Radar Characteristics and Occurrence Conditions of Jinghai Tornado in Tianjin on 13 August 2018
Using conventional observation data, ground automatic weather station data, Doppler weather radar and wind profiler radar data, etc., this paper analyzes the EF2 tornado that occurred in Jinghai, Tianjin at about 17:30 BT 31 August 2018. The focus is on the analysis of environmental background conditions, local instability conditions, trigger uplift conditions and radar echo characteristics, etc. before and after the occurrence of tornado. The results show that this tornado weather occurred when the 500 hPa high-altitude trough moved eastward, the subtropical high strengthened its westward movement and Typhoon Capricorn was moving northward. Before the tornado occurred, the environmental background conditions were obviously unstable, the CAPE value was 1797 J·kg-1 (CIN was 0 J·kg-1) and the wind vector difference in the height of 0-6 km was approximately 14 m·s-1 and there was lower lifting condensation level (LCL). At the same time, there was high CAPE (maximum value exceeded 4000 J·kg-1), lower LCL. Before the tornado occurred, LCL showed a sudden drop (lowest to around 897 hPa). With the occurrence of tornado, the vertical wind shear values near the ground (0-1 km height and below) at both upwind and downwind stations showed the characteristics of rapid increase first and then rapid decrease. The rapid decrease of the vertical wind shear value started from the upper layer and descended rapidly, while the vertical wind shear value above the 0-1 km height did not change significantly. Doppler weather radar observations show that the tornado echo had a small individual scale (with a diameter of about 2 km at the positive and negative velocity centers), a maximum rotational speed of 24 m·s-1 at the positive and negative velocity centers, and a vertical vorticity of 2.4×10-2 s-1. The echo of this tornado had obvious overhanging structure, bounded weak echo zone and tornado vortex. Moreover, the low-level inflow at the rear side was very strong, so it should be a micro supercell tornado. The collision between the sea breeze front moving to the locale and the gust front formed by the upwind thunderstorm cell outflow triggered the vigorous release of unstable energy locally, stimulating the regeneration of thunderstorm cells, and developing into organized strong thunderstorm cells, which should be the direct trigger conditions for the occurrence of this tornado.