Numerical Sensitivity Study Of Terrain Mechanism In The Process Of Rainstorm Triggering In Tianshan Are

The WRF V4.1 model with a high resolution was used to numerically simulated the heavy rainstorm process that occurred on 9 to 10 September 2019 near the Tianshan mountain south to Ili river valley in Xinjiang.By using this simulation,topographic sensitivity tests were performed to study the mechanism of topographic disturbance during the occurrence of the rainstorm and the key physical process which played a important role in the occurrence topographic rainstorm from aspects of moisture condition,instability condition and vertical movement.The results show as follows:(1)The conceptual model of the rainstorm in Tianshan mountains: Tianshan mountains’ blocking led to the meso-scale wind convergence so that the vapor converged before the rainstorm occurred.The vertical movement was mainly caused by marked meridional pressure gradient force coupled vertical shear of meridional wind.The effect of meridional divergence took the second place.The vertical shear of meridional flow-over wind also affected the meridional gradient of generalized potential temperature,which changed the vertical gradient of generalized potential temperature.It resulted in the development of potential stability,and the rainstorm occurred at last.(2)As for the rainstorm in Tianshan mountains,the moisture flux divergence was mainly caused by the convergence of mesoscale wind induced by topographic disturbance.The topographic disturbance of Tianshan mountains could result in obvious convergence of mesoscale wind so that the area of moisture converged would be expanded.The area would apparently reduce when the scale of filtered topographic disturbance up to 384 km.(3)Vertical shear of meridional flow-over and meridional gradient of generalized potential temperature were changed by topographic disturbance to influence the tendency of potential stability during this rainstorm.During the trigger phase,vertical shear of meridional flow-over coupled meridional gradient of generalized potential temperature term was the dominant term of the terrain-following tendency equation of potential stability.It meant that the vertical shear of meridional flow-over would act on the meridional gradient of generalized potential temperature and affect the potential stability.It was getting weaker and weaker with the development of the rainstorm while vertical wind shear coupled vertical gradient of generalized potential temperature term’s contribution was getting more and more.After filtering the topographic disturbance,the influence of vertical shear of zonal flow-over coupled zonal gradient of generalized potential temperature term on the tendency of potential stability increased.(4)The influence of topographic disturbance on the occurrence of vertical movement during this rainstorm was mainly realized by changing the function of meridional pressure gradient force coupled meridional divergence.The topographic disturbance could observably change pressure gradient force,vertical shear of meridional wind and meridional divergence.The function of meridional pressure gradient force coupled vertical shear of meridional wind got weak,and the diabatic heating process tended to be a significant physical process of leading to vertical movement when the scale of filtered topographic disturbance up to 384 km.(5)The meridional divergence was a key factor of changing vertical velocity by topographic disturbance during this rainstorm.The surface pressure would be changed by meridional convergence,which would further change the dry air column perturbance term and affect the tendency of vertical velocity at last.