The Research On Climate Characteristics Of Rainstorms In The South Of Eastern China And Their Relationship With Symmetric Instability Index

Based on the European Centre for Medium-Range Weather Forecasts（ECMWF）ERA-interim reanalysis data,the daily precipitation data collected at 741 stations in China from1981 to 2010 and May to June in 2011 and the time series of South China Sea monsoon onset,moist potential vorticity（MPV）,slantwise convective available potential energy（SCAPE）,water vapor flux,water vapor flux divergence and other physical diagnostic quantities are calculated,and the climate characteristics of the rainstorms in the south of eastern China and their relationship with symmetry instability under the influence of summer monsoon are studied by means of mathematical statistics and weather dynamics diagnostic method.The main conclusions are as follows:（1）The statistical results show that,on average,the rainstorm centers in the south of eastern China are mainly distributed in the windward slope at the southeast side of the coastal area of South China where the large terrain interacts with the summer monsoon,and in the mountainous areas of north Jiangxi and southeast Anhui.The annual precipitation of the rainstorm centers in South China coastal area exceeds 1000 mm,accounting for 40～50%of the total annual.Furthermore,there is a high incidence of heavy rainfall in summer,and the heavy rainfall areas in summer is consistent with the average annual rainstorm centers.Monthly rainfall anomaly fields during the rainy season from April to September show the northward leap of heavy rainfall center from South China（April to June）to Yangtze–Huaihe region（May to July）,and then to northern China.Characterization of rainy season shift from the Yangtze–Huaihe to northern China between July and August is well represented by the rainfall anomaly field than by the mean field.From 1981 to 2010,there were 388 regional rainstorm days and 287 rainstorm processes in southern China,with annual frequency ranging from 6 to 14 and an average annual frequency of 9.5.The highest proportion（73%）was caused by upper trough,low vortex and low-level shear line.（2）During the rainy season,the seasonal variation of the negative MPV vertical flux is a good indicator of the northward leap of heavy rainfall from South China（April to June）to Yangtze–Huaihe region（May to July）.Conditional symmetric instability（CSI）zones within the rainband centers or nearby are mainly located between 925 and 600 h Pa in April,May and September,whereas they vertically shift to 700–600 h Pa from June to August.The CSI zones are also in phase with the northward leap of summer monsoon rainband and its intensification as well as its southward retreat and weakening.During the late spring and early summer,the thermal wind vertical shear contribution to SCAPE is more than the contribution of CAPE in the magnitudes,which is 800-1200 J·Kg^-1,and plays a major role in rainband in South China from April to June and that in Yangtze–Huaihe from May to July.The thermal wind vertical shear contribution is in phase with the northward seasonal leap of heavy rainfall from South China in April to June to northern China in July to August,while CAPE has no such relationship with the heavy rainfall center,just CAPE large value zone expanding to the north with the northward leap of the summer monsoon.Moreover,the conditional MPV flux（CMF）index representing the strength of CSI is consistent with the regional heavy precipitation in the pre-flood period in South China and the Meiyu period in Yangtze–Huaihe region.（3）According to the south China Sea summer monsoon onset date,the April-June period of pre-flood season rainfall over South China is divided into frontal rainstorm period and monsoon rainstorm period.The number of annual average frontal rainstorm stations before monsoon onset is 75,with an annual average rainfall of 5,829 mm.After onset,the number of annual average monsoon rainstorm stations is 139,with an annual average rainfall of 11,673mm,which is almost twice that of the frontal rainstorm.The synthesis analysis shows that strong divergent airflow is maintained at the upper level in both periods,but the convergence of the lower level and vertical transport of the latter are stronger than the former.In terms of circulation pattern,the location of the South Asia high（SAH）and the West Pacific subtropical high（WPSH）is mainly the difference.In the frontal rainstorm period,at 200 h Pa,South China is located at the right back of the westerly jet entrance region,and high-level divergence is caused by subtropical westerly jet.At 500 h Pa,WPSH controls the South China Sea,blocking the airflow over the Bay of Bengal.The water vapor channel in the Bay of Bengal mainly ranges from 0 to 10°N,with the maximum water vapor flux value only 6g·cm^-1·h Pa^-1·s^-1.At 850 h Pa,the pseudo-equivalent potential temperature in the frontal zone north of South China is steep,with an inverted"V"shape at the front.Also,the specific humidity presents an inverted"V"shape,and the humidity axis inclines to the north with the height.While in the monsoon rainstorm period,SAH moves up to the Qinghai-Tibet Plateau from 90°N,South China is located in the distributary area between the right side of the subtropical westerly jet entrance region and northwest airflow east of SAH.At 500 h Pa,WPSH retreats eastward,thus connecting the water vapor in the Bay of Bengal to the South China Sea.The water vapor channel expands from 0-10°N to 0-20°N,the central value of water vapor flux at 10-15°N increases to 14-16 g·cm^-1·h Pa^-1·s^-1,and water vapor transport by the south wind is enhanced.At 850 h Pa,the pseudo-equivalent potential temperature gradient in South China is small.The pseudo-equivalent potential temperature first inclines to the south,then bends to the north with height,resulting in a structure similar to that of the Meiyu front.The atmospheric instability in both periods is dominated by convective instability（CI）.Whereas CSI also strengthens the vertical circulation of the mesoscale system in the frontal rainstorm period.The CMF index is consistent with the annual change of the rainfall amount,and the correlation of the frontal rainstorm period is greater than that in the monsoon period.（4）The Meiyu-period rainstorms are caused by summer monsoon meeting with cold air when it pushes to the middle and lower reaches of the Yangtze River.The mechanism of Meiyu-period rainstorms in Yangtze–Huaihe region making shift from drought to flood is analyzed,and the results show that in the extreme drought period,dry and cold northeast airflow prevails,WPSH locates eastward,water vapor flux in the lower layer is weak and water vapor is mainly transported by north wind,and downdraft mainly occupies the lower and middle layer,plus no low level jet,sparse pseudo-equivalent potential temperature lines and the lack of boundary uplift mechanism,which all lead to drought intensified.Whereas in Meiyu period,the southwest airflow is strengthened,WPSH extends westward,the low-level airflow converges in Yangtze River,and water vapor flux in low level is increased and water vapor is mainly transported by southwest and southeast wind.Accompanied by the coupling of high and low level jets and deep ascending motion,the Meiyu front is formed by the dense pseudo-equivalent potential temperature lines,which enhances the forced uplift and vertical transport of unstable warm and humid air,resulting in frequent rainstorms and regional floods.The unstable layer with negative MPV is below 600 h Pa in the rainstorm center.The joint action of CI and CSI is the instability mechanism of heavy precipitation.The CMF index is in phase with the change of drought and flood and the evolution of rainstorm.In the extreme drought period,the CMF index is small and changes gently,whereas in Meiyu period,the opposite is true,CMF index increases gradually at the beginning of the rainstorm and decreased rapidly at the end.