Numerical Simulation on the Formation of Mesoscale Vortex in Col Field

There exist typically two kinds of low-level col fields over the middle and lower reaches of the Yangtze River of China during summer. One is associated with the mesoscale vortex embedded in the Meiyu front; the other is related to tropical cyclones making landfall over eastern or southern China. The first one is the focus of this study. The meso-β scale vortex (MβV) causing heavy rainfall usually forms in a col field or within a shear line associated with the mesoscale low-level jet (mLLJ). The MβV, triggered by mesoscale wind perturbation in a col field, is simulated by using a three-dimensional η-coordinate mesoscale model. This col field represents the circumstance of the “98.7” heavy rainfall event over eastern Hubei Province. The results show that the MβV triggered by wind perturbation was weak and maintained only several hours if the latent heat feedback was switched off. The wind perturbation also weakened rapidly. However, when the latent heat feedback was included, precipitation became more intense and the mLLJ and MβV quickly developed. The MβV maintained quasi-stationary during its life cycle under the stable col field. The MβV triggered by the southwesterly perturbation was located closely to that by the northeasterly perturbation. They were both located in the weak wind region near the col point. The stronger the perturbation was, the more intense and longer the dynamic MβV lived. The 24-h accumulated precipitation in different experiments showed a similar pattern, which indicates that the relatively stable intensity and range of precipitation were the intrinsic characters of the stable col field. Furthermore, it is found that mesoscale perturbations had some impacts on the location and intensity of the rainfall. The fluctuation of large-scale LLJ to the south of the col field might produce a perturbation, causing instable stratification and rainfall within the low-level col field or shear line. The mLLJ near the rainfall was enhanced due to the latent heat feedback and then caused formation of the MβV. The stable col field provided favorable thermodynamic conditions for the formation and development of the MβV. Therefore, it is an “incubator” for MβVs and the associated consecutive heavy rainfall.