Mechanism Analysis Of The Upscaling Process Of A Squall Line In South China

Hourly precipitation gid data set combined by Chinese station observations and CMORPH products(0.1°×0.1°),NCEP reanalysis data(1°×1°),regular observation data,radar obsevations and high-resolution WRF numerical simulation data are used to analyze the dynamic and thermal environmental conditions of the upscaling growth process of a squall line in China around April 13,2016,and terrain sensitivity tests are conducted.The results show that :(1)Before the formation of the squall line,there is a jet stream in the upper troposphere,and South China is located on the right side of the jet stream entrance area,which is conducive to high-altitude divergence.There are weak trough at high altitude,and the temperature trough lags behind the height trough,which is conducive to the development of the trough.There is a low-pressure system near the ground,and there is convergence of the wind field,which strengthens the lifting movement.(2)Compared with the previous squall line process,during this squall line process,the effective potential energy at the upscaling stage is higher than that in the initial convection stage,but the convective effective potential energy in the entire South China region is at a medium-to-low level.In addition to the influence of the synoptic scale system,the cooperate of various mesoscale conditions is very beneficial to the upscale development of the squall line.The low-level vertical wind shear,low potential vorticity and high temperature decline rate are the triggering conditions,which provide certain unstable conditions and uplifting conditions.The low-level wind field turning,the vertical wind shear strengthened,the sufficient water vapor supply and the deepening of the rear inflow aggravate the instability and vertical uplifting movement,which are important conditions for promoting the upscaling of the ? mesoscale squall line.The cold pool is not obvious at the upscaling stage,but the cold pool is very important to maintain structure of the ? mesoscale squall line at the mature stage.(3)The topography affects the scale and organization of the squall line,and the high topography disperses the convection in the north.Terrain can change the horizontal flow field,water vapor field,vertical movement,and vertical wind shear at the low level,which are mainly as follows: no terrain obstruction is conducive to the northward advance of the jet stream,and the water vapor transportation is more convenient;A certain terrain height is beneficial to the vertical movement of the low level,while the terrain height is too high to weaken the vertical movement of the low level.If the terrain exceeds a certain height,the low-level convergence field will be correspondingly weakened.