The relation of moist symmetric instability and upper-level potential-vorticity anomalies to the observed evolution of cloud heads

The cloud-head phenomenon has been known for some time as a precursor of rapid cyclogenesis. Based upon satellite imagery, two stages of cloud-head development are identified in this study that can be related to early and late stages of the associated cyclone. Weakly convex cloud heads emerge from the baroclinic zone at an early stage in cyclogenesis and strongly convex cloud heads develop in association with the rapid deepening of the cyclone. Trajectories are used to highlight the role of an upper-level potential-vorticity (PV) anomaly in the growth of the strongly convex cloud head and the main flow associated with the growth of the strongly convex cloud head is viewed using isentropic analysis, The possible role of moist symmetric instability in the development of both weakly and strongly convex cloud heads is discussed and a ne diagnostic for measuring the vertically integrated extent of realizable symmetric (VRS) instability, based on the moist geostrophic PV, is introduced.