A Case Study of the Influence of the Western Pacific Subtropical High on the Torrential Rainfall in Beijing Area

Summer is a pluvial season in Beijing area.There is high correlation between the total amount of precipitation and the number of days when there are heavy rain events during the rainy season.72 percent of heavy rain events appear in July and August,and 46 percent among them are affected directly by the Western Pacific Subtropical High(WPSH) when it interacts with westerly trough.Furthermore,the precipitation in the Beijing area during summer is related to the strength,the area and the location of the ridge of the WPSH.In order to further reveal the impacts of the WPSH on the moisture transfer and the circulation situation under which heavy rain occurs,based on dynamic and thermodynamic diagnosis,a case study is performed by employing a 6-hourly and 1°×1° reanalysis data achieved at standard pressure levels for the period of 18-20 August 2001.Results show that during the heavy rain period in the lower troposphere,abundant water vapor over ocean is transferred towards continent by the easterlies along the south of the WPSH.The water vapor fluxes are then turned northward and converged with northerlies in the Beijing area.A strong energy front is formed between the westerly trough and the WPSH.The thick layer of moist available potential energy is found to the south of the front,which is then transformed to kinetic energy by the strong ascending motion associated with slantwise vorticity development.In the middle troposphere,the vertically differential vorticity advection and the warm advection to the west of the WPSH destroy the quasi-geostrophic balance in the Beijing area and its neighborhood,the secondary circulation is therefore forced both dynamically and thermodynamically in association with baroclinic perturbations,and in the area the vertical ascent is intensified.Torrential rain then occurs along such a strong energy front with strong low-level cyclonic vorticity development and high-level divergence.Furthermore,along the northward transfer of warm and moist current over the western flank of the WPSH,the stronger vertically differential advection of pseudo-equivalent potential temperature results in the development of convective instability,and is in favor of the intensification of rainfall.All these contribute to the occurrence of torrential rain in the Beijing area,and should be taken into consideration for predicting this kind of heavy rain events.