Rainy-season Precipitation over Sichuan Basin

The precipitation features in the rainy-season (June 20- September 11) over the Sichuan Basin are summarized using a long-term mean precipitation estimate, with a particular emphasis on the propagation and the intensity variation of the precipitation. The rainfall pattern is obtained from the high-resolution CMORPH data in 2003-2010. The corresponding dynamics are discussed using the NCEP reanalysis data.

The result shows that, not exactly like the eastward rainfall propagation emphasized in \citet{bao2011}, the movement of rainfall signal from the Tibetan Plateau to the Sichuan Basin (in the basin) is southeasterly (northeasterly). Based on the features of the rainfall propagation and the eastern Tibetan Plateau slope, a new slantwise averaging method is introduced. The averaging direction is southwest-northeast almost parallelling to the eastern slope of the Tibetan Plateau. The corresponding time-distance {{H}ovm{\"{o}}ller} diagram reveals the two isolated rainfall centers located in the eastern Tibetan Plateau and the Sichuan Basin respectively. The precipitation intensity tends to get significantly strengthened in the basin. Comparison results between the slantwise averaging and the along-longitude averaging show that for the study of the precipitation in an area with dramatic terrain height transition, the averaging calculation considering the terrain shape can better exhibit the local rainfall characteristics.

Details of the dynamics are described to explain the precipitation features. The shifting of the active rainfall area is tied to the variation of the diurnal mountain-plain circulation, the low-level vorticity and the surface moisture distribution. Inconsistence between rainfall location and the strongest upward motion is due to the distribution of the surface water vapor convergence and the low-level vorticity. The positive vorticity and the surface moisture channel are shown to be shallow systems and under the effect of diurnal variation.

The seasonal variation of the rainy-season precipitation is also investigated by comparing to the rainfall in the pre-rainy period (May 5-June 19). In the pre-rainy period the rainfall in the Sichuan Basin is no longer concentrating along the foothill of the eastern Tibetan Plateau, but propagates easterly. The obvious amplitude increase in the basin rainfall only appears in the rainy period. The intensity and the eastward extent of the mountain-plain circulation and moisture convergence are responsible for the seasonal rainfall variation.