Examining extratropical transition of Ernesto wind fields and developing storm surge

Hurricane Ernesto weakened quickly after it came ashore in North Carolina. It was downgraded to a tropical depression within 12 hours after landfall. In the meantime it transitioned to an Extratropical cyclone by the time it reached the North Carolina -Virginia border. During the transition phase the wind field greatly expanded and hurricane force wind gusts were observed along the Chesapeake Bay. The strength of this wind field was not forecast well by any of the NCEP models. This expanded and strengthening wind field produced significant storm surge damage as well as wind damage along the Chesapeake Bay.

In this study we use analyses and local mesoscale models to examine the precursors for the changing and strengthening wind fields which could be used to improve forecasts of this type of event. We also examine the impact of using various parameterization methods to improve the local WRF model forecast. The produced wind fields are compared with the verifying observational data.

From these improved wind fields, the storm surge inside the Chesapeake Bay was examined. A large domain un-structured grid, hydrodynamic model ELCIRC was set up to study storm surges induced by both remote wind field (outside the Bay), and the local wind fields (inside the Bay) by Tropical Storm Ernesto. Nine tidal harmonics were used at the open boundary condition, and the model time step used was 5 minutes. The model results indicate: (1) The combined high pressure to the north and the low pressure system of Ernesto from the south generated unusually strong easterly winds which were the main cause of the set up in the lower Chesapeake Bay, (2) the temporal and spatial water level response inside the Bay can be explained by the supposition of off-shore surge propagating into the Bay and the surge generated by the local wind, (3) When a suitable forcing wind field was provided, the Eulerian-Lagrangian-based, unstructured grid model ELCIRC is capable of simulating storm surge in a large-scale model domain while maintaining detailed, high resolution results inside the Bay. These results will be shown.