TROPICAL CYCLOGENESIS VIA CONVECTIVELY FORCED VORTEX ROSSBY WAVES IN A 3-DIMENSIONAL QUASI-GEOSTROPHIC MODEL

This work investigates the problem of tropical cyclogenesis in three d imensions. In particular, the authors examine the interaction of small -scale convective disturbances with a larger-scale vortex circulation in a nonlinear quasigeostrophic balance model. Convective forcing is p arameterized by its estimated net effect on the potential vorticity (P V) held. Idealized numerical experiments show that vortex intensificat ion proceeds by ingestion of like-sign potential vorticity anomalies i nto the parent vortex and expulsion of opposite-sign potential vortici ty anomalies during the axisymmetrization process. For the finite-ampl itude forcing considered here, the weakly nonlinear vortex Rossby wave mean-flow predictions for the magnitude and location of the spinup ar e in good agreement with the model results. Vortex development is anal yzed using Lagrangian trajectories, Eliassen-Palm flux vectors, and th e Lorenz energy cycle. Using numerical estimates of the magnitude of P V injection based on previous observational and theoretical work, the authors obtain spinup to a 15 m s(-1) cyclone on realistic timescales. Simulation of a midlevel vortex with peripheral convection shows that axisymmetrization results in the spinup of a surface cyclone. The axi symmetrization mechanism demonstrates the development of a warm-core v ortex. The relative contribution from eddy-heat and eddy-momentum flux es to the warm core structure of the cyclone is investigated. The vort ex spinup obtained shows greater than linear dependence on the forcing amplitude, indicating the existence of a nonlinear feedback mechanism associated with the vortex Rossby waves. Building on recent work by s everal authors, this work further clarifies the significance of the ax isymmetrization process for the problem of tropical cyclogenesis. The theory is shown to be consistent with published observations of tropic al cyclogenesis. Further observational and modeling tests of the theor y, specific to the dynamics examined here, are proposed.