| Tibetan Plateau has complex topography and unique climate. Due to particular heat and dynamic in summer, heavy rainfall occur suddenly and locally. Coupled with the lack of information on plateau region, it’s very difficult to forecast heavy rainfall. Based on NCEP/NCAR1°×1°gridded analyses data, the conventional observation and TBB data, the temporal and spatial distribution characteristics of Qinghai-Tibet Plateau heavy precipitation with more than20mm/6h during May to September in the period of2007-2011was statistically analyzed. These heavy precipitations can be divided into four categories according to circulation patterns and precipitation clouds position:in front of the upper-level trough type, diffusion in periphery of the South Asia High type, high-pressure ridge type and others type, and the former three types each accounts for the percentage of13.3%,54.3%and26.7%。. According to the statistics, heavy precipitation on Plateau is closely connected with the position of South Asia high. The average distance between precipitation center of in front of the upper-level trough type and High ridge line is about12.5latitude and longitude. This type mainly occurs in June and the moisture come from both south and north. Precipitation of high-pressure ridge type is near the ridge line of South Asia high and the average distance between precipitation center and ridge line is about1.6latitude and longitude, and this type mainly appears in July and August while no cases in May. The main moisture channel for high-pressure ridge type is "S" pattern of Bengal Bay transportation. The average distance between precipitation center of diffusion in periphery of the South Asia High type and the ridge line is about5.6latitude and longitude. Cases of this type frequently occur in August and the moisture can be from two channels:one across Xinjiang province to Qinghai-Tibet Plateau, another is the "S" pattern of Bengal Bay transportation. Cases decline rapidly except that in front of the upper-level trough type in September. Tibetan heavy precipitation has obvious diurnal variation, and precipitation occurred during12:00-18:00(UTC) accounted for the most, with a percentage of47.5%; cases during06:00-12:00and18:00-00:00, respectively with24.4%and24.1%, while no cases occurred during00:00-06:00. The northeastern part of the plateau is the region with the highest occurrence of heavy precipitation, followed by the southeastern plateau. It’s worth noting that high-pressure ridge is the main precipitation type,71.1%of which occur at the eastern ridge line of South Asia High, and28.1%of which have no obvious surface system. Numerical simulation of the weather systems in plateau region is often difficult. Parameterization schemes provided herein by WRF3.5mode are collocated into42groups, and precipitation ETS grade are given for combinations’ simulation results.By objective comparison of simulation results and scores, the optimal combination is elected with the WSM-3microphysics scheme, KF cumulus parameterization and RUC land surface process solutions. Through further simulation on cases of in front of the upper-level trough type, diffusion in periphery of the South Asia High type, high-pressure ridge type, further analysis of the causes of various types of precipitation is been done. According to analysis, the main factor triggering heavy rainfall in the high-pressure ridge and in front of the upper-level trough type cases seems to be the positive wet vorticity down from high level and the convective instability in low level, the high-level inertial instability generated in the high-pressure ridge type substantially increases precipitation, diffusion in periphery of the South Asia High type cases, different from before two types, inertia instability caused by high-level high pressure peripheral anticyclonic flow, in conjunction with the low-level convective instability is the main reason to rainfall; the twice transmision of positive moist potential vorticity in high level increases precipitation. |
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