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.

• explosive extratropical cyclone
• potential vorticity
• explosive development
• tropopause folding
• latent heat release

7. Acknowledgements

The authors thank the JMA, NCEP and NOAA for providing the data. Sincere thanks are also extended to Prof. Christopher A. Davis for his valuable suggestions about the PPVI method, as well as Profs. Fuqing Zhang and Jimy Dudhia for their insightful suggestions about the sensitivity experiments and configurations of the simulations. This research was supported by the National Key Basic Research and Development Project of China (2012CB417201) and the National Natural Science Foundation of China (41205027 and 41375053). The authors also thank two anonymous reviewers for many useful comments and suggestions.

Notes

¹Yoshida and Asuma (2004) proposed that 1.2 Bergerons is the most common deepening rate of PO–O cyclones, whereas 2.6 Bergerons is a really rare and extreme deepening rate.

²Actually, the sensitivity to KA selection was tested as follows: in calculating Table 1, the KA-average was calculated three times: (1) using the rectangles shown in Fig. 8; (2) using smaller rectangles [moving all four boundary lines of the KAs shown in Fig. 8 inward by 1° (latitude/longitude), except for Fig. 8a, which used 0.5°]; and (3) using bigger rectangles [moving all four boundary lines of the KAs shown in Fig. 8 outward by 1° (latitude/longitude), except for Fig. 8a, which used 0.5°]. After calculations based on all three types of KAs, although the exact KA-averaged value changed moderately, the ranking of the relative importance of different factors remained unchanged.