The Characteristic and Mechanism of an Extreme Gale in Beijing

Abstract：A review was conducted of the subjective and objective forecasts and their biases for the extreme thunderstorm and gale event in Beijing on May 30, 2024. Additionally, using multi-source observational data such as Beijing's S-band dual-polarization radar and automatic weather stations, an analysis was performed on the convective characteristics, causes, and mechanisms of the gale. The results indicate: (1) This thunderstorm and gale event was influenced by a cold vortex, characterized by significant temperature differences between the upper and lower troposphere and moderate-to-strong vertical wind shear conditions, but with extremely poor humidity. The gale exhibited extreme characteristics. (2) Scattered convection rapidly intensified into a squall line as it moved downslope. Radar monitoring revealed widespread velocity aliasing, rear inflow jet (RIJ), and local small-scale vortices, all indicating straight-line strong winds at the surface and locally possible vortex-induced strong winds. (3) The primary reason for the forecast bias was inadequate consideration of the intensity of convection downslope in a dry environment. In conditions of poor humidity unfavorable for convection enhancement downslope, the coupling of thermodynamic processes can rapidly saturate the relative humidity near the storm, thereby enhancing convection downslope in conjunction with strong thermodynamic instability and dynamic processes. (4) The RIJ forms as a compensation for the intense downward divergent airflow within the storm, which subsequently further entrains dry air into the cloud. The evaporation, cooling, and temperature decrease processes of cloud and rain particles within the storm result in the formation of strong dry convective available potential energy (DCAPE), which enhances surface winds. This is the reason for the intensification of winds leading to extreme thunderstorm and gale events.

Keywords：Extreme Gale, Cold Vortex, Dry Environment, Enhanced Downslope Convection , Rear Inflow Jet（RIJ）