An objectively defined continuum of cyclone phase space is proposed and explored through examples and a climatology. Cyclone phase is described using three parameters: 1) storm-relative thickness asymmetry (asymmetric /frontal vs. symmetric/nonfrontal) 2) 900-600hPa thermal wind (cold versus warm core structure), and 3) 600-300 hPa thermal wind.

Four basic types of cyclone phase result from the phase space: asymmetric/frontal cold-core, symmetric/nonfrontal cold-core, asymmetric/frontal warm-core, symmetric/nonfrontal warm-core. A cyclone's lifecycle can be analyzed within this phase space, providing substantial insight into the cyclone evolution and an objective classification of cyclone phase that can be helpful in diagnosing phase transitions, such as extratropical transition.

Various cyclone lifecycles are illustrated through historical cases of conventional and unconventional cyclones using 1.125 deg ECMWF and 2.5deg NCEP reanalyses and 1deg NOGAPS operational analyses. The conventional tropical cyclone lifecycle is represented as the development of a symmetric warm-core cyclone whose warm-core signal intensifies bottom-upward and decays top-downward. The conventional extratropical cyclone lifecycle from development to occlusion is also well- represented.

The various transitions between cyclone phases are also illustrated, including extratropical transition (e.g. Floyd 1999), tropical transition (e.g. Bertha 1990), multiple phase transitions (e.g. Micheal 2000) and warm-core seclusion (e.g. Danielle 1998).

The resulting cyclone phase space is a continuum with only a minority of cyclones existing within one phase throughout their lifecycle. The inhabitance of the proposed phase space was examined using 15,000 cyclones between 1986 and 2000. Frontal warm-core cyclones are the most rare of cyclone types to occur, but are also the most intense on average. The majority of intense extratropical cyclones (< 970hPa) reaching Europe from the western Atlantic do so with a partial or complete warm core dynamic structure. This warm core structure results from oceanic surface fluxes of heat and moisture from below, often in concert with a warm-seclusion process, dominating the quasi-geostrophic forcing from above in determining the vertical profile of cyclone intensity (geopotential perturbation).

Further, an empirical observed minimum pressure (~945hPa) for land-based northern Hemisphere cold-core cyclones is evident from this analysis (based upon 2.5 deg resolution reanalyses). All cyclones in this geographic region intensifying beyond this threshold have a component of lifecycle as a warm-core cyclone. The existence of warm-core characteristics within intense extratropical cyclones over oceanic regions is a fundamental component of their lifecycle and illustrates there are no sharp boundaries between the known cyclone types.

The phase analysis proposed here can be completely derived from the analyzed or forecast 4-D geopotential field. As a result, the phase diagram can be (and is) readily created from operational numerical output. Forecast phase diagrams derived from operational model output have been used several times experimentally by the National Hurricane Center and the Canadian Hurricane Center this hurricane season to anticipate the warm-core development of Hurricanes Karen and Noel (2001) and the extratropical transition of Gabrielle, Humberto, and Michelle (2001) within gridded analyses and forecasts.

Reference web site: eyewall.met.psu.edu/cyclonephase