京津及河北平原凌晨强降水形势和环境条件

Synoptic patterns and characteristics of environmental parameters for early-morning heavy rainfall over Beijing,Tianjin,and most of Hebei Plains

北京、天津及河北平原大部(37°~41°N,115°~119.5°E)(简称BTH-P)为夏季强降水夜发性较突出的区域。基于2010—2021年国家级地面气象站逐小时降水数据和CFSR再分析数据集,对该区域6—8月42个凌晨时段强降水个例的天气形势进行了分类,统计了不同天气形势下强降水的环境物理量特征。结果表明:BTH-P区域凌晨强降水的天气形势可分为热带气旋类、低涡切变类、低槽冷锋类和副高边缘弱天气尺度强迫类,且低涡切变类凌晨强降水最为频发。统计不同类天气形势的降水环境物理量场特征分布发现,水汽条件都非常充沛,热带气旋类凌晨强降水水汽条件最优,其大气可降水量的中位数达72 mm;热带气旋类热力和不稳定能量条件不如其他类突出,对流有效位能、K指数均最小,而副高边缘弱天气尺度强迫类的上述指数显著强于其他3类;热带气旋类降水相比其他类降水凌晨低层的辐合更为明显、垂直涡度也更大,低涡切变和低槽冷锋类降水的0~6 km垂直风切变最大、850 hPa与500 hPa温差显著大于其他类,表明其对流层中低层具有更强的斜压性。总体来看,该区域凌晨时段强降水的主要天气形势为低涡切变、低槽冷锋和热带气旋,具有特别充沛的水汽条件、不太强的条件不稳定和中等偏弱的0~6 km垂直风切变环境条件等。

Beijing,Tianjin and most of the Hebei plains(37°-41°N,115°-119.5°E)(BTH-P region)are regions with frequent nocturnal precipitation in summer.Based on hourly precipitation dataset from national observation stations during 2010-2021,42 heavy precipitation cases in the early morning period in this region were classified according to synoptic patterns.The distributions of environmental physical parameters of heavy precipitation cases under different synoptic patterns were then calculated and obtained.The results show that the cases can be classified into four types,i.e.tropical cyclone type,low vortex or shear line type,low trough with cold frontal type and weak synoptic forcing at the edge of West Pacific Subtropical High type.It was found that the tropical cyclone type has much better water vapor conditions,with median PWAT(Precipitable Water)up to 72 mm,but weaker thermal and energy conditions than the other three,with the smallest CAPE(Convective Available Potential Energy)and K index values.The type of weak synoptic forcing in the edge of West Pacific Subtropical High has the most favorable thermal and energy conditions.In terms of dynamic and uplift conditions,the convergence of tropical cyclone type is more obvious and its vertical vorticity is greater than that of others.The 0~6 km vertical wind shear of low-vortex-shear type and frontal type are the more violent,with temperature difference between 850 hPa and 500 hPa being more relatively larger than that of the other two types,which indicates their middle and low troposphere has stronger baroclinity.In summary,the main weather patterns of heavy precipitation in the early morning of BTH-P region are low vortex or shear line,low trough with cold frontal and tropical cyclone,with particularly abundant water vapor conditions,unstable conditions that are not too strong,and moderate weak 0~6 km vertical wind shear environmental conditions.