The unique features in the 4 d 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 our understanding of the formation of extreme rainfall events over this region. This heavy rainfall event, lasting from 29 July to 2 August 2023 (referred to as the “23 · 7” event), is characterized by a long duration, widespread coverage, and high accumulation of 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 extension of the western North Pacific subtropical high (WNPSH), 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 cycle of the 23 · 7 event is divided into two stages. In the first stage, the western part of the WNPSH was weakened by Tropical Storm Doksuri, where it appeared that the WNPSH retreated eastward with decreasing height. The marginal zone of this subtropical high then became inclined below 500 hPa. Therefore, convection was limited by the tilted WNPSH, with a warm, dry cover embedded in the low-to-middle troposphere. Meanwhile, mountain areas in the western part of North China were occupied by cold air masses above approximately 3.0 km. Combining the orographic and cold-air blocking, only a thin layer of southeasterly wind (between 1.3 and 3.0 km) was able to pass over the mountains. Although the warm and moist southeasterly flows were lifted by orography, no convection was triggered because of the local capped cold and dry air masses overhead. Under this regime, 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 generating widespread but low-intensity rainfall. However, the lifting was too weak to allow convection to be highly organized. In the second stage, the WNPSH was further weakened by the enhancement of Khanun, and thus the embedded warm, dry cover associated with the tilted WNPSH was significantly thinned. Consequently, convection triggered by orographic blocking was able to extend upward and develop further, forming deep convection. Generally speaking, the convection in the second stage is much deeper than that in the first stage. The results may shed new light on a better understanding and forecasting of long-lasting extreme rainfall.