The Transformation of Tropical Storm Agnes into an Extratropical Cyclone. Part II: Moisture, Vorticity and Kinetic Energy Budgets

Abstract

Moisture, vorticity and kinetic energy budgets are constructed to diagnose the transformation of tropical storm Agnes (June 1972) into an extratropical cyclone in this second of two papers on Agnes. The vertical motions and divergent wind components used in the computations are taken from the solution of the nonlinear balance model described in Part I [DiMego and Bosart (1982)]. The budget equations are formulated in a quasi-Lagrangian reference frame centered with respect to the moving surface cyclone for several storm volumes. The results are displayed spatially as well as in time-section format. Synoptic-scale transport and moisture convergence dominate the moisture budget and all terms together define well the areas of observed precipitation. Both budget and model-computed precipitation, particularly the latter, underestimate the observed precipitation. The discrepancy is attributed to the sub-grid scale convective processes and model underestimation of the divergent wind components. Advection of vorticity by the non-divergent wind in the upper troposphere, vertical advection and convergence in the middle troposphere and low-level convergence in the presence of intense precipitation are the principal source terms in the vorticity budget. Reintensification of Agnes to tropical storm strength and the subsequent transformation to an extratropical cyclone with a westward displacement of the surface center is a near-classic example of a Petterssen and Smebye (1971) type-B cyclone development with one crucial difference, namely, the presence of a tropical storm with its large in situ vorticity beneath a region of prominent cyclonic vorticity advection aloft. Cyclonic rotation of areas of vorticity generation by diabatic processes and destruction by thermal advection effects in the lower troposphere are consistent with the observed westward looping of the storm center accompanying transformation. Near Agnes the prominent kinetic energy source is in situ generation through cross-contour flow, primarily by the non-divergent wind. The generation of kinetic energy during the reintensification stage of Agnes (18?20 W m2) is about 40% of that computed by Palmén (1958) in his study of the transformation of hurricane Hazel. Most of the generated energy is used to increase the storm kinetic energy content and offset dissipation. Horizontal flux convergence of kinetic energy is the dominant source term in regions surrounding Agnes where the generation is negative. Upscale exchange processes associated with convection near the storm center and anticyclonic conditions in the storm periphery, coupled with an underestimation of vertical motion and divergent wind components, are associated with positive residuals prior to transformation and occlusion.