monsoon region are comprehensively investigated based on the Tropical Rainfall Measuring Mission (TRMM) satellite data during 1998–2008. The topographic influence on the diurnal variations and phase propagations of maximum precipitation are identified according to spatiotemporal distributions of the amplitude and peak time of the diurnal precipitation. The amplitude and phase of diurnal precipitation show a distinct geographical pattern. Significant diurnal variations occur over most of continental and coastal areas including the Maritime Continent, with the relative amplitude exceeding 40%, indicating that the precipitation peak is 1.4 times the 24-h mean. Over the landside coasts such as southeastern China and Indochina Peninsula, the relative amplitude is even greater than 100%. Although the diurnal variations of summer precipitation over the continental areas are characterized by an afternoon peak (1500–1800 Local Solar Time (LST)), over the central Indochina Peninsula and central and southern Indian Peninsula the diurnal phase is delayed to after 2100 LST, suggesting the diurnal behaviors over these areas different from the general continental areas. The weak diurnal variations with relative amplitudes less than 40% exist mainly over oceanic areas in the western Pacific and most of Indian Ocean, with the rainfall peak mainly occurring from midnight to early morning (0000–0600 LST), indicating a typical oceanic regime characterized by an early morning peak. However, apparent exceptions occur over the South China Sea (SCS), Bay of Bengal (BOB), and eastern Arabian Sea, with the rainfall peak occurring in daytime (0900–1500 LST). Prominent meridional propagations of the diurnal phase exist in South Asia and East Asia. Along the eastern Indian Peninsula, there is not only the southward phase propagation with the peak occurring around 25°N but also the northward phase propagation with the peak beginning with the southernmost continent, and both reach the central Indian continent to finish. Along the same longitudes where southern China and Kalimantan are located, the diurnal phase of the former propagates from the oceanic area (northern SCS) toward the inland continent, while the phase of the latter propagates from the land area toward the outside sea, showing a landward or seaward coastal diurnal regime. A distinct zonal propagation of the diurnal phase is observed over the BOB oceanic area. The maximum precipitation zone originates from the land-sea boundary of the eastern coast of the Indian peninsula at around 0300 LST, and then propagates eastward with increasing time to reach the eastern coast of the BOB on 1800 LST, finally migrates into the Indochina continent on about 2100 LST.