Analysis of precipitation evolution and mesoscale characteristics for a severe torrential rain event in Hebei from 29 July to 2 August 2023

Abstract

Affected by the weakening of the low-pressure system caused by the northward movement of the typhoon "Doksuri", Hebei encountered an unprecedented severe torrential rain event (hereinafter referred to as "23.7" severe torrential rain) from 29 July to 2 August 2023, in which the accumulated rainfall in 52 hours is up to 1 003.4 mm at Liangjiazhuang station in Xingtai of southern Hebei. Using routine upper-air and surface weather observations, regional automatic weather station data, radar mosaic of reflectivity factor, ERA5 reanalysis data, and other relevant data, we conducted the analysis on the precipitation evolution characteristics, circulation patterns, environmental conditions, and mesoscale features of the "23.7" severe torrential rain. The results indicate that the "23.7" severe torrential rain is characterized by long duration, large accumulated rainfall and wide coverage. After landing and moving northward, Typhoon "Doksuri" is blocked by the high pressure barrier in northern North China at 500 hPa and stagnates. Total content of water vapor over most areas of Hebei reached more than twice the climate average state. All the factors, that low lifting condensation level, low free convection height, moderate convective instability energy, as well as the lasting convergence uplift provided by the stability and stagnation of inverted trough at 850 hPa, are the causes of maintaining precipitation echo for a long time in central Hebei and in the foothills of Taihang Mountains in southwestern Hebei. The southeasterly boundary layer jet moves northwards along with the typhoon inverted trough, and the jet core is located in central Hebei, with the wind speed exceeding 22 m·s-1 and terrain plays a significant role in the rainfall event. During the influence of boundary layer jet on southwestern Hebei, multiple convective cells with echo intensity of 30~40 dBz and centroid height lower than 6 km successively pass over the Liangjiazhuang station, causing a total of 24 hours of short-term severe rainfall at the station. After the boundary layer jet moving northwards, the rainfall overlapping, caused by the continued long-term maintenance of stratocumulus mixed cloud precipitation echo in the foothills of Taihang Mountains due to the interaction of easterly wind of about 8 m·s-1 and terrain, is the main reason of the 52-hour accumulated rainfall exceeding 1 000 mm at the Liangjiazhuang station.

Keywords

• severe torrential rain
• typhoon "doksuri"
• boundary layer jet
• topographical effect