Preliminary study on the causes of a downburst event in Tianjin in the summer of 2017

A severe thunderstorm gale event with several short-term rainfall,hail and downburst occurred in the southwest of Tianjin on 9 July 2017, during which two downbursts attacked successively Duliu Town,Jinghai District,Tianjin. Based on the Doppler weather radar data,the three-dimensional variational wind retrieval data,the intensive observations from the automatic weather stations and other data,we have conducted a preliminary study on the causes of downburst during this severe thunderstorm gale event. The results indicate that (1) this downburst event is due to the cold vortex at 500 hPa and the forward-tilted trough,as well as a statically unstable stratification formed by the dry and cold air in the high and the warm and humid air in the low. There are obvious conditional instability and large CAPE and DCAPE in the lower and middle troposphere before the convection,which is triggered by the mesoscale convergence line formed by the southwesterly and southeasterly winds with high temperature in the low atmosphere. The weak vertical wind shear environment makes the convective storm appear in the form of pulse storm in the early stage. (2) Before the downburst occurs,it corresponds to the decline of reflectivity factor core and the MARC and the low-level mesocyclone in the radar echo map. (3) The dragging effects of precipitation particles play an important role in the occurrence of two downbursts in Duliu Town. In addition,the occurrence of the first downburst gale is also related to the negative buoyancy and the entrainment of dry air,and that of the second downburst gale is related to the vertical gradient of disturbed air pressure behind the mesocyclone.