A classification of FASTEX cyclones using a height‐attributable quasi‐geostrophic vertical‐motion diagnostic

Abstract

SUMMARYA systematic and objective procedure is developed for applying the simple cyclone classi cation scheme of Petterssen and Smebye. This method uses a height-attributable solution of the quasi-geostrophic ! equation to identify and quantify the relative importance of upper-and lower-tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels. By applying this classi cation method to a sample of cyclogenesis events, during their maximum intensi cation stage, from the Fronts and Atlantic Storm-Track EXperiment eld experiment, the Type A and Type B scheme of Petterssen and Smebye is reproduced and extended to include a Type C. Type C consists of upper-level dominated cyclones that form at high latitudes and in their initial stages resemble comma-cloud-type polar lows. Detailed examples of each of the three types are presented. Some cyclones can undergo more than one period of development and it is found that each development period can be classi ed as a different cyclogenesis category, A or B: these cyclones are classi ed as hybrid Type A=B. There is some evidence that cyclone forecast accuracy depends on cyclone type, thereby suggesting the potential for this method to be used to assign con dence levels for forecasts of cyclogenesis produced by numerical weather-prediction models. Type B cyclones appear more dif cult to predict, because their development depends, initially, on the interaction between signi cant features in the upper and lower troposphere.

Impact of increasing resolution and a warmer climate on extreme weather from Northern Hemisphere extratropical cyclones

“…Other studies, for example, Pinto et al (2007Pinto et al ( , 2009, have shown an increase in the upper air baroclinicity, as well as an increase in the polar jet intensity, specifically for the North Pacific basin. Dacre and Gray (2009) also suggest an increase in high-latitude cyclones that are formed when a large upper level trough moves over the ocean (Deveson et al, 2002), due to a warmer climate. A better understanding of the diabatic processes that occur in storms is needed to better inform whether the increase in precipitation will lead to such consequences.…”

Factors contributing to the development of extreme North Atlantic cyclones and their relationship with the NAO

“…Fig. 4c) was also identified by Gray and Dacre (2006) as main source region of cyclones of type B following the threefold classification scheme of extratropical cyclogenesis by Deveson et al (2002), an extension of the classification by Petterssen and Smebye (1971). This type B of cyclone development is predominantly upper-level forced.…”

Quantifying the climatological relationship between extratropical cyclone intensity and atmospheric precursors

“…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]. Dacre and Gray [2009] found, in a 6-year climatology of North Atlantic cyclones, that a larger proportion of east than west Atlantic cyclones showed characteristics of type C development (39% compared to 25%), consistent with the generally weaker low-level frontal gradients in the east North Atlantic.…”