Theoretical Analysis And Numerical Study On The Development Mechanism Of Squall Line In The Northeast Cold Vortex

Based on the dynamic frame of CM1 model,the main factors affecting the occurrence and development of severe convective weathers are obtained by deducing the water vapor equation.A typical squall line process under the background of Northeast Cold Vortex(NECV)is selected and the numerical simulation is carried out.Through the scale analysis of model data,the main factors affecting the occurrence and development of squall line are obtained.Through the contrast test of cold vortex stations in different regions and different initial fields,the theoretical results are verified and the conditions for the formation of a squall line under the NECV are obtained.Different cloud microphysical parameter schemes are adopted for simulation,the structure and evolution process of squall line are investigated from the distribution and transformation of water matter particles,and the main process of squall line occurrence and development is obtained.It is found that advection,convection and hydrometeor precipitation have the greatest influence on the change of moisture variables in the squall line system,the influence of water vapor phase change is the secondary,and the turbulence effect is relatively small.In the southwestern side of the NECV,upper levels are affected by cold air,which,when combined with warm advection in lower levels,leads to the formation of unstable stratification.The unstable stratification and transverse gradient pattern of humidity field can jointly lead to the formation of the squall line.Under the influence of water vapor concentration and temperature,strong radar echoes can easily form in the middle and upper levels with strong updrafts.Under the influence of precipitation,strong radar echoes can also be found in lower levels.The increase of horizontal grid spacing mainly slows down the evolution of the system,and the strength of system is also weakened.The decrease of unstable energy of the initial field will cause the squall line to weaken obviously and the squall line can not be generated when the unstable energy is reduced to 0.The vertical wind shear has little effect on the formation of the squall line,which mainly changes the structure of the squall line,and when there is no vertical wind shear in the initial field,the squall line is more loosely in structure.The transformation of water particles in different cloud microphysical parameter schemes are different,which affect the content and distribution of water particles in the squall line and further affect the precipitation.Before the formation of the squall line structure,the cloud water content increased rapidly.After the squall line take shape,a large amount of cloud water is transformed into rain,hail and graupel.After the squall line reached the mature stage,the proportion of water substance particles tended to be stable.