“96.1”高原暴雪过程湿对称不稳定的数值研究

NUMERICAL STUDY OF CONDITIONAL SYMMETRIC INSTABILITY OF “96.1” SNOWSTORM

利用一次较成功地模拟了“96 .1”青藏高原东北部暴雪过程的MM4中尺度模式输出资料 ,根据条件对称不稳定 (CSI)的非线性理论判据对这次暴雪过程进行了诊断 ,结果表明 ,CSI是“96 .1”暴雪发生和发展的一种动力机制。在诊断的基础上 ,用非线性对称不稳定和非纬向非平行基流对称不稳定模式对“96 .1”暴雪发生发展过程的动力学机制进行了数值试验 ,结果指出 :用两种方法进行数值模拟时 ,都会出现对称环流 ,不仅Ψ′场和w′场彼此协调 ,而且与观测分析的结果大体一致 ,进一步揭示了对称不稳定确实是这次暴雪发生发展的一个动力学机制 ,尽管非线性模拟中的上升支位置在移动过程中落后于切变线的位置。

With the criteria of nonlinear symmetric instability theory, the mechanism of the heavy snowstorm of 15～17 January 1996 (“96.1”) occurred in the northeast region of the Qinghai Xizang Plateau is diagnosed by using the output data of a MM4 mesoscale numerical model which fairly well simulated the developing structure and the evolution of the “96.1” snowstorm, it suggested that the CSI is one of the mechanisms of triggering and organizing the shear line on which the snowstorm formed. Based on the diagnosis, numerical simulation experiments are done by nonlinear, non zonally and non parallel basic flow CSI models to further validate the possible effect of CSI on the formation of the snowstorm, the results showed that the circulations of symmetric perturbation fields emerged in each model, not only the fields of vertical velocity are coincident with that of streamfunction, but also with the observations, the results in the CSI is substantially a formative mechanism for the occurrence of the “96.1” snowstorm, though the vertical velocity fields are behind the shear line during the evolution.