高空急流对青藏高原切变线影响的数值试验与动力诊断

Numerical experiments and dynamic diagnosis of the upper-level jet stream impact on the Tibetan Plateau shear line

利用NCEP 1°×1°FNL分析资料和中尺度数值模式WRF对一次青藏高原(简称高原)切变线过程进行了数值试验,主要研究高空急流强度对高原切变线的影响,并结合ω方程分析了影响高原切变线上垂直上升运动的若干因子。研究得出高空急流的强度对低层风场有重要影响,急流增强会使高原切变线上的风切变增大,切变线变长,同时高空急流强度的增强也有利于高原切变线上水汽的辐合。高空急流可通过影响高层辐散、低层辐合的散度场垂直配置对高原切变线上的正涡度柱与辐合上升运动产生作用。ω方程的诊断分析表明,温度平流的拉普拉斯项对高原切变线上的垂直上升运动起主导作用,低层暖平流有利于切变线上产生上升运动。高空急流强度的变化对差动涡度平流项的影响要大于温度平流拉普拉斯项,高空急流强度的增强会放大差动涡度平流项和温度平流项的正贡献,从而更加有利于上升运动及高原切变线的维持。

Using the NCEP 1°×1°FNL analysis data and the mesoscale model WRF,numerical experiments have been conducted to simulate a Tibetan Plateau shear line process.The study focuses on the impact of the upper-level jet stream intensity on the Tibetan Plateau shear line.In combination with theωequation,factors affecting the ascending motion in the Tibetan Plateau shear line are also analyzed.The results show that the intensity of the upper-level jet stream has an important influence on low-level wind fields.The increases in jet stream will intensify the wind shear along the Tibetan Plateau shear line,and the shear line will become longer.Meanwhile,the enhancement of the upper-level jet intensity is also favorable for the convergence of water vapor along the Tibetan Plateau shear line.The upper jet can affect positive vorticity,convergence and ascending motion along the Tibetan Plateau shear line by influencing the vertical configuration of lower-level convergence and upper-level divergence.The diagnostic analysis of theωequation shows that the Laplace term of temperature advection plays a dominant role in the ascending motion along the Tibetan Plateau shear line,and the low-level warm advection is favorable for the development of ascending motion along the shear line.The influence of the variation of upper-level jet stream intensity on the differential term of the vorticity advection is greater than that of the Laplace term of temperature advection,and the enhancement of the upper-level jet intensity will enlarge the positive contributions of the differential vorticity advection term and the temperature advection term,which is more favorable for the development of ascending motion and the maintenance of the Tibetan Plateau shear line.