Genesis of Hurricane Sandy (2012)

In this paper, we examine the tropical cyclogenesis sequence of Hurricane Sandy (2012) 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 often referred to as the `marsupial paradigm'.

Sandy's cyclogenesis provides a useful illustration of the marsupial paradigm within a partially open recirculating region, the opening being present to the 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, the accretion of low-level cyclonic vorticity filaments into the developing vortex from several sources is documented (including the South American Convergence Zone, a weak easterly wave located west of the pre-Sandy wave, and cyclonic vorticity generated along Hispaniola). Organization and growth of the nascent storm is suggested to be enhanced by this accretion of low-level cyclonic vorticity during genesis and intensification of the storm. A Lagrangian trajectory analysis and angular momentum flux calculations are used to quantify the potential contributions to Sandy's spin-up from a Caribbean Gyre in the southwest of the Caribbean sea and an easterly wave that eventually formed Hurricane Tony. Our 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 both the United States and European operational numerical weather prediction models and discuss their effectiveness for this case. We suggest that the ability of these models to indicate genesis in the Sandy case is limited by the intrinsic multi-scale nature of this formation event and complexity of the vorticity accretion process.