Dynamical and thermal problems in vortex development and movement. Part I: A PV-Q view

Abstract

Based on the Lagrangian change equation of vertical vorticity deduced from the equation of threedimensional Ertel potential vorticity (PVe), the development and movement of vortex are investigated from the view of potential vorticity and diabatic heating (PV-Q). It is demonstrated that the asymmetric distribution in the vortex of the non-uniform diabatic heating in both vertical and horizontal can lead to the vortex’s development and movement. The theoretical results are used to analyze the development and movement of a Tibetan Plateau (TP) vortex (TPV), which appeared over the TP, then slid down and moved eastward in late July 2008, resulting in heavy rainfall in Sichuan Province and along the middle and lower reaches of the Yangtze River. The relative contributions to the vertical vorticity development of the TPV are decomposed into three parts: the diabatic heating, the change in horizontal component of PVe (defined as PV2), and the change in static stability θ z . The results show that in most cases, diabatic heating plays a leading role, followed by the change in PV2, while the change of θ z usually has a negative impact in a stable atmosphere when the atmosphere becomes more stable, and has a positive contribution when the atmosphere approaches neutral stratification. The intensification of the TPV from 0600 to 1200 UTC 22 July 2008 is mainly due to the diabatic heating associated with the precipitation on the eastern side of the TPV when it uplifted on the up-slope of the northeastern edge of the Sichuan basin. The vertical gradient of diabatic heating makes positive (negative) PVe generation below (above) the maximum of diabatic heating; the positive PVe generation not only intensifies the low-level vortex but also enhances the vertical extent of the vortex as it uplifts. The change in PVe due to the horizontal gradient of diabatic heating depends on the vertical shear of horizontal wind that passes through the center of diabatic heating. The horizontal gradient of diabatic heating makes positive (negative) PVe generation on the right (left) side of the vertical shear of horizontal wind. The positive PVe generation on the right side of the vertical shear of horizontal wind not only intensifies the local vertical vorticity but also affects direction of movement of the TPV. These diagnostic results are in good agreement with the theoretic results developed from the PV-Q view.