Examining the Roles of the Easterly Wave Critical Layer and Vorticity Accretion During the Tropical Cyclogenesis of Hurricane Sandy

We examine the tropical cyclogenesis sequence of Hurricane Sandy and find that genesis occurs within a recirculating Kelvin cat's eye flow of a westward propagating tropical wave. The cat's eye flow is able to provide a protective environment for the mesoscale vortex to grow and is characterized by gradual column moistening and increased areal coverage of deep cumulus convection. These findings are generally consistent with a recently proposed tropical cyclogenesis sequence referred to as the marsupial paradigm'. Sandy's cyclogenesis provides a useful illustration of the marsupial paradigm within a partially open recirculating region, with the opening located south of the pouch center. It is suggested that the opening acts to enhance the genesis process because it is adjacent to an environment characterized by warm, moist air, conditions favorable for tropical cyclogenesis. From a dynamical perspective, we document accretion of low-level cyclonic vorticity filaments into the developing vortex from several sources (the South American Convergence Zone, an easterly wave located west of the pre-Sandy wave, and cyclonic vorticity generated along Hispaniola). Organization and growth of the nascent storm is enhanced by this accretion of cyclonic vorticity. A Lagrangian trajectory analysis is used to assess potential contributions to Sandy's spin-up from a Caribbean Gyre and the easterly wave that formed Hurricane Tony. This analysis indicates that these features are outside of the Lagrangian flow boundaries that define the pre-Sandy wave and do not directly contribute to spin-up of the vortex. Finally, we examine forecasts from the United States and European operational numerical weather prediction models and discuss their effectiveness for this case. Prepared in collaboration with NorthWest Research Associates, Bellevue, WA.