Objective Identification And Analysis Of Warm-shear-type Warm-area Rainstorms In The Jianghuai Basin

The warm-sector rainstorm(WR)with warm shear pattern over Yangtze-Huaihe river region were studied in this paper(WSWR).Based on the definition of WR and shear-line,and the objective identification criteria of meso-scale rainstorm cluster,front and shear-line,this study use the merged rain gauge-satellite gridded hourly precipitation dataset and ERA5 reanalysis data to identify the WSWR over Yangtze-Huaihe River Basin(YHRB)during2010?2017,A total of 1268 WR clusters and 451 WSWR clusters have been identified over the8 years.Spacially,the WSWR clusters exhibit similar horizontal distribution with the WR clusters over YHRB,both located at the junction of the Anhui,Jiangxi,and Hubei province and the coastal areas of Jiangsu and Zhejiang province.In terms of time,WSWR clusters occur mainly from mid-June to mid-July,during this period,which account for more than 40% of the total WR clusters.The two types of clusters gradually move northward,consistent with the migration of the main rainfall belt.The occurrence of WSWR clusters show significant diurnal variation,with one peak from morning to early-afternoon(0800?1300 LST)and the other after sunset(2000?2200 LST).The maximum precipitation of WSWR clusters appeared during afternoon(1300?1900 LST)and early morning(0100?0500 LST),respectively.Based on the70 WSWR clusters which occurred in the coastal areas of YHRB,32 WSWR events were detected.The weather circulation and thermodynamic characteristics of the WSWR event in the coastal areas of YHRB were analyzed,and the results showed that,the coastal areas of YHRB is located in the northwest of the western Pacific subtropical high,and in the south of the warm shear line.The results of the analysis of moisture conditions showed that,in the lower troposphere(875?700h Pa),the water vapor over the coastal areas of YHRB came from the South China Sea along the western edge of the Western Pacific Subtropical High,and in the bottom layer of the troposphere(1000?900h Pa),water vapor is transported from the eastern coast of China through the southeasterly wind,the water vapor is abundant in the middle and lower troposphere.Thermal dynamic analysis shows that,in the bottom layer of the troposphere,under the action of moisture advection and warm advection,before the start of WSWR events,the pseudo-equivalent potential temperature of 950 h Pa continued to strengthen and reached the maximum at-3h under the advection of warm-moist air,the energy of warmmoist in the bottom of troposphere were strengthened.Under the effects of coastal terrain,the wind of boundary layer decreased when it entering the inland,and then induced the convergence at the coastal area,which were conducive to triggering convection and producing WSWR.The scale analysis shows that the large-scale dynamic field and the small and medium scale thermal disturbance field play a leading role in the formation of the WSWR in the coastal areas of YHRB.To evaluate the forecast technique of PWAFS(Precision Weather Analysis and Forecasting System)through 5 WSWR events,the result shows that the model had poor forecasting results on WSWR clusters and WSWR processes,and it got low TS scores.The comparison of WSWR event,which failed to be forecasted,between the observations and model results shows the model has a positive deviation in the wind forecast of boundary layer,and the model failed to forecast the variation of the boundary layer wind in coastal areas of YHRB and the convergence of vapor in precipitation area.The diagnose of observation and numerical data shows that under the effect of coastal terrain,the convergence of the wind in boundary layer is the main trigger condition for WSWR events in the coastal area of YHRB.