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

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 ageost rophic 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.