Impact Of Vertical And Horizontal Turbulent Mixing On Simulation Of Two Types Of Precipitation In Yangtze-Huaihe Region

Aiming at the problem that the impact of turbulence mixing intensity on small scale precipitation needs to be further studied,YSU parameterization scheme and 2D Smagorinsky horizontal turbulence diffusion model(combination scheme)in mesoscale numerical model WRF3.8 were used,and a local thermal convective precipitation and a shear line precipitation in Yangtze-Huaihe region were simulated to discuss the simulation effects of different turbulent mixing intensity for vertical and for horizontal on two types of precipitation by adjusting the turbulent mixing intensity for vertical and horizontal.Then,closed the boundary layer parameterization scheme,and only the 3D Smagorinsky turbulent diffusion model was used to compare with the combination scheme with better simulation to explore the difference between them in the simulation of two types of precipitation.The results show that:The initiation and evolution of convection are improved either weakening vertical mixing or horizontal mixing in the local thermal convective precipitation simulation,which makes the results closer to the observation,while it is completely impossible to simulate the thermal convection process by increasing the horizontal mixing intensity.Reducing vertical mixing,the height of the boundary layer decreases obviously,in where the air becomes colder,which is conductive to get larger energy,at the meanwhile,the wind speed and vertical wind shear increase,and the turbulent dissipation weakens,these changes are conducive to trigger convection earlier.It is completely opposite by increasing the horizontal mixing intensity.Combination scheme with smaller mixing intensity for vertical and horizontal get a result closest to observation,whose quantitative evaluation value is best.When simulated the shear line precipitation,the vertical motion and momentum exchange in the vertical direction are enhanced after increasing the vertical mixing,result in upward growth of the convection storms and stronger intensity.With the increase of horizontal mixing,the horizontal momentum is significantly enhanced,the large momentum and vertical motion are concentrated inside of the storms,and the vorticity and divergence center are broader and more concentrated,forming a clustered cyclonic with convergence lower and divergence upper,which makes the storm grow laterally,more organized,and the upper cloud develops wider.Enhancing horizontal and vertical mixing at the same time synthesizes the advantages of both,however,due to the limited water vapor and the dynamic effect,the storms cannot continuously grow.Compared with the combined scheme,3D Smagorinsky scheme failed to simulate the thermal convective precipitation,because it failed to simulate the vertical distribution of potential temperature,humidity,horizontal wind field and the turbulent transport process in the atmosphere reasonably.Both schemes can simulate the rainbelt position well in the shear line precipitation simulation,the three-dimensional scheme simulate a broader range of stratospheric precipitation,while the central falling of mass grade precipitation simulated by the combination scheme whose mixing intensity for vertical and horizontal were increased is closer to the observation,and the quantitative evaluation shows that the combination scheme is better.Further analysis indicates that the three-dimensional scheme simulates stronger vertical motion,vertical water and heat flux transport and the vorticity and divergence distribution polycentric,which are conducive to the upward development and to excited new convective cells continuously,but not obvious cyclonic structure,and then simulates more radar echo cylinders,which generates a broader cloud coverage and precipitation.The combination scheme shows stronger lateral development,more organized because more concentrated flux transportation for heat and water and obvious cyclonic dynamic structure.In conclusion,combination scheme and simultaneously weakening the horizontal and vertical mixing intensity is more suitable for local thermal convective precipitation under the kilometer scale(it is the better when p increases 0.5-1.0 for vertical and C_sdecreases 0.125 for horizontal).In the simulation of shear line precipitation,the combination scheme with enhanced mixing intensity improve the accuracy of mass grade precipitation(it is greater when C_s belong 0.5-1.0),while the three-dimensional turbulence scheme simulated broader stratospheric precipitation region,so it is necessary to increase horizontal turbulent mixing in medium scale convection.