A Retrospective Evaluation of the Storm Surge Produced by Hurricane Gustav (2008): Forecast and Hindcast Results

Abstract

The evolution and convergence of modeled storm surge were examined using a high-resolution implementation of the Advanced Circulation Coastal Ocean and Storm Surge (ADCIRC) model for Hurricane Gustav (2008). The storm surge forecasts were forced using an asymmetric gradient wind model (AWM), directly coupled to ADCIRC at every time step and at every grid node. A total of 20 forecast advisories and best-track data from the National Hurricane Center (NHC) were used as input parameters into the wind model. Differences in maximum surge elevations were evaluated for ensembles comprised of the final 20, 15, 10, and 5 forecast advisories plus the best track. For this particular storm, the final 10-12 forecast advisories, encompassing the last 2.5-3 days of the storm's lifetime, give a reasonable estimate of the final storm surge and inundation. The results provide a detailed perspective of the variability in the storm surge due to variability in the meteorological forecast and how this changes as the storm approaches landfall. This finding is closely tied to the consistency and accuracy of the NHC storm track forecasts and the predicted landfall location and, therefore, cannot be generalized to all storms in all locations. Nevertheless, this first attempt to translate variability in forecast meteorology into storm surge variability provides useful insights for guiding the potential use of storm surge models for forecast purposes. Model skill was also evaluated for Hurricane Gustav by comparing observed water levels with hindcast modeled water levels forced by river flow, tides, and several sources of wind data. The AWM (which ingested best-track information from NHC) generated winds that were slightly higher than those from NOAA's Hurricane Research Division (HRD) H*Wind analyses and substantially greater than the North American Mesoscale (NAM) model. Surge obtained using the AWM more closely matched the observed water levels than that computed using H*Wind; however, this may be due to the neglect of the contribution of wave setup to the surge, especially in exposed areas. Several geographically distinct storm surge response regimes, some characterized by multisurge pulses, were identified and described.

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“…Thus, we suggest that the large spatial variability of the measured estuarine wind field can be attributed to very local (<1 km) variability in the atmospheric boundary layer, especially in estuaries characterized by a mosaic of open water, forested wetlands, marshes, and barrier islands. Accordingly, the spatial distribution of U 10 in the Mississippi Delta should be more “noisy” (i.e., high spatial variability at small scales) than what is depicted by large scale atmospheric models (Forbes et al, ).…”

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“…Forecasts are run using storm track information that includes the latitude and longitude of the storm's center, intensity (maximum sustained 1-min wind speed), pressure drop and radius of maximum winds from NHC's Best Track operational data and parameters from all of the model information available to the Hurricane Specialists at NHC. The SLOSH parametric wind model is used to ensure that the parameters in the SLOSH wind formulation are consistent with those in the model guidance, i.e., the resulting wind speed in the SLOSH wind model is in accordance to the NHC's Best Track and the model guidance intensity, in a manner similar to other storm surge forecast systems [12,13].…”

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