The Contribution of Environmental Mid-Level Cyclonic Vorticity to the Genesis of Hurricane Sandy (2012)

The contribution of environmental cyclonic vorticity to the genesis of Hurricane Sandy, especially in terms of mid-level cyclonic vorticity, is examined using cloud-resolving WRF simulations. As Hurricane Rafael moved north of Puerto Rico between 15 and 17 October 2012, a region of mid-level vorticity separated from a northeast-southwest oriented trough axis on the southeast side of Rafael and remained nearly stationary over the northeastern Lesser Antilles. Starting on 18 October, the area of cyclonic vorticity moved southwest until early the following day when it was located over a weak and convectively-inactive easterly wave. Over the next 48 hours this wave became more convectively active and genesis occurred 18 UTC 21 October.

The goal of this research is to test the hypothesis that externally-generated mid-level cyclonic vorticity had a significant impact on the evolution of the pre-Sandy disturbance by transforming the system from a weak easterly wave devoid of convection to a viable tropical convective system. Cloud resolving WRF model simulations, initialized with the ECMWF operational analysis, will be used to assess the impact of environmental mid-level cyclonic vorticity on the development of the pre-Sandy disturbance. Potential vorticity inversion will be used to remove the cyclonic vorticity feature formerly associated with Rafael from the initial conditions and the evolution of the pre-Sandy disturbance after this vortex removal will be compared to a control run with unmodified initial conditions. Additionally, the effect of South American and Caribbean orography on the pre-Sandy disturbance will be assessed in simulations with and without vortex removal.