The Transformation of Tropical Storm Agnes into an Extratropical Cyclone. Part I: The Observed Fields and Vertical Motion Computations

Abstract

This is the first of two papers dealing with the transformation of tropical storm Agnes (June 1972) into an extratropical cyclone. Synoptic analyses and vertical motion patterns are used to describe the behavior of Agnes over a five-day period subsequent to initial landfall during which time Agnes regained tropical storm strength prior to transforming into an extratropical cyclone and dissipating. Input data for all calculations are obtained from optimum interpolation objective analyses with a resolution of 1° latitude-longitude in the horizontal and 100 mb in the vertical. A modified, 9-level version of Krishnamurti's (1968a) diagnostic balance model is discussed and applied to the data set to determine the fields of vertical motion. Redevelopment of Agnes subsequent to initial landfall is the result of the spread of an area of appreciable cyclonic vorticity advection aloft over the periphery of the low-level circulation devoid of significant baroclinicity. Unlike a corresponding midlatitude storm development, however, the initial presence of a warm, moist, high vorticity environment at low levels greatly aids the development. Ascent induced by thermal advection becomes increasingly important as the circulation intensifies. It is responsible in large part for the observed cyclonic rotation of the total vertical motion ascent-descent dipole around the storm. The unique aspect of the regeneration of Agnes is provided by diabatic (primarily latent heat release) heating in the area of heavy rains extending well north of Agnes to the east of the Appalachian mountains. In conjunction with ascent due to thermal (warm) advection in the lower troposphere and cyclonic vorticity advection in the upper troposphere, a quasi-stationary region is produced along the eastern slopes of the mountains favorable for height falls, low-level convergence and vorticity generation. Agnes, after regaining tropical storm strength over open water, responds to this favorable forcing by redeveloping westward and rapidly transforming into an extratropical cyclone.