18E.2 Storm surge measurement with an airborne scanning radar altimeter

Friday, 2 May 2008: 11:15 AM
Palms E (Wyndham Orlando Resort)
Edward J. Walsh, NASA/GSFC, Wallops Island, VA; and C. W. Wright, W. B. Krabill, W. A. Shaffer, S. R. Baig, M. Peng, L. J. Pietrafesa, A. W. Garcia, F. D. Marks Jr., P. G. Black, J. Sonntag, and B. D. Beckley

The NASA Scanning Radar Altimeter (SRA) has a long heritage in measuring the energetic portion of the sea surface directional wave spectrum. SRA wave spectra have been used to assess the performance of the WaveWatch III numerical wave model. This paper demonstrates that an airborne scanning radar altimeter could also routinely provide targeted measurements of storm surge for assessing and improving the performance of storm surge models.

The storm surge associated with a landfalling hurricane is a mound of water generated by the high winds and pushed toward the shore. Some of the factors affecting the magnitude of the surge are the maximum wind speed, the radius of maximum wind, the forward speed of the storm, its angle of track relative to the coastline, and the characteristics of the bathymetry and coastline, including roads, levees and other physical features that can modify the storm surge flow pattern. The locally raised sea level gives the storm waves riding on top of the surge access to cause extensive damage to coastal structures and beach erosion.

The storm surge from Hurricane Katrina exceeded 8 m. With much of the densely populated Atlantic and Gulf Coast shorelines less than 3 m above mean sea level, storm surge has the potential to destroy lives and property and cut off escape routes. Because the public needs to evacuate long before landfall, sophisticated modeling efforts have been developed for predicting the spatial and temporal variation of the surge so emergency managers can issue timely evacuation orders and effectively position response resources.

Over the years, hurricane track and intensity forecasts and surge models and the digital terrain and bathymetry data they depend on have improved significantly. Strides have also been made in knowledge of the detailed variation of the surface wind field driving the surge from airborne measurements using GPS dropwindsondes and Stepped Frequency Microwave Radiometers as well as wind data gathered from temporary towers set up along the coast in the hurricane's projected path.

The area of least improvement has been in obtaining data on the details of the temporal/spatial variation of the storm surge dome of water as it evolves and inundates the land to evaluate the performance of the numerical models. Tide gauges in the vicinity of the landfall are frequently destroyed by the surge. Survey crews dispatched after the event provide only indirect indications of the maximum surge envelope over land and no temporal information.

The landfall of Hurricane Bonnie on 26 August 1998 provided an excellent opportunity to demonstrate the potential benefits of direct airborne measurement of the temporal/spatial evolution of storm surge. Bonnie was a slow moving storm with a large radius of maximum wind, minimizing both the temporal and spatial gradients. The peak of the Hurricane Bonnie storm surge was less than 2 m, providing an opportunity to demonstrate that even a minimal surge can be well documented.

Despite a 160 m variation in aircraft altitude and an 11.5 m variation in the elevation of the mean sea surface (MSS) relative to the ellipsoid over the flight track, and the tidal variation over the 5 hour data acquisition interval, a survey-quality Global Positioning System (GPS) aircraft trajectory allowed the SRA to produce storm surge measurements that generally fell between the surge values computed by the NOAA SLOSH model and the North Carolina State University storm surge model.

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