Comparing the mechanisms of two types of summer extreme precipitation in Beijing-Tianjin-Hebei region, China: Insights from circulation patterns and moisture transports

Abstract

Beijing-Tianjin-Hebei (BTH) is undergoing huge risks from severe precipitation extremes, but their climate features and underlying mechanisms are not fully understood and warrant in-depth investigations. Here, the summer extreme precipitation events in BTH are objectively divided into two types according to the spatial distribution i.e., Northeast precipitation (NEP) and Southwest precipitation (SWP) and their underlying mechanisms are revealed and compared from perspectives of circulation patterns and moisture transports. In the case of Type NEP, the anomalous deep low-pressure (high-pressure) systems respectively cover over the west (east) of BTH, which jointly induce strong anomalous southwesterly and southerly airflows converging over BTH. The converging airflows strengthen water vapor transports from the western and southern boundaries of BTH and result in a strong convection over northeastern BTH, thereby triggering Type NEP precipitation. Compared with Type NEP, the circulation pattern of Type SWP is characterized by an anomalous deep (shallow) high-pressure (low-pressure) system over northeast (southwest) of BTH, respectively. The circulation patterns could induce strong anomalous southerly and easterly airflows converging over BTH and thus strengthen water vapor transports from the southern and eastern boundaries of BTH, resulting in a strong convection over southwestern BTH. Over the long-term period, the summer extreme precipitation days with multiple return periods extracted by the Generalized extreme value distribution theory show significantly increasing trends in Beijing-Tianjin and surrounding areas, particularly in urban regions, indicating that summer extreme precipitation events are becoming more frequent. These findings provide theoretical basis for summer extreme precipitation forecasting and scientific insight for taking effective measures to mitigate the corresponding disasters in BTH.