Unique moisture transport under abnormal circulations for producing the extreme rainstorm over Beijing on July 31, 2023

Abstract Extremely strong water‐vapor supply is a necessary condition for generating an extreme rainstorm; however, this condition is rarely satisfied for the semi‐humid regions, such as North China. To clarify how the extreme moisture condition is satisfied within semi‐humid areas is of crucial importance to further the understanding of this type of event. From July 29 to August 2, 2023, an unprecedented extreme rainfall event hit Beijing (an 83‐hr accumulated precipitation of ˜1,025 mm appears) and broke its rainfall record that has stood the past 140 years. We conduct synoptic analyses and three types of quantitative moisture budgets to explore the key features of the moisture transport. It is found that the displacement and evolution of Typhoon Doksuri result in an abnormal circulation, under which the West Pacific subtropical high and Typhoon Doksuri play the dominant roles for driving the direct moisture transport for the extreme rainstorm in Beijing. Most of the moisture is transported into the targeted region (i.e., a box covering the rainfall region) across its eastern and southern boundaries in the lower troposphere, with the inflow efficiency of the former ˜3.9 times that of the latter. The northwest Pacific contributes significantly (37.1%) to the moisture supply for the extreme rainstorm region. Eastern China, mainly located over land, also provides a substantial portion (33.8%) of the moisture, implying that earlier heavy rainfall in one region can notably affect subsequent heavy rainfall in adjacent areas by providing moisture.