DYNAMIC DIAGNOSES OF THE GENESIS AND DEVELOPMENT FOR THE MESOSCALE SHEAR LINE DURING “96.1” SNOWSTORM. I: DIAGNOSES OF VORTICITY AND VORTICITY VARIABILITY

The structure of the genesis and development of “96.1” plateau snowstorm shear line was diagnosed in kinematics and dynamics using the output data of a mesoscale model (MM4), in which a three phase cloud explicit precipitation scheme has been introduced. The evolution of vorticity field showed that the genesis and development of the local vorticity center or zone not only was directly related to a snowstorm shear line, also predicted its genesis in advance. The vertical section structures of vorticity, divergence, vertical velocity and equivalent potential temperature showed that the inter allocation and coupling relationship of kinetic and thermodynamic fields were very favorable to the development of snowstorm shear line and the genesis and persistence of the snowstorm. Diagnoses of the vorticity variability revealed that the positive vorticity variability center originated near the snowstorm shear line, its space time evolution was basically consistent with that of the positive vorticity which was associated with the genesis and development of shear line. In the factors of contributing to the vorticity variability the relative numeric of nonlinear interaction vorticity variability was the most component; the time mean vorticity variability was less important; the contribution of the perturbation vorticity variability related with strong disturbed flow was small. In the forcing terms of the total vorticity variability, the divergence term contribute most; less was the horizontal vorticity advection term; the vertical vorticity transport and the twisting term played the opposite role, and roughly offset.