Organization of Clouds and Precipitation in Extratropical Cyclones

Abstract

A number of models accounting for the distribution of cloud and precipitation in extratropical cyclones were proposed during the 19th and early 20th centuries. A history of these has been recounted by Bergeron (1959, abridged version in Bergeron 1981) and also by Ludlam (1966) in his inaugural lecture as professor of meteorology. These models culminated in the classical Norwegian polar-front cyclone model of the Bergen school Bjerknes and Solberg 1922) in which the patterns of cloud and precipitation were related to vertical air motions resulting from the relative movement of different air masses along inclined frontal surfaces. This model is still widely used today. During the past quarter-century, however, the availability of imagery from satellites and radars has revolutionized the capability to observe cloud and precipitation. The imagery has drawn attention to many synoptic-scale and mesoscale features not explained by the classical model, as discussed by Reed in Sec(tion 3.3.3. It is now clear that the Norwegian model, despite its popularity, is a broad-brush model that fails to explain many important variations in structure among cyclones.