The miscellaneous synoptic forcings in the four-day widespread extreme rainfall event over North China in July 2023

Abstract. Synoptic forcings have traditionally played a pivotal role in extreme rainfall over North China. However, there are still large unexplained gaps in understanding the formation of extreme rainfalls over this region. The heavy rainfall event, lasting from 29 July to 2 August 2023 (referred to as “23·7” event), is characterized by long duration, widespread coverage, and high accumulated rainfall over North China. Overall, the persistent extreme rainfall is closely associated with the remnant vortex originating from typhoon Doksuri(2305), tropical storm Khanun(2306), and the unusual westward extended western Pacific subtropical high (WPSH), as well as quasi-stationary cold dry air masses surrounding North China on the west and north sides. Based on wind profiles and rainfall characteristics, the life history of the “23·7” event is divided into two stages. In the first stage, the western boundary of the western Pacific subtropical high (WPSH) was destroyed by the tropical storm Doksuri, appearing that the WPSH retreated eastward with decreasing height. As a result, an inclined vertical distribution on the western boundary was established below 500 hPa. Therefore, convections were limited by the tilted WPSH with warm-dry cover embedded in the low-to-middle troposphere. Meanwhile, the orography in the west of North China was controlled by cold air masses above nearly 3.0 km. Combining the orographic and cold air blockings, only a shallow southeasterly layer (between 1.3 and 3.0 km) can overpass mountains. Although the warm and moist southeasterly flows were lifted by orography, no convections were triggered because of the local capped cold and dry air masses overhead. Under this framework, equivalent potential temperature (θe) gradients were established between warm humid and dry cold air masses, similar to a warm front, causing warm air to lift and generate widespread rainfall but low intensity. However, the lifting was too weak to allow convection to be highly organized. In the second stage, the WPSH was further destroyed by enhanced Khanun, and thus the embedded warm-dry cover associated with the tilted WPSH was significantly thinned. Consequently, convections triggered by orographic blocking can move upward and consequently further develop, forming deep convections. Comparatively speaking, the convections in the second stage are much deeper than those in the first stage. The results gained herein may shed new light on better understanding and forecasting of long-lasting extreme rainfall.