Cause Analysis of an Extreme Gale Event with a Bow Echo in Southern Shandong Under the Background of a Cold Vortex
On the night of May 17,2020,an extreme gale event with a rare strong bow echo,large hail,and extreme wind speed exceeding 12 on the Beaufort scale,was recorded in southern Shandong Province,China.To improve the understanding of the environmental conditions of such extreme gales,the present study analyzed the characteristics of storm structure and the mechanism of evolution by using ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts,and data from conventional observation,ground automatic weather station intensive observation,and Doppler radar.The results indicate that the transverse trough at the rear of the North China cold vortex turned vertical to guide the cold air southward,which enhanced the vertical shear of mid-and low-level tropospheric winds,promoted the development of potential instability,and provided favorable environmental conditions for the generation of strong convective weather.The invasion of dry and cold air in the mid-level and the low-level temperature stratification curve being nearly parallel to the dry adiabatic line were conducive to the formation of thunderstorm gales.The initial convection was triggered near the place where the 850 hPa shear line coincided with the surface convergence line.The potential of the maximum ground wind speed calculated by using WINDEX(an index for forecasting microburst potential)was closer to the observed surface extreme wind speed.The bow echo had significant structural features such as echo overhang,weak echo region,and strong rear inflow.The strong rear inflow and abnormally dry mid-troposphere conditions were conducive to the formation of strong downdrafts,with extreme gale occurring at the place corresponding to the convex part of the bow echo.The strengthening of the cold pool was accompanied by the development of the inflow jet behind the storm,which had a direct impact on the formation of the bow echo and surface extreme gale.Pressurization due to the small-scale cold pool at the land surface and depressurization due to synoptic scale cyclones led to isallobaric winds.The superposition of isallobaric wind and cold pool density flow enhanced the intensity of surface extreme gale.The surface extreme gale was the result of the combined influence of cold pool density flow,rear inflow jet,and downward momentum transportation.
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