The Influence of Mountains on the Vorticity Sources of the Southwest Vortex

• 1、Department of Atmospheric Sciences, Nanjing University

Abstract：Southwest vortex (SWV) is the most important perturbations due to the large scale orographies in China. The michanism of SWV formation and their movement are studied using mesoscale numerical model ARPS. The main parts of China topography, which are very important for the formation of SWV, are simplified as three parts, the Tibetan plateau, the Hengduan cordillera and the Sichuan basin. Ideal constant-stability flows with different wind speed and/or direction are applied to the real and the simplified topography. The simulations show that: There are three vorticity sources contributing to the formation of the SWV, one is along the steep slope on the conjoint region between Tibetan plateau and Sichuan basin (I), one is tailing from the southeast edge of Hengduan cordillera (II), and the third is from the lee trough near the northeast side of Tibetan plateau (III). The distribution and importance of the three sources are corresponding to the climatic statistical regions where SWV formed in high frequency. The contribution of part I is direct and quick, where the Sichuan basin attracts and collects the vorticity around, and finally form a very strong vortex, also it prevents the vortex from moving out the basin till the vortex is violent enough. Part II is main and important for the early-stage movement of SWV. Part III is not obvious as part I and part III, but the lee trough is significant for the subsequent movement and development of the SWV. It is the best condition for the formation of SWV when Sichuan basic locaded leeside of Tibetan plateau and Hengduan cordillera. Both southwest and nothwest wind do not favor the formation of SWV while stronger west flow can make the vorticity inside the SWV a more complex pattern and reduce the stability of the vortex, thus, it willl drift away from Sichuan basin more easily.

Keywords： southwest vortex, the Tibetan plateau, the Hengduan cordillera, the Sichuan basin, vorticity source, movement track