Analysis of heavy precipitation supercell environment condition and structure in Tianjin coastal area

Abstract

In the afternoon of 24 July 2016,a local short-term heavy rainfall occurred in Dagang of Tianjin. The maximum rainfall intensity was 95 mm·h-1,which broke the extreme record in the past ten years. The formation environment and structure of the supercell thunderstorm which caused the short-term heavy rainfall are qualitatively and quantitatively analyzed by using a variety of observation data from automatic station,sounding,radar,microwave radiometer,etc. The results show that:(1) The evolution of supercell is that ground convergence line triggers the generation of old thunderstorms,cold pool outflow boundary triggers the generation of new thunderstorms,and new thunderstorms merge,develop and strengthen into supercell. (2) Three hours before the extreme heavy rainfall,the sea breeze from the Bohai is cold and wet,which played the role of "cold cushion" and vapor transport. (3) Super cells exhibit high centroid development with strong echo cores migrating to the right with increasing altitude. The bounded weak echo area,the backward inflow gap and the drape structure are obvious. The weak mesocyclone maintains four volume scans,with a rotation speed 16 m·s-1,a diameter of the core 40 km and a spreading height 13 km,which is always in the parcel of heavy rainfall. Before the radar echo parameter V50 (volume of reflectivity greater than 50 dBz) reaches its maximum,the echo variable parameter FV50 (change of strong echo volume of 50 dBz in 6 min) appears a sudden drop,which has warning significance for the development of supercells. (4) Within 6 minutes,the heavy precipitation corresponds well with the vertical decline rate of vapor density,the effective thickness and the effective area of the unit in the lower levels.

Keywords

• heavy precipitation
• supercell
• ground convergence line
• cold pool
• echo parameter
• vertical decline rate of vapor