MECHANISMS OF CIRCULATION CHANGE DURING A LEE CYCLOGENESIS

Abstract

The life cycle of a destructive cyclone over the United States during 16 to 21 November 1948 is investigated. The vorticity advection at 300 millibars, as an indicator of divergence in the upper troposphere, is shown to be closely related to the organization, maintenance and final collapse of the surface cyclone. The early period of most rapid cyclogenesis is investigated with reference to orographic factors. It is found that the cyclogenesis took place when the region of strongest upward motion at 500 mb, directly beneath the upper-tropospheric jet stream, became superimposed over the band of maximum descending motion at the earth's surface, giving maximum vertical stretching in the lower troposphere. Computations of the contributions of horizontal convergence, and of lateral differences of friction, to circulation changes indicate that, during the period of most rapid development, the latter was of about equal importance with the former in increasing the circulation. The mechanism of circulation production, by friction, and orographic influences on the path taken by the cyclone, are discussed qualitatively.