A cyclone phase space dedicated to extratropical cyclones

Abstract. Despite intensive research on midlatitude cyclones since the mid-twentieth century, open questions on their structure and development remain, like the question of their core temperature. It is not clear yet what the proportion of cold-core and warm-core cyclones in midlatitudes is, if occluded cyclones are cold-core or warm-core cyclones and how different the processes leading to cold-core and warm-core cyclones are. To address these questions, a new cyclone phase space denoted as ETC-CPS and adapted to extratropical cyclones is developed by introducing three parameters: the core temperature, the thermal asymmetry and the baroclinic conversion rate. Differences with existing cyclone phase spaces are detailed by analyzing two consecutive storms in the North Atlantic, one ending with a warm seclusion and another with an occlusion. ETC-CPS is then applied to all midlatitude cyclones of the Northern Hemisphere (NH) tracked during winter and summer using ERA5 reanalysis.

The results highlight that most of midlatitude NH cyclones are asymmetric warm-core cyclones. At the time of maximum intensity, the fraction of cyclones with a cold core temperature in the lower troposphere fluctuates around 10–15 % depending on the season while that of warm core cyclones is around 85–90 %. It indicates that, in addition to warm-seclusion cyclones, most occluded cyclones have also a warm core. Both cold-core and warm-core cyclones undergo a well-marked baroclinic growth phase before reaching their maximum intensity but their vertical structure differs during that phase. Warm-core cyclones exhibit a clear vertical westward tilt of the geopotential height anomaly contours as in the classical picture of a developing baroclinic unstable mode. In contrast, cold-core cyclones have a funnel-like vertical structure with the anomalous geopotential height field leaning more westward than eastward which makes them also grow baroclinically but with a non-standard baroclinic structure. Differences between seasons are also noticeable. During winter, cold-core cyclones have much weaker intensity than warm-core cyclones and preferentially develop over continental regions whereas warm-core cyclones develop over the oceanic storm tracks. During summer, both types of cyclones preferentially develop over the oceanic storm tracks and have similar intensities.