Characteristics of the July 2023 extreme rainfall in North China and its water vapor supply

Characteristics and water vapor condition of the extreme rainstorm in North China from 29 July to 1 August in 2023 are investigated based on surface meteorological observations of precipitation and the ERA5 reanalysis data. It is found that the “23·7” extreme rainstorm in North China has the characteristics of long precipitation period and large cumulative amount, exhibiting a significant extremity. The precipitation was mainly located in front of the Taihang mountain and the Yanshan mountain. The maximum precipitation center was basically consistent with the orientation of the mountains, showing a banded pattern. There were significant circulation anomalies during the extreme rainstorm. The northward shifted upper-level subtropical jet stream and the abnormally northerly western Pacific subtropical high pressure were the key circulation factors that affected the extreme rainstorm, while the remnant circulation of northward-moving typhoon Doksuri, typhoon Khanun and the low-level jet stream were the major weather systems affecting the extreme rainstorm. The water vapor condition of the extreme rainstorm showed obvious phase features. From 08:00 BT 29 to 08:00 BT 31 July, low-level convergence, high-level divergence and upward motion were strong in the main precipitation area, and the low-level cyclonic wind further deepened. Water vapor was transported by the remnant circulation of typhoon Doksuri over a short range and continued by typhoon Khanun, with the former being the main source. The entire layer of water vapor in Beijing-Tianjin-Hebei region was in a net inflow state, with the maximum net inflow reaching 1.5×10⁸ kg/s. From 08:00 BT 31 July to 08:00 BT 1 August, the ascending motion, the lower-level convergence and upper-level divergence significantly weakened, the thickness of low-level cyclonic wind field shrank, and water vapor was transported only by typhoon Khanun over long distances. The entire layer of water vapor in Beijing-Tianjin-Hebei region was in a state of net outflow, with the maximum net outflow reaching 5×10⁷ kg/s. The blocking effect of the Taihang mountain and Yanshan mountain caused the water vapor convergence center to remain for a long time, and their frictional effect may be favorable for the amplification of the ascent motions in front of the mountain. These effects provided favorable conditions for this extreme heavy rainfall.