A Diagnostic Study of the Flateyri Avalanche Cyclone, 24–26 October 1995, Using Potential Vorticity Inversion

Abstract

The evolution of a deep North Atlantic cyclone, which caused devastating avalanches in northwest (NW) Iceland in October 1995, was investigated. As the main tool for this investigation, potential vorticity analysis was used. This allows the quantification and comparison of the roles of different processes that contribute to the cyclone deepening at different stages. Interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of the cyclone development. The thermal field over the North Atlantic had acquired strong west?east gradients due to a combination of advection of cold air southeastward from a cold cyclonic gyre south of Iceland and advection of warm air northward on the westward flank of a warm anticyclonic ridge over central Europe. A low-level baroclinic wave forming just south of Ireland was rapidly reinforced due to interaction with a descending, high-value, upper-level potential vorticity anomaly and was isentropically advected from the low south of Iceland. As the wave deepened, diabatic heating in association with the frontal systems became a major source of cyclonic vorticity. Cross sections of the height fields associated with potential vorticity anomalies reveal the baroclinic nature of some of the anomalies. The isentropic trajectory analysis shows strong ascent of warm air taking place over Iceland and thereby explaining the heavy precipitation in NW Iceland. The advection of rather warm, humid air overlying very cold air from a persistent high over Greenland, together with orographic lifting, seems to be responsible for the snowfall that together with heavy winds produced the unusual avalanches in Iceland.