Abstract:

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.

Key words: Early morning, Short-term precipitation cases, Synoptic patterns, Environmental parameters