The Circulation Patterns, Variation Characteristics And Its Possible Mechanism Of Mean Daily Precipitation Extremes Of Autumn Persistent Rain Over Western China

Based on the daily precipitation observation over 743 stations in China during 1961-2010 and the NCEP/NCAR reanalysis data, the circulation patterns, variation characteristics and its possible mechanism of mean daily precipitation extremes of autumn persistent rain over western China have been investigated. The result shows that:1) We used percentile method to sort out 161 regional mean daily precipitation extreme (DPE) events over western China (WC) during the autumn persistent rainy season. Then we divided 161 DPE events into two types and investigated the features of the circulation anomalies that are compared with ordinary precipitation events. For type 1 (Ⅱ), an anomalous cyclone (a cyclonic wind shear) over the WC forms convergence at lower troposphere, an anomalous divergence at higher level, thus causing strong upward motion. The vapor transported from tropical oceans is stronger (weaker) and forms convergence over the WC. The Western Pacific subtropical high is west- and north-ward of its mean position, thus causing stronger SW winds. The positive (negative) geo-potential height anomalies over north of the Balkhash Lake generate "one trough and one ridge" ("two troughs and one ridge") circulation pattern, thus leading to stronger NW winds north of the WC. The confrontation between warm-moist SW and cold-dry NW winds over the WC will produce the DPE events. Condensation heating from heavy precipitation of the anomalous circulation patterns for the DPE events makes the atmospheric apparent heat source and apparent vapor sink being the maximum over the WC. In addition, the SSTA of North Pacific and North Atlantic is abnormally high. But typeⅠ in numerical size and scope is greater than type Ⅱ.The anomalous circulation patterns for the DPE events are similar to that of ordinary precipitation but with stronger intensity. Such a anomalous circulation patterns can be found one day ahead of the occurrence of the DPE events and can be regarded as a strong signal for short-range forecast.2) The rotated empirical orthogonal function (REOF) is employed to reveal the patterns of extreme precipitation during the autumn persistent rainy season over western China. Pick out extreme precipitation events that more than 95% and the interannual and interdecadal variation of extreme precipitation frequency and total rainfall were analyzed for each area. The reasons of interannual variability over the eastern part of western China during the autumn persistent rainy season is:The surface temperature over most regions of China of DPE frequently occur years is lower than that of occasional occur years. Sea level pressure in frequently occur years is higher. The atmospheric circulation during the autumn persistent rainy season in occasional occur years is closer to winter and the southeast of airflow from the south China sea-the western Pacific Ocean is weaker.In addition, the Western Pacific subtropical high is west-and north-ward,the subtropical westerly jet position is east-ward, the SST in Middle East equatorial Pacific is lower, all of that causes precipitation and extreme precipitation events increase in some years and vice3) The possible mechanism of interdecadal variation of the DPE frequency over the eastern part of western China is especially discussed. Results show that the DPE frequency (total precipitation) in the past 50a exhibit interdecadal variations of high (more rainfall) from 1960’s to the mid of 1980’s-low (less rainfall) from the mid of 1980’s to the end of 20th century-high (more rainfall) in the 21th century. Such an interdecadal variations are closely related to the changes in surface temperature over western China and its surrounding areas under global warming background with its possible mechanism as followings:From the mid of 1980’s to the end of 20th century, the cooling of surface temperature over western China and its surrounding areas can cause an anomalous higher sea level pressure and cold anticyclonic circulation at lower troposphere. The northerly wind is thus strengthened over central and eastern China, and the summer monsoon circulation as well as the associated water vapor transport is weakened. This will further reduce the southeasterly to the east of the study domain, making its convergence with southwesterly weakened, and finally leading to the declines both in the total precipitation and the DPE frequency. With the global warming strengthened in the 21th century, the surface temperature over western China is no longer lower, and the atmospheric circulation demonstrates an opposite features, the DPE frequency is thus higher again.