Characteristics of plateau-scale precipitation in Tibet estimated by satellite data during 1993 monsoon season

Abstract

Precipitation distribution around the Tibetan Plateau was examined by using satellite data of the Geostationary Meteorological Satellite Infrared channel (GMS/IR) and Special Sensor Microwave/Imager (SSM/I). These data were compared with observed precipitation and a product of Xie and Arkin (1997), in 1993 monsoon season. Three kinds of algorithms were used based on an index of fractional coverage of clouds, meso-scale convection centers, and scattering signal of the microwave. IR-based estimates showed two increased centers in the southern and southeastern plateau with a zonal gap behind the Himalayas, but the SSM/I-based estimate failed to yield representative distribution due to infrequent data sampling. IR-based estimates were combined, and the differences between daytime and nighttime precipitation and distribution variability were clarified, in relation to the onset of the rainy season. The differences between the daytime and nighttime precipitation depend on the larger-scale topography. The stepwise onset of the rainy season is known to be related to different circulation patterns around the plateau. In the middle of June, the first heavy precipitation was observed in the southeastern plateau, associated with a strong southwesterly moisture inflow from the meandering of westerlies at the 500 hPa level south of the plateau. After the middle of July, a stable Tibetan anticyclone was established, and continuous precipitation prevails only in the southern plateau. The importance of studies on the regional activities of meso-scale convection and the transitional period ushering in the mature rainy season are discussed.