Hurricane Sandy (2012): Vortex Intensification during Warm Core Seclusion

Hurricane Sandy's landfall along the New Jersey shoreline at 2330 UTC 30 October 2012 produced a catastrophic storm surge stretching from New Jersey to Rhode Island that contributed to damage in excess of $50 billion – the sixth costliest U.S. hurricane on record since 1900 (with inflation) – and directly caused 72 fatalities. The complex life cycle of Hurricane Sandy began on 21 October with the genesis of an easterly wave over the southwestern Caribbean. After making landfall over Cuba as a category-3 hurricane on 25 October, Sandy moved northward and interacted with an upper-level trough embedded in the subtropical waveguide on 26 October and a second upper-level trough embedded in the polar waveguide on 28–29 October. While interacting with the first subtropical trough, Sandy weakened to tropical storm intensity as the convection became asymmetric in response to the introduction of drier air and increased vertical wind shear. As Sandy moved northeastward it regained hurricane strength as convection became re-established near the center. During the second trough interaction, Sandy turned northwestward and intensified (sea-level pressure decreased by ~20 hPa to 942 hPa in the 48-h period ending 1800 UTC 29 October; equivalent to a sea-level rise of ~0.2 m) as cold continental air swept cyclonically around the vortex and Sandy acquired characteristics of a warm seclusion. The aim of this presentation is to examine and diagnose Sandy's secondary peak in intensity on 29 October 2012 just prior to landfall using high-resolution (4-km horizontal grid spacing) numerical simulations from the Advanced Hurricane Weather Research and Forecasting (AHW) model.

The AHW simulations produced a vortex structure evolution that agrees well with observed dropsondes from Air Force reconnaissance flights on 28–29 October. While the mid- and upper-tropospheric part of the vortex weakened, low-level vorticity and maximum tangential wind (below 700 hPa) increased. Intensification of the low-level vortex occurred in response to an increase in the radial gradient of potential temperature. Using the Sawyer-Eliassen balanced vortex model to diagnose the underlying dynamics, the enhancement of the radial gradient of potential temperature and attendant secondary circulation was driven primarily by radial advection of cold air inward and a shift of the diabatic heating maximum to inner radii (from 350 to 100 km during 0000–2000 UTC 29 October). Much of the convection within Sandy's circulation occurred on the west side and was driven by low-level frontogenetical forcing on the cyclonic shear side of the cold air surge. The radially inward shift of diabatic heating occurred as the cold air surge and axis of frontogenesis moved radially inward. From a vorticity-based perspective, vortex intensification occurred in response to shallow low-level convergence (below 850 hPa; contribution from vortex stretching) that was consistent with the Sawyer-Eliassen solution for the radial wind component of the secondary circulation. Generation of cyclonic vorticity and an increase in circulation also occurred in the 850–700 hPa layer in conjunction with tilting of horizontal vorticity along the cyclonic shear side of the cold air surge.