Analysis and Numerical Simulation Research on Severe Surface Wind Formation Mechanism and Structural Characteristics of a Squall Line Case

An unusually severe squall line resulted in significant losses of lives and property on June 3, 2009, in Henan, China. To better understand the characteristics and production mechanism of the squall line, the data of satellite, radar, intensive surface observation, and the mesoscale Weather Research and Forecasting (WRF) model were used to investigate the atmospheric background, macrostructure, and microstructure of the squall line and the formation mechanism of the damaging surface wind. The results show that a northeast cold vortex was the main influencing system of the squall line. The transversal trough located at the back of the northeast cold vortex induced a strong cold airflow, which met with a relatively weak southwesterly warm and moist airflow to produce convection. The system further developed in the study region as a severe squall line. The atmosphere contained weak southwesterly winds and water vapor at low layers; thus, the atmospheric environment of the squall line formation was dryer. The atmosphere was conditionally unstable with a convective available potential energy (CAPE) index of approximately 1300 J/kg and adequate wind shear. A relatively cold and moist high with thunderstorms and a strong cold pool on the surface field occurred concurrently with the squall line to produce severe surface wind rather than heavy rain. The results of the WRF model showed that although the maximum downdraft of the squall line was only -13 m/s, the surface outflow wind speed was 35 m/s, which exceeds the maximum downdraft by a factor of 2.7. Further investigation revealed that the cooling processes of rain evaporation and graupel melting are the major contributors to the decrease in surface temperature and strong wind production. Among them, the cooling rate due to rain evaporation was approximately -3 K/min while that due to graupel melting was approximately -0.7 K/min. Therefore, the key factor to influence the cold pool intensity was rain evaporation; this cold pool played a critical role in the formation of the severe surface winds during the squall line event.