地转动量近似下的Q矢量及其在天气图上的应用

Geostrophic momentum approximation Q-vector and its application in synoptic charts

将Hoskins等提出的准地转Q矢量表达式推广到地转动量近似下的Q矢量表达式,并讨论了如何在天气图上用地转动量近似下的Q矢量表达式定性估计垂直运动方向的方法.地转动量近似下的Q矢量与准地转Q矢量相比,表达式中多了与位势高度极值和风速极值相关的两项,分别将地转动量近似下的Q矢量表达式中的每一项写成在天气图上便于判断的表达式.结果表明,在天气图上可以通过以下三种方式定性判断垂直运动方向:一是当等高线呈波状槽脊形式及通过急流的入口和出口区时,使用地转动量近似下的Q矢量的第一项(即准地转Q矢量)加以判断;二是当存在闭合气压系统且等高线与风向有交角时,使用地转动量近似下的Q矢量的第二项加以判断;三是在急流核上使用地转动量近似下的Q矢量的第三项加以判断.

In this paper, we extend the expression of quasi-geostrophic Q-vector brought forward by Hoskins et al. into the Q-vector equation under the condition of geostrophic momentum approximation, and discuss the method of how to quantitatively estimate vertical motion in synoptic charts with Q-vector expression under geostrophic momentum ap-proximation. Compared with the expression of quasi-geostrophic Q-vector, the Q-vector expression under geostrophic momentum approximation has two more terms which are related to geopotential height extremum and wind velocity extremum. Then, each term of Q-vector under geostrophic momentum approximation is transformed into the expres-sion which is conveniently judged in synoptic charts. The results show that to judge vertical motions in synoptic chart quantitatively, one of the three ways can be used： when contour line presents the form of wave-like trough-ridge and passes through the entrance and exit areas of jet stream, correspondingly use the 1st term of Q-vector under geostrophic momentum approximation （i.e., quasi-geostrophic Q-vector）; when there exist a closed pressure system and an angle between contour line and wind velocity, correspondingly use the 2nd term of Q-vector under geostrophic momentum approximation; at the jet streak, correspondingly use the 3rd term of Q-vector under geostrophic momentum approxi-mation.