Characteristic Analysis And Cause Analysis Of Horizontal Vorticity And Vertical Velocity Variation In The Development Process Of Squall Line

In the convective weather,the horizontal vorticity has a great influence on the vertical velocity,but the quantitative relationship between the horizontal vorticity and the vertical velocity and the influence mode are not completely solved at present.In this paper,through the analysis of the numerical simulation results of a squall line process in Guangxi on June 19,2010,the quantitative relationship between horizontal vorticity and vertical velocity during the development of squall line is studied emphatically.The following conclusions are drawn.First of all,the present work provides a novel method for calculating vertical velocity based on continuity equations in a pressure coordinate system.The method overcomes the disadvantage of accumulation of calculating errors of horizontal divergence in current kinematics method during the integration for calculating the vertical velocity,and consequently avoids the subsequent correction of the vertical velocity.At the same time,through the deformation of the continuity equations,it can be obtained that the rotation intensity of the horizontal vorticity vector caused by the horizontal wind shear vertically is proportional to-?.On condition that co fluctuates in the vertical direction,the updraft occurs when the vector of horizontal vorticity rotates counterclockwise;the downdraft occurs when rotating clockwise.The validation of the vertical velocities in the development of the squall line using the numerical simulation results shows that the method in this paper is overall better than the ? equation method using the wet Q-vector divergence as the forcing term and the kinematics method modified by the O'Brien method.The plus-minus sign of the vertical velocity obtained from this method is not correlated with the intensity of reflectivity,but the absolute error has an increase when reflectivity is>=40.This method demonstrates that it is a good reflection of the direction and intensity of the vertical velocity.Secondly,the horizontal vorticity local change equation used in this study was obtained using the equations of motion in the P coordinate system without considering friction,which reveals the relationship with vertical shear.By diagnostically analyzing each term in the horizontal vorticity equation during a squall line process occurred on 19 June 2010 and the relationship between horizontal vorticity and vertical velocity,we found that the contribution of the rotation intensity of the non-thermal wind term to the local changes of the vertical motion is low in the low level,but the contribution increases with the increase of the height.The contribution of the rotation intensity of the vertical transport was the largest in the low level,but had negative contribution in the high level.These features were most evident in the development and maturity stage of the squall line.Based on analysis of convection cells along a squall line,we found that,in the process of cell development,diabatic heating caused subsidence of constant potential temperature surface and non-geostrophic motion,which then triggered stronge convergence of horizontal acceleration in the mid-level atmosphere and divergence of horizontal acceleration in the upper-level atmosphere.These changes of horizontal wind field could cause a counterclockwise increment of the horizontal vorticity around the warm cell,which then generated an upward motion increment.This was the main reason why the non-thermal wind term had the largest contribution to strengthening the upward motion in the mid-and upper-level atmosphere.The vertical transport of large value of horizontal vorticity was the key to trigger convection.At last,based on the simulation results,the relationship of the MVs and the rear inflow in the squall with the surface high winds are analyzed.The Vorticity Budget and the analysis of vortex line show that there are east-west vorticity couplet in the bow echo,which are mainly caused by the divergence term,and the middle relatively weak anticyclone vorticity is generated due to the conversion of horizontal vorticity to vertical vorticity caused by the arch of vortex line.The contribution of vorticity couplet to the high winds on the ground is quantified by calculating cyclostrophic wind caused by the vorticity couplet.The results show that the cyclostrophic winds account for 40%-50%of the ground winds in this squall line process.When the cyclostrophic winds are removed,the intensity of the surface high winds will be weakened and the position will be shifted.It can be seen from the three-dimensional streamline that the subsidence of the rear inflow is another factor that produces the surface high winds.Through the diagnosis of the buoyancy acceleration and dynamic acceleration,it is found that the sink of rear inflow jet is caused by the negative buoyancy acceleration caused by the drag of the solid water condensate and the precipitation.