Analysis of environmental condition and convective storm evolution of a thunderstorm gale event under cold vortex in Hebei

A regional thunderstorm gale weather event occurred in central and southern Hebei on 9 July 2017. It is a typical severe convective weather with the cold advection forcing in upper air. The convective cloud clusters successively affected the south area (I) and north area (II) of central and southern Hebei during this event. Based on the routine upper-air and surface observations, satellite cloud images, Doppler weather radar data, observations from regional automatic weather stations and NCEP reanalysis data with 1°×1° resolution, we have conducted an analysis of the environmental conditions and the evolution characteristics of the convective storm when this event occurred. The main results are as follow. (1) This event occurs under the background of the Mongolian cold vortex weather. The cold air at the rear of the cold vortex and the warm and wet air in the low-level jointly form an unstable stratification with the dry and cold air in high level and the warm and humid air in low level, in which vertical wind shear is strong in southern Hebei Province. The convection in area I is triggered by topographic uplift, and moves southeastwards under the action of northwest airflow in the high level. Nevertheless, the convection in area II is directly triggered by the cold front, and moves eastwards and northwards under the combined action of advection and propagation. (2) The convective systems causing the gale in area I include the squall line and the supercell associated with mesocyclone. The rear inflow jet below 1 km exceeds 31 m·s^-1 in mature stage of the squall line. The surface gale appears at the front of the center of high wind speed, the front of the thunderstorm high and the vicinity of hourly positive pressure center. There are various types of convective systems causing the gale in area II, including supercell, block-shaped echo and squall line. The gale squall line appears between the rear side of gust front and the hourly positive pressure center. After the echo intensity of the squall line is decreased, the combined effect of the cold pool density flow, the downward momentum transportation and the allobaric wind can still cause surface gale. (3) The fact that high wind speed core exceeding 30 m·s^-1 in the radar radial velocities at the low elevation angle corresponded to the hourly positive surface pressure change of over 5 hPa, and the descending velocity of 7-10 m·s^-1 below 5 km followed by the strong northwest wind below 1 km in the wind profiler radar product can be used as a reference indicator for the 0-2 h nowcasting and warning of the thunderstorm gale above level 8 on the ground.