低空急流形成发展的一种可能机制——重力波的惯性不稳定

A POSSIBLE DEVELOPING MECHANISM OF LOW LEVEL JET——THE INERTIAL INSTABILITY OF GRAVITY WAVE

当斜压大气在高空急流轴附近满足条件 f(f- u/y) <0时 ,非地转运动激发出的重力惯性波将得到进一步的发展。此时 ,斜压大气的地转适应过程无法实现 ,非热成风和垂直环流之间将发生正反馈作用 ,负的非热成风将激发并加强南部上升北部下沉的垂直环流 ,垂直上升流的加强将导致低层低压系统的发展和低层流场的辐合 ,使得低层低压系统南侧的气压梯度力增大 ,结果在辐合区南侧形成低空急流。此外 ,非热成风的分布对垂直环流和低空急流的形成发展也具有非常重要的作用。

In this paper, the relationship of low level jet with gravity waves generated near upper level jet is investigated by using an extended normal mode method. The analytic solutions indicate that the gravity waves stirred by ageostrophic disturbances near upper level jet will develop as the baroclinic atmosphere satisfies the inertial unstable condition . It is impossible to complete baroclinic geostrophic adjustment under the condition, this unbalance process leads to the positive feedback interaction between non thermal wind and vertical motion, and negative non thermal wind stirs and consolidates thermally direct circulation (i.e. descending cold air and rising warm air). For the negative non thermal wind under the entrance of upper level jet, the developing of vertical ascending motion leads to deepen the low and intensify the convergence of low level atmosphere, and indirectly increases the pressure gradient force in the southern part of the low. As a result, intensifying pressure gradient force causes large wind speeds, and finally the low level jet develops in the southern part of the vertical ascending area. In addition, the vertical and horizontal distributions of non thermal wind are also very important to the development of vertical circulation and low level jet. For a given weak non thermal wind distribution, the result indicates that a large wind speed center located in the southern part of the ascending area emerges in low level atmosphere after several hours.