On a threefold classification of extratropical cyclogenesis

Abstract

SUMMARYA recently proposed extension to the twofold extratropical cyclogenesis classi cation scheme of Petterssen and Smebye is discussed. A third class of extratropical cyclone (type C) is described, in which initial development is controlled by a pre-existing upper-level potential vorticity (PV) anomaly. In its early stages, such a system is indistinguishable from the classical type B cyclone of the Petterssen and Smebye scheme. However, subsequent development cannot be understood in terms of a co-operative interaction of the upper-level feature with a lowlevel baroclinic zone. Rather, the cyclogenetic dynamics of type C systems are dominated by the action of strong midlevel latent heating. Such heating can generate important anomalies of PV that act to suppress the formation of a low-level thermal anomaly and that interact destructively with the pre-existing upper-level feature.Candidate type C events are identi ed using recently developed, height-attributable, quasi-geostrophic, vertical-motion diagnostics. The application of one such diagnostic to a climatology of instantaneous cyclonic features suggests that type C events may occur with reasonable frequency. The generic behaviour of system types in the proposed threefold classi cation scheme is compared with the actual dynamics of some cyclones from the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The analysis is based on piecewise PV inversions and numerical simulations. This approach is able to provide a good description of a case of standard type B development, consistent with the qualitative description of Petterssen and Smebye and with the quasi-geostrophic diagnostics. Within the same framework, two systems are discussed whose behaviours do not t the simple A/B classi cation, but are consistent with the proposed type C mechanism.

Using satellite and reanalysis data to evaluate the representation of latent heating in extratropical cyclones in a climate model

“…The heating contours in HiGEM can be related to the enhanced low level PV anomaly in HiGEM relative to ERAI. Though it is not possible to attribute causality within the framework of this study, a strengthened low level PV anomaly has been shown to induce stronger low level flow within WCBs (Plant et al 2003). The effect of such enhancement provides a positive feedback to the latent heating.…”

“…Latent heat release generates (positive/ cyclonic) PV anomalies below and (negative/anticyclonic) above the location of heating (Hoskins et al 1985;Wernli and Davies 1997). Cyclonic PV anomalies below the heating maxima can act to enhance the low level circulation (Plant et al 2003) and strengthen flow in and downstream of the warm conveyor belt (WCB, Grams et al 2011), a band of ascending moist air responsible for much precipitation in ETCs. Anticyclonic PV generation above the heating maxima can help to slow the movement of the upper level PV maxima (the trough), thus maintaining the vertical tilt of the ETC (Stoelinga 1996), and can also enhance downstream ridging (Davis et al 1993).…”

Quantifying the climatological relationship between extratropical cyclone intensity and atmospheric precursors

“…Davis [1992] and Wernli et al [2006] both showed that the superposition of stratospheric PV anomalies and moisture-induced midlevel PV could enhance cyclone intensity. Finally, Plant et al [2003] showed that midlevel PV anomalies, generated in response to latent heating, were important for the development of two socalled type C cyclones. Type C cyclones are characterised by strong (quasigeostrophic) upper-level forcing, very weak low-level forcing and a vertical upstream tilt that remains constant or increases as the cyclone intensifies [Deveson et al, 2002].…”

Extratropical Cyclones in Idealized Simulations of Changed Climates

“…In particular, the EKE includes contributions from both cyclones and anticyclones, and it is possible that the role of moisture and latent heating (LH) is more pronounced for certain classes of cyclones (cf. Plant et al 2003) or for deepening rates rather than measures of intensity. LH has been shown to be important for cyclone intensification in theoretical studies (e.g., Emanuel et al 1987), idealized life cycle experiments (Boutle et al 2011;Booth et al 2013;Schemm et al 2013), and various case studies (e.g., Kuo et al 1991;Stoelinga 1996;Ahmadi-Givi et al 2004), but climatologically the role of moisture for the statistical distribution of cyclone intensities is less clear.…”