Circulation Background and Genesis Mechanism of a Cold Vortex over the Tibetan Plateau during Late April 2018

A cold vortex occurred over the northeastern Tibetan Plateau (TP) on 27 April 2018 and subsequently brought excessive rainfall to the spring farming area in southern China when moving eastward. This study investigates the genesis mechanism of the cold TP vortex (TPV) by diagnosing reanalysis data and conducting numerical experiments. Results demonstrate that the cold TPV was generated in a highly baroclinic environment with significant contributions of positive potential vorticity (PV) forcing from the tropopause and diurnal thermodynamic impact from the surface. As a positive PV anomaly in the lower stratosphere moved towards the TP, the PV forcing at the tropopause pushed the tropospheric isentropic surfaces upward, forming isentropic-isplacement ascent and reducing static stability over the TP. The descent of the tropopause over the TP also produced a tropopause folding over the northeastern TP associated with a narrow high-PV column intruding downwards over the TPV genesis site, resulting in ascending air in the free atmosphere. This, in conjunction with the descending air in the valley area during the night, produced air stretching just at the TPV genesis site. Because the surface cooling at night increased the surface static stability, the aforementioned vertical air-stretching thus converted the produced static stability to vertical vorticity. Consequently, the cold TPV was generated over the valley at night.