Abstract:To investigate the environmental characteristics, storm structures and formation mechanism of the thunderstorm that occurred on 27 July 2016 in Beijing Area, the data of conventional observation, wind profile, Doppler weather radar, Variational Doppler Radar Analysis System (VDRAS) and automatic weather station (AWS) data are analyzed. The case analysis shows that this windstorm developed under a weak synoptic forcing background. The unstable thermodynamic condition enhanced the development of linear convection into bow echo, resulting in thunderstorm. The characteristic in sounding data such as the temperature laps rate in the lower troposphere was almost equal to the dry adiabatanda sudden increase of DCAPE, which are indicative of windstorm weather. The cold pool outflow and warm moist southerly flow at front of the mountain formed a convergence zone in the western region of Beijing. Under the combined action of significant disturbance temperature gradient and topographic forcing new thunderstorms were triggered. Because of the uplift of mountain area, the lifting of thunderstorm cold pool outflow caused significant disturbance temperature gradient, contributing to the development of the thermal instability, and triggered unstable energy, enhancing thunderstorm intensity in the process of coming down the hill. The Doppler radar product also showed a strong reflectivity core, an overhanging echo, boundary weak echo range and a book end vortex in radial velocity images. Under the combined action of the intensive cold pool and the book end vortex, the rear inflow was intensified, becoming rear inflow jet, which was characterized by a strong wind zone in the low elevation radial velocity images. The dry and cold air were trapped into the cloud by the rear inflow, creating a negative buoyancy through evaporation. Combined with the dragging of precipitation particles, eventually, strong surface winds were formed.