Impact Of Qinling-Bashan Mountains On The Movement And Development Of Southwest Vortex

Using WRF mesoscale model,high resolution numerical simulation of a northeastward Southwest vortex rainstorm process happened on August 9-12,2014 was conducted.On the basis of successful simulation,six topographic experiments about Qinling-Bashan Mountains were designed to study the impact of topography on the movement and development of SWV and rainstorm.Impact of topography on vortex was analysed mainly from the structure features,maintenance,movement,and development.Impact of topography on rainstorm was analysed mainly from the precipitation features,water vapor conditions,vertical motion and development of instability.The main conclusion can be summarized as follows:(1)SWV is a weak shallow system in its primary stage and the vorticity axis tilts westward with height.There is a weak cold region below 850 h Pa and significant warm wet structure in middle layers with the axis tilting norwestward with height.Qinling-Bashan Mountains have no effects on the structure features of vortex at this moment.It's a deep intensely developing system in SWV's mature stage and the vorticity axis is nearly vertical.The divergence in upper layers is greater than the convergence in lower layers.There is warm heart structure in middle-upper layers and the warm center extends from the surface to the upper.The thermodynamic structure of vortex is uneven at this moment.After elevation of the topography,the vortex became deep and convergence ascending motion increased.And the baroclinity is relatively weak.Conversely,with the decreasing of terrain height,the vortex became shallow and convergence ascending motion decreased.And the baroclinity became stronger.SWV became a shallow baroclinic shallow system in its weakening stage.The vorticity value became negative above 500 h Pa.There is a divergence region from the ground to 800 h Pa and descending motion below 850 h Pa.It's not conducive to the vortex's development because the lower layers have been controlled by the cold air from north and the warm wet air has been lifted.After elevation of the topography,the vortex became relatively stronger and it is still convergence in lower layers and divergence in upper layers.It's still warm wet structure and the baroclinity is not strong.Conversely,with the decreasing of terrain height,it's divergence region below 800 h Pa and the descending motion in lower layers became stronger.The warm structure disappeared so that the vortex weakened faster.(2)Qinling-Bashan Mountains have an important effect on maintenance of the complete form of SWV and only moderate terrain is conducive to the complete form of vortex.The topography has an effect on vortex's pathway by the blocking and friction effects on vortex and the meridional airflows of both sides of the mountains.With the increasing of terrain height,the pathway moved southwestward and the moving speed was relatively slow.The vortex maintained stably so that it has not moved out of northeastern Sichuan in 48 h.With the decreasing of terrain height,the pathway moved northeastward and the speed was rather fast so that it has moved to western Hubei in 48 h.Under the impact of Qinling-Bashan Mountains,stretching of positive vorticity caused by convergence of airflows on windward slope and vertical transport of positive vorticity caused by orographic uplift on windward slope have played the dominant roles in vortex strengthening in the process of SWV's strengthening and development.The horizontal transport and twisting of vorticity is also affected by the topography but the contribution is much less.Furthermore,SVD caused by isentropes stiffly tending to the topography is also the important reason of vortex strengthening.(3)The rainfall process of SWV is accompanied by several strong mesoscale convective activities that strengthened in nights on August 9,10 and weakened in days.The topography's amplitude effect to the precipitation is significant.With the increasing of terrain height,rainfall began to increase and rain belts moved southwestward.Conversely,with the decreasing of terrain height,rainfall began to decrease and rain belts moved northeastward.(4)A great deal of water vapor from southwest gathered in Sichuan Basin is hard to transport to the northeast because of the blocking effect of topography,forming the strong convergence of the water vapor flux on windward slope and the location and intensity of rainbelts are consistent with the convergence region of water vapor flux.Vertical motion forced by the orography is rather strong on windward slope.Orographic uplift plays the dominant role while frication convergence of the boundary layer also takes a considerable proportion.Vertical velocity forced by orography and 6h accumulated rainfall increased with the terrain height.The rainfall has two peaks and the heavy rain belt lies to the south of the ascending motion center.The topography has made the warm wet high-energy air from south and the dry cold air from north accumulate on both sides of the mountains so that the instability energy has not been released immediately but stored temporarily.This made the isentropes so stiff to develop the CSI intensely,forming the inclined convection which can trigger the rainstorm.The latent heat release from convective rainfall in turn increased the horizontal moist baroclinity and decreased the convective stability so as to promote the development of positive vorticity again.The positive feedback mechanism like this could be the important reason of the vortex and rainstorm sustainably developing and strengthening on windward slope.(5)The rainfall process is dominated by inertial instability,convective instability and symmetric instability.The heavy rain belt lies in a warm wet environment where is the junction region of convective instability and symmetric instability.Convective instability plays a dominant role above the heavy rain belt which is close to the convective instability region and there exists inertial instability in middle-upper layers to strengthen the convection while moist symmetric instability plays a dominant role above the heavy rain belt which is close to the symmetric instability region and there exists weak convective instability in middle-upper layers to develop the circulation together with moist symmetric instability in lower layers.