A regional model simulation of the 1991 severe precipitation event over the Yangtze-Huai River valley. Part I: Precipitation and circulation statistics

The summer Mei-yu event over eastern China, which is strongly influenced by large-scale circulation, is an important aspect of East Asian climate: for example, the Mei-yu frequently brings heavy precipitation to the Yangtze-H uai River valley (YHRV). Both observations and a regional model were used t o study the Mei-yu front and its relation to large-scale circulation during the summer of 1991 when severe hoods occurred over YHRV. This study has tw o parts: the first part, presented here, analyzes the association between h eavy Mei-yu precipitation and relevant large-scale circulation. while the s econd part, documented by W. Gong and W.-C.. Wang, examines the model biase s associated with the treatment of lateral boundary conditions (the objecti ve analyses and coupling schemes) used as the driving fields for the region al model. Observations indicate that the Mei-yu season in 1991 spans 18 May-14 July, making it the longest Mei-yu period during the last 40 yr. The heavy precip itation over YHRV is found to be intimately related to the western Pacific subtropical high, upper-trospospheric westerly jet at midlatitudes, and low er-tropospheric southwest wind and moisture flux. The regional model simula tes reasonably well the regional mean surface air temperature and precipita tion, in particular the precipitation evolution and its association with th e large-scale circulation throughout the Mei-yu season. However. the model simulates smaller precipitation intensity, which is due partly to the colde r and drier model atmosphere resulting from excessive low-level clouds and the simplified land surface process scheme used in the present study.