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The col field (a region between two lows and two highs in the isobaric surface) is a common pattern leading to the generation of mesoscale vortex and heavy rainfall in China. The mesoscale vortex usually forms near the col point and the dilatation axis of the col field in the low-level troposphere.The Mesoscale model WRF was used to numerically simulate a rainfall process in col field. A temperature perturbation column (TPC) was introduced into the low-level col field near the col point, and the effects of TPC on mesoscale vortex and rainfall was analyzed.It was shown that in the region of strong wind background, the TPC moves downstream and has little effect on the environment, while near the col point, the wind speed and the vertical wind shear are small, the TPC can stay in the col field for a long time, which can have a greater impact on the environment. The strong TPC near the col point can trigger the vortex. As the temperature of the air column increases, the pressure drops, leading to the low-level convergence and the upper-level divergence, and the low-level cyclonic vorticity form under the effect of ageostrophic winds, which is favor of the formation of mesoscale vortex in the weak wind field. The formation of vortex promotes the intensification of precipitation. The release of the latent heat of the condensation induced by the TPC makes a positive feedback for the mesoscale vortex. The southwestly low-level jet enhances through the thermodynamic action, resulting in convergence of the leeward low-level jet and increase of precipitation, and divergence of the upwind low-level jet and decrease of precipitation, respectively. The col field is a favorable circumstance for the formation of mesoscale vortex.Acknowledgements. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41975128 and 41275099). |
