| Abstract: | An explosive extratropical cyclone (EC) over the Eastern Asian region that caused two shipwrecks is analyzed using ERA-Interim reanalysis data from the European Center for Medium-Range Weather Forecasts. Analyses of the evolution of the EC reveal that the positive potential vorticity (PV) at the upper-tropospheric level displays a hook-shaped structure during the mature period of the cyclone. The PV distribution forms a vertically coherent PV structure called a PV tower. The vertical distribution of the PV can induce and strengthen cyclonic circulation from the lower- to upper-levels of troposphere, which is an important deepening mechanism of explosive cyclone. The PV tower occurs approximately ten hours prior to the development of surface occlusion in the cyclone. The evolution of surface fronts closely follows the development of the horizontal upper-tropospheric PV. This tandem de-velopment is largely attributed to the ability of the positive upper-tropospheric PV and the PV tower to induce cyclonic circulation simultaneously. The kinematic wrap-up process of cyclonic circulation also accelerates the formation of warm occlusion. A concep-tual model of the distributions of positive PV and potential temperature combining the perspectives of dynamic tropopause folding, PV tower, and atmospheric stability, including westward tilting and baroclinicity, is proposed. This model can illustrate the explosive deepening mechanism of ECs. The regions of convective instability and rainfall determined by this model are consistent with those identified from the actual observation. |
