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 sto...