A Cyclone Phase Space Derived from Thermal Wind and Thermal Asymmetry

Abstract

An objectively defined three-dimensional cyclone phase space is proposed and explored. Cyclone phase is described using the parameters of storm-motion-relative thickness asymmetry (symmetric/nonfrontal versus asymmetric/frontal) and vertical derivative of horizontal height gradient (cold-versus warm-core structure via the thermal wind relationship). A cyclone's life cycle can be analyzed within this phase space, providing substantial insight into the cyclone structural evolution. An objective classification of cyclone phase is possible, unifying the basic structural description of tropical, extratropical, and hybrid cyclones into a continuum. Stereotypical symmetric warm-core (tropical cyclone) and asymmetric cold-core (extratropical cyclone) life cycles are illustrated using 1Њ Navy Operational Global Atmospheric Prediction System (NOGAPS) operational analyses and 2.5Њ NCEP-NCAR reanalyses. The transitions between cyclone phases are clearly illustrated within the phase space, including extratropical transition, subtropical and tropical transition, and the development of warm seclusions within extratropical cyclones. The planet's northwestern hemisphere inhabitance of the proposed phase space between 1980 and 1999 is examined using NCEP-NCAR 2.5Њ reanalyses. Despite the inability to adequately resolve tropical cyclones at the coarse 2.5Њ resolution, warm-core cyclones (primarily warm-seclusion extratropical cyclones) have a mean intensity that is 10 hPa lower than that of cold-core cyclones. Warm-core cyclones also have a much larger variability for intensity distribution, with an increased occurrence of lower MSLP. Further, at 2.5Њ resolution the lowest analyzed MSLP for a warm-core cyclone was 14 hPa lower than that for a cyclone that remains cold core. These results suggest that cyclones that maintain solely a cold-core structure (no warm-seclusion or tropical development) may be associated with a significantly weaker minimum observed intensity at 2.5Њ resolution, although further examination using higher-resolution data is required to refine this. Phase diagrams are being produced in real time to improve the forecasting of cyclone phase evolution and phase transitions, and to provide measures of phase predictability through ensembling of multiple models. The likelihood of warm-core development in cyclones can be anticipated by applying the diagnostics to various model forecasts, illuminating the potential for large intensity changes when the explicit model intensity forecasts may be insufficient.

Climate perspective on the large‐scale circulation associated with the transition of the first South Atlantic hurricane

“…When the BI is high, the westerlies are weaker than normal at midlatitudes in the South American sector. Therefore, a positive BI is associated with less large-scale baroclinicity (which also depends on the static stability) and is physically consistent with a TT-prone environment (Davis and Bosart, 2003;Hart, 2003). The association between the BI and the shear is not direct because the former pertains, in the first instance, to conditions at higher latitudes.…”

“…However, with insights on hybrid systems and their tropical and extra-tropical transition (Beven, 1997;Davis and Bosart, 2003;Hart, 2003;Jones et al, 2003 and references therein) becoming increasingly important, and particularly after Catarina's genesis, there has also been an increasing awareness of the need for a rethinking of empirical constraints, as recently pointed out by McTaggart-Cowan et al (2006). Different aspects of Catarina's life cycle have been investigated Simmonds, 2005, 2008;McTaggart-Cowan et al, 2006;Veiga et al, 2008), but many facets of its transition are still puzzling, particularly due to lack of observational data in the SA sector.…”

Counting Atlantic tropical cyclones back to 1900

“…Quikscat [Atlas et al, 2001 ], the advanced microwave sounding unit [Brueske and Velden, 2003], and the cyclone phase space analyses [Hart, 2003] (quiet up to 1925, active from 1926 to 1970, quiet from 1971 to 1994, and active from 1995 onward) associated with the Atlantic multidecadal oscillation [Goldenberg et al, 2001]. A reanalysis of the trends with the inclusion of these additional tropical cyclones leads to an insignificant trend for both the periods between quiet eras and active eras.…”

Characteristics and development of European cyclones with tropical origin in reanalysis data

“…This has the potential of describing different parts of a particular cyclone evolution, but does not qualitatively distinguish one cyclone life cycle from another. Hart (2003) proposed a three-dimensional cyclone phase space using relative thickness symmetry, which is an approximate measure of frontal nature of the cyclone and the vertical derivative of the horizontal height gradient (in two different atmospheric layers), which is a measure of the cyclone thermal wind and thus the thermal nature of the core. Using this phase space analysis, Hart showed the potential to distinguish different cyclone life cycles and therefore to categorize these life cycles.…”