A Study on the Dynamic Mechanism of the Formation of Mesoscale Vortex in Col Field

The mesoscale vortex associated with a mesoscale low-level jet (mLLJ) usually causes heavy rainfall in the col field. The col field is defined as a region between two highs and two lows, with the isobaric surface similar to a col. Using a two-dimensional shallow water model, the meso- scale vortex couplets (MVCs) induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated. With the sizes of ～100 km, the MVCs induced by northerly perturbation (NP) and southerly perturbation (SP) moved toward the col point. The sizes of MVCs induced by southwesterly perturbation (SWP), southeasterly perturbation (SEP), northwesterly perturbation (NWP), and northeasterly perturbation (NEP) were relatively small for the perturbations moving toward dilatation axis. The MVC induced by easterly perturbation (EP) and westerly perturbation (WP) could not develop because they quickly moved away from the col point, before the circulation could form. The size of the circulation was determined by the distance between the vortex and the col point. The closer to the col point the vortex was, the larger the size of vortex. The comparisons of maximum vorticity and vorticity root mean square error (RMSE) of the NP, the SWP, and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations. Therefore, the weak environment of the col field favors the maintenance of vorticity and the formation of vortex. When a mesoscale vortex forms near the col point or moves toward the col point, it may maintain a quasi-stationary state in the stable col field.