Diagnostic analysis of the evolution mechanism for a vortex over the Tibetan Plateau in June 2008

Based on the final analyses data (FNL) of the Global Forecasting System of the NCEP and the observational radiosonde data, the evolution mechanism of an eastward-moving low-level vortex over the Tibetan Plateau in June 2008 was analyzed. The results show that the formation of the vortex was related to the convergence between the northwesterly over the central Tibetan Plateau from the westerly zone and the southerly from the Bay of Bengal at 500 hPa, and also to the divergence associated with the entrance region of the upper westerly jet at 200 hPa. Their dynamic effects were favorable for ascending motion and forming the vortex over the Tibetan Plateau. Furthermore, the effect of the atmospheric heat source (Q (1)) is discussed based on a transformed potential vorticity (PV) tendency equation. By calculating the PV budgets, we showed that Q (1) had a great influence on the intensity and moving direction of the vortex. In the developing stage of the vortex, the heating of the vertically integrated Q (1) was centered to the east of the vortex center at 500 hPa, increasing PV tendency to the east of the vortex. As a result, the vortex strengthened and moved eastward through the vertically uneven distribution of Q (1). In the decaying stage, the horizontally uneven heating of Q (1) at 500 hPa weakened the vortex through causing the vortex tubes around the vortex to slant and redistributing the vertical vorticity field.