Analysis on the features and synoptic systems of the rainstorm during Plum Rains Season in 2011 in Zhejiang province

Using NCEP reanalysis data and rainfall data of automatic weather stations, the features and synoptic systems of rainstorm during Plum Rains Season in 2011 in Zhejiang province are analyzed. The results indicate that the Plum Rains Season in 2011 in Zhejiang province is composed of four severe precipitation processes with stable rain belt, overlapping distribution of severe precipitation in the upper and middle reaches of the Qiantang River and accumulative precipitation more than that in the same period of the past years, which lead to frequent flash floods in the Qiantang River basin and its all tributaries. The stability and continuance of South Asia high, subtropical high over the west Pacific and“two trough and one ridge”circulation pattern are favorable to the formation of rainstorm during Plum Rains Season in Zhejiang.Different from the stability of subtropical high gradually stretching westwards and moving northwards to the same location as Meiyu period approached in the past, the subtropical high in 2011 is located at west first, and moves eastwards and stretches westwards and moves northwards,and then it is stable after the first severe precipitation process took place; the precipitations from other three processes present a typical circulationpattern of Meiyu period, and the subtropical high is westward during the first process, but very severe precipitation emerges in Zhejiang.The precipitation is most significant in the 3rd process when the dry and cold air from the north intruded into front zone. There are three upper-level jet cores moving eastwards during Plum Rains Season in 2011 in Zhejiang, and the severe precipitation during the four processesemerges on the right side of entrance region of the three upper-level jets successively. The stability and continuance of southwest monsoonmoving northwards close to 30°N determine the distribution of severe precipitation during Plum Rains Season in 2011 in Zhejiang.