Case analysis of damaging high winds generated by bow echoes in the presence of a cold vortex over the North China Plain

In order to improve the study of environmental evolution and mechanism of strong winds induced by bow echo, multi-source observations (Doppler radar, wind profiler, meteorological tower in Tianjin and automatic weather station data at 5 min interval) and reanalysis data of the ECMWF ERA5 (spatial resolution of 0.25° and temporal resolution of 1 h) are comprehensively used to analyze the nocturnal bow echo event in North China Plain on 25 June 2020. Results indicate that this bow echo event developed under favorable weather conditions. The cold dry air behind the 500 hPa cold vortex superimposed on the warm moist southwesterly airflow at 850 hPa, which is conducive to the occurrence of convective storm. Characteristics of the convective storm evolution can be summed up as three stage: Supercell, bow echo and comma echo. The environmental condition at the supercell stage is characterized by moderate Convective Effective Potential Energy (CAPE) and 0—6 km wind shears, while at the bow echo and comma echo stages CAPE is low and 0—6 km wind shears are high. In the supercell stage, the sounding curve shows an "X"-shaped distribution. The negative buoyancy effect is the primary contributor to the surface gale. Meanwhile, the downward transport of momentum and cold pool high-density air are helpful for the formation of surface gale. In the bow echo stage, due to the combined effects of low-level warm advection and surface radiative cooling, a strong temperature inversion layer occurred near the surface, and the vertical temperature lapse rate between 850—500 hPa increased. The effects of negative buoyancy, downward transportation of momentum and cold pool high-density air all strengthened compared with that in the previous stage, resulting in the formation of the 13th grade disastrous gale. In the comma echo stage, the 850—700 hPa dry layer weakened, and the negative buoyancy effect is similar to that in the supercell stage. The effects of downward transport of momentum and cold pool density are similar to that in the bow echo stage, resulting in the formation of surface gale. Finally, the environmental evolution and mechanism of bow echo conceptual model are summarized.