Numerical Study On The Response Of Squall Lines To Initial Environmental Fields And Its Formation And Development Characteristics

Through high resolution numerical simulations,this study conducts two-dimensional idealized simulation of squall lines and a real case modeling study of a squall line event using the Weather Research and Forecasting(WRF)model to investigate the response of squall line to key environmental factors,analyze the impact of initial low-level moisture,vertical wind shear and temperature profile on the structure and intensity of squall lines,explore the sensitivities of the structure and intensity of squall lines to initial environmental features and reveal the precursors to squall lines formation and development and evolution characteristics of the structure and intensity of squall lines.The main conclusions of this study are as follows:The ideal sensitivity experiments reveal that the change of moisture and temperature has an effect on the vertical distribution of convective available potential energy(CAPE),which affects the intensity of squall lines.Surface CAPE remains unchanged,in the environment of low-level drier(moister)and entire air column colder(warmer),the depth of CAPE increases(decreases),more(less)CAPE releases when the convection is triggered,the intensity of convective system increases(decreases),and rainfall increases(decreases)leading to stronger(weaker)cold pool.Vertical wind shear affects the strengths of the cold pool by mainly affecting the height of the cold pool.Low-level vertical wind shear increases(decreases),the height of cold pool significantly increases(decreases)when the squall line reaches into mature stage and the intensity of convection at the leading edge of the cold pool increases(decreases).A squall line system in South China is investigated using numerical simulations and sensitivity experiments.Sensitivity experiments are performed by increasing low-level moisture of the initial conditions used for the control simulation.It is found that the precipitation rate increases with the increase of low-level moisture.With the layer of moisture increasing to 850 h Pa,the convective system tends to organize linear structure earlier,and the intensities of radar echo and surface high wind increase in all stages of the convective system.The increase of low-level moisture of the initial conditions has an effect on the strength and structure of the cold pool,which affects evolution characteristics and structure of the squall line.With the increase of low-level moisture,more water vapor is transported to middle and upper-level,and more latent heat is released by condensation of water vapor.With the increase of precipitation,the strength of the cold pool is enhanced due to diabatic cooling caused by evaporation of raindrops.The cold pools on the east and west sides of simulated squall lines in each sensitivity experiment show different evolution characteristics.With the layer of moisture increasing to 850 h Pa,the cold pool on the west side of the squall line develops faster and tends to organize linear structure,which is conductive to enhancement of convective system and rear inflow on the west side.With the strength of the cold pool on east side of the squall line increasing,the intensity of the rear inflow on the east side also increases due to upshear tilt.With the increase of low-level moisture,the strength of the cold pool is the main factor for the variability in the the structure and intensity of the squall line.An additional sensitivity experiment is performed by reducing diabatic cooling caused by evaporation of raindrops in the experiment of increasing of low-level moisture of the initial conditions.It is found that the strength of the cold pool is weakened,which leads to the rainfall area and radar echo structure closer to the results of control simulation,and the rear inflow weakened accordingly.