Simulated precipitation structure of the extratropical transition of tropical cyclones

The second, or reintensification, stage of the extratropical transition (ET) of tropical cyclones occurs when a transformed tropical cyclone reintensifies if the midlatitude conditions are favorable for extratropical cyclogenesis, which is usually as part of an interaction with a midlatitude upper-level trough (e.g., DiMego and Bosart 1982, Harr and Elsberry 2000, Klein et al. 2000). Klein et al. (2000) noted in their study of 30 western North Pacific ET events that this re-intensification stage was very dependent on the details of the midlatitude circulation structure and how the transformed tropical cyclone interacted with that midlatitude circulation. In particular, poleward-moving tropical cyclones underwent a variety of re-intensification rates. Of those that did significantly re-intensify, the ability of forecast models to accurately predict the rate of intensification and future track of the storm was at times poor, with large discrepancies from forecast to forecast even by the same model (P. Harr 2000; personal communication). This led to the Harr and Elsberry (2000) hypothesis that the details of the structure of the transformed tropical cyclone has relatively little influence on the intensification processes; rather, it is the structure of the midlatitude environment that determines the rate and nature of re-intensification. If the forecast model poorly handles the timing of the transformed tropical cyclone with the midlatitude environment, a poor re-intensification forecast results. Prior studies of ET (DiMego and Bosart 1982, Sinclair 1993, Foley and Hanstrum 1994, Harr and Elsberry 2000, Harr et al. 2000) have described reintensification of the transformed tropical cyclone in terms of a Type-B development following Petterssen and Smebye (1971). In this context, low-level cyclone development occurs when an area of mid-level positive vorticity