East Asian Subtropical Monsoon Precipitation In The Spring Of Diagnostic Analysis And Numerical Simulation

We analyzed the rainband moving north and south in subtropical East Asian and then investigated the circulation and the other forcing factors of the spring precipitation anomaly over Southeastern China and Yangtze-Huai River (YHR) area and mainly discussed the influence of land-sea thermal contrast on precipitation area and precipitation anomaly, by use of NCEP–NCAR reanalysis dataset and 740 stations daily rainfall datasets in China from 1960 to 2008. Finally, we using the NOAA's Climate Prediction Center's merged analysis of precipitation, studied the impacts of surface temperature differences between East Asian and seas around on the rainfalls for southwesterly winds in spring over Eastern China, by the fifth-generation PSU/NCAR Meso-scale Model version 3 (MM5v3). The major conclusions are as follows:(1)The Rainy season first appears at the 14th pentad over Southeastern China, where the precipitation which accounts the largest part ,about 38%, for the rainfall in whole year becomes monsoonal and deep convective in the first third of April, and then over South China and YHR area, North China. As the precipitation in Southeastern China increase, the southwest winds appear and strengthen, which means the outbreak of the East Asian sub-tropical monsoon. Sub-tropical monsoon and precipitation develops and strengthens, with the earlier reversing of the zonal land-sea thermal contrast over sub-tropical Eastern China.(2) There is a great difference between the spring circulation in more and less precipitation year over Southeastern China and that over YHR area. In spring rainy years over the Southeastern China, the sub-tropical high is stronger and pressure gradient is shaper, which causes the southerly winds and water vapor flux transmission anomaly and low-level convergence, obvious increase of updraft. More spring rainfall in YHR area is mainly due to the ridge is obviously weak over the Qinghai-Tibet Plateau and its northern area, which reduces the northwest wind in front of the ridge and forms anomaly field that the east higher than the west, that is obvious larger subtropical high. Southwest winds of the western subtropical high can affect the region further north,which can change the circulation that the area of low-level moisture convergence and upward motion is more north than usual, which results in positive anomaly center of rainfall moving to YHR region.Rainfall anomalies over Southeastern China and YHR area caused changes southwest winds and atmospheric circulation in by have a lot of thing to do with local anomalies of zonal land-sea thermal contrast. Precipitation increases caused by the southwest winds strengthening in the region of the same and higher latitude, which caused from zonal land-sea thermal contrast varying to the summer feature earlier. When the zonal thermal gradient increase more south than usual, it lead to precipitation enhance in Southeastern China that the south-west wind and water vapor transmission and low-level flow convergence, updraft increasing in Southeastern China. When the zonal thermal gradient increase more north than usual, it results in increased precipitation YHR area that the southwest winds can affect the region further north and flow convergence zone and the center of the upflow are also located in YHR region.(3)Numerical simulation result confirms diagnosis that: the surface temperature differences in spring exert strong influence on the occurrence of the southwesterly winds and rainfall over southern China and their northward advances. When surface temperature increases over East Asia and decreases over the oceans around, the temperature gradient with a winter feature earlier changes to that with a summer feature, with the strengthening of the east of the Tibetan Plateau low and the subtropical high over the western Pacific, the lower-tropospheric southwesterly winds over Eastern China. Accordingly, it leads to an increase of spring rainfall over the YHR valleys and a decrease over southern China that the upward motion increases over the YHR valleys and decreases over southern China. Thus, the rain belt over eastern China appears over the YHR valleys but not over southern China. Under a weaker condition of the thermal contrast, the rain belt does not occur over eastern China. When the spring thermal contrast pronouncedly strengthens, the rain belt over southern China may advance northward to the YHR valleys during spring although there is no onset of the tropical monsoon over the South China Sea. This forms a rain belt similar to that of the YHR valleys during the Meiyu period.(4)The topography of East Asia can enhance southwest wind and precipitation in spring over Southeastern China. The topography of the Qinghai-Tibet Plateau make spring rainfall more concentrated in the Southeastern China. However, sub-tropical and tropical monsoon rainfall can still appear, without East Asia topography.