Numerical Simulation And Diagnostic Analysis During A Squall Line Process

The eastern China experienced a strong squall line with heavy rain from afternoon on May 2nd,to early morning May 3rd in 2016,which caused severe flooding and huge economic losses in passing provinces.In order to deepen the understanding of the squall line's process,improving the forecast ability of the strong convective weather.In this paper,the atmospheric circulation background and unstable environment of the squall line were analyzed by using the FNL reanalysis data,the ground encrypted observation data,and the sounding data.On this basis,the high resolution numerical simulation of WRF(Weather Research and Forecast)model was run,and the model output data was used to analyze three aspects of this squall line process specifically.First and second,the evolution characteristics of the various stages of the squall line and the squall line rainfall in the micro-physical process of sensitivity test were analyzed.Finally,this paper carried out a diagnosis and analysis of the evolution characteristics of convection stability in this process,and probed into the direction and prediction of the potential divergence of the severe convection weather.The main conclusions were as follows:(1)The squall line was caused by the combination of weather systems on the high and low levels.The jet on 200 hPa isobaric surface and the shear line on 850 hPa were coordinated to form the dynamic configuration of the divergence on the higher levels and the convergence on the lower levels,promoting the development of vertical motion.The low vortice on 500 hPa guided the cold air southward with the large trough and raises the warm and humid air,which is conducive to releasing unstable energy and triggering the strong convection.The warm and humid air currents in the southwest and the south have been pouring water vapor into the squall line.(2)It could be seen from the simulation results,the squall line existed the wind field convergence line which continuously stimulates the generation of new convection and causes the squall line to maintain and move forward.The meso-scale features of the low pressure in the front of squall,low pressure and ground thunderstorm high pressure are obvious.With the development of the squall line,the tail inflow was gradually enhanced,which can lead to the engendering of the gust wind on the ground and the enhancing of the cooling pool.The inter-phase configuration of the divergence field and the convergence field was good for increasing vertical speed and enhancing updraft.The interaction of vertical wind shear and cold pool conformed to RKW theory.(3)The presence of graupel particles has an important influence on the squall line structure.In the developing period of the squall line,the graupel particles caused the cold pool in the system and improved the cooling rate of the system rapidly.It promoted the generation of the bow echo and the large area of the strong convection region early.In the later,because of the graupel particles falling quickly,leaving the updraft,so echo dispersal velocity was faster than the micro-physical process without graupel particles.Graupel particles was mainly from the contribution of rainwater particles,snow particles and cloud water particles(4)The potential divergence was the main physical factor for the change of atmospheric potential stability.Its negative value was conducive to enhance potential instability and its positive value tended to inhibit potential instability.In the higher level of troposphere,the local variation of potential divergence affected mainly by the Coriolis force.While in the lower level of troposphere,the co-action of Coriolis force and wind velocity gradient coupling played an important role in the variation of potential divergence.In this paper,we tried to using potential divergence to forecast precipitation during the squall line.The results displayed that potential divergence were in good agreement with the observed one hour precipitation in in time and space.Combined it with other characteristics of the squall line,the forecast decision tree of heavy precipitation in airport was proposed initially.