Accelerating two-stage explosive development of an extratropical cyclone over the northwestern Pacific Ocean: a piecewise potential vorticity diagnosis

Abstract

An extreme explosive extratropical cyclone over the northwestern Pacific Ocean (NPO) that formed in winter 2004 and went through two distinct rapid deepening periods was successfully simulated by a non-hydrostatic mesoscale model (MM5). Based on the simulation, the cyclone's rapid deepening was investigated in detail using the piecewise potential vorticity (PV) inversion method which successfully captured the characteristics of the cyclone and its associated background circulations. Results indicated that explosive development of the cyclone was dominated by forcings in the extended surface layer (ESL), which were closely related to baroclinity (temperature advection) and boundary layer processes (sensible heat exchange). In the interior layer (IL), direct effects of condensation were mainly conducive to the cyclone's development, whereas indirect effects (interactions with other layers) mainly acted conversely. Processes associated with latent heat release (LHR) were characterised by nonlinearity. Features of the precipitation, including intensity, duration, range and relative configuration to the cyclone determined the influences of condensation on the cyclone. In the upper layer (UL), tropopause-folding processes and horizontal PV advection were main influencing factors to the evolution of the cyclone. Upper-level forcings firstly exerted slight effects on the cyclone's development, since upper-level positive PV anomalies were far from the cyclone; then, as the influencing short-wave trough and the cyclone both moved northeastward, upper-level positive PV anomalies merged, enhanced and entered key areas of the cyclone, and thus both direct and indirect effects associated with the upper-level forcings strengthened significantly around the cyclone, and this dominated the cyclone's transition from a moderate explosive cyclone to an extreme one.