In the western North Pacific, the transition of a tropical cyclone as it encounters the strong baroclinic zone associated with midlatitude westerlies is accompanied by striking structural modifications. Typical changes include the re-organization of the strong core convection and rainbands into asymmetric patterns particularly in the north segment of the tropical cyclone core. In addition, outflow is favored in the northern sector of the storm, and the central pressure begins to rise. Re-intensification occurs in some systems when, as part of an interaction with an upper-level trough, re-deepening of the cyclone to a significant midlatitude storm takes place with consequences for coastal and maritime interests. Improved understanding of the structural changes accompanying the transformation of a tropical cyclone into an extratropical system should both help explain why some storms re-intensify while many do not, and improve the ability of numerical weather prediction models to forecast such events.
In this paper, the primary effects of the interaction between a baroclinic zone and a vortex with the structure of a tropical cyclone are explored. Simulations are performed using the U.S. Navy's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS). This system is nonhydrostatic, has nested grids, and sophisticated physical process parameterizations and is capable of resolving both the large-scale features associated with the midlatitude, baroclinic zone, and the dynamic features of the tropical cyclone down to the rainbands and core.
In the set of simulations, the effects of idealized environments, similar to that encountered in the midlatitudes, on a tropical cyclone-like vortex are investigated. No attempt is made to simulate an interaction with an upper-level trough, and so the focus is on the structural changes of the tropical cyclone up to when re-intensification might begin (this is known as transformation). The results of the interaction are explored with attention focussed on the structural changes of the tropical cyclone as the degree of idealization is reduced. The simulated system that is the result of the interaction is compared to existing analyses. In the presentation particular attention is directed upon the physical characteristics of the transformed system, particularly the vertical structure of the tropical cyclone remnants