Characteristics Of Short-Time Heavy Rainfall And Convective Organizations In The Early Morning Over Beijing,Tianjin,and Most Of The Hebei Plain Areas During The Warm Season

The Beijing-Tianjin-Hebei region and its surrounding areas(34~°-43~°N,113~°-123~°E)(BTH-N region)are the regions with frequent heavy precipitation and more obvious nocturnal characteristics but less afternoon occurrence in northern China.Based on hourly precipitation data of 87 national level stations from 1966 to 2021 and298 stations from 1980 to 2021,the geographical distribution,interannual and diurnal variations of total,light and short-term precipitation from May to September in the region were statistically analyzed,with emphasis on the early morning precipitation.Combined with CFSR reanalysis data,heavy precipitation cases in the early morning from June to August in the region of the Beijing,Tianjin,and most of Hebei Plain(37~°-41~°N,115~°-119.5~°E)(BTH-P region)were screened out and then classified into four categories based on the weather backgrounds.Their environmental conditions and the sources of systems and organizational forms were analyzed afterwards.The results show that there exists an area in the region of Beijing,Tianjin and most of the Hebei Plain(37~°-41~°N,115~°-119.5~°E)(hereafter BTH-P region)with convective and nocturnal characteristics of precipitation over more obvious than that in surrounding BTH-P＿EX regions(BTH-N region excluding BTH-P region).The precipitation in BTH-P＿EX region has an increasing trend generally,while the short-term precipitation is more obvious increase;The interannual variations of rainfall amount,frequency and intensity in the early morning during the warm season both increase in the BTH-P＿EX and BTH-P regions.Compared to BTH-P＿EX region,the BTH-P region shows more obvious convective characteristics during midsummer.Compared with the precipitation in the evening,the precipitation after midnight over BTH-N region is characterized by the weakening convective activity and the significant weakening intensity during the warm season.The total rainfall amount and frequency of two sub-regions in different intra-day periods have different interannual variations.Compared with other years,the short-term heavy precipitation in these two regions after 2005 decreased in the afternoon,and increased from midnight to morning.Based on the climatic characteristics above,124 early morning strong precipitation cases in BTH-P region were screened out.Meanwhile,42 cases after 2010 was classified,with shear type precipitation cases the most.In terms of environmental conditions,the tropical cyclone type has the greatest water vapor conditions but the worst heat and energy conditions;the convergence of tropical cyclone type is more obvious and its vertical vorticity is greater than that of other types.The middle and low troposphere of cold frontal type and low-vortex&shear type has stronger baroclinity.Finally,based on the radar combined reflectivity data,satellite data,etc.,it was found that the convective organizations of TC-type precipitation is mainly developed from upstream,and its average duration of influencing is longer than other types;In low vortex&shear precipitation,there are relatively more cases developed from upstream and local generated,with the TS type appearing the most frequently and LS type the least;the convective organizations of frontal-type and weak-forcing-type precipitation are mostly developed locally,with influencing time between 3-4 h and the convective organizations mostly in the form of TS type.Focusing on statistical research on the characteristics and organizational forms of early morning heavy precipitation in Beijing,Tianjin and most of Hebei plains,this article aims to obtain the distribution and diurnal variation characteristics.The environmental physical conditions of early morning heavy precipitation under different weather conditions were analyzed,and found that the low vortex shear type and low trough cold frontal type have stronger baroclinic characteristics in the middle and lower layers and etc.In addition,it was found that most heavy early-morning precipitation in BTH-P region are migrated from upstream and locally regenerated.Linear type is the main form casing heavy rainfall there,with TS type being the most common.