Sea State Dependent Surface Stress in ADCIRC for Storm Surge Predictions

Storm surge in most coastal areas is driven predominantly by surface stress due to winds blowing across the ocean surface. Accurately representing this surface stress is therefore essential for robust predictions of storm surge and associated flooding. Surface stress is typically computed in surge models as a function of the 10 meter wind velocity using a quadratic drag law. In an attempt to account for the effect of sea state in this relationship, a number of empirical equations have been proposed expressing the scaling coefficient in the quadratic drag law (i.e., the drag coefficient) as a function of wind speed. It is reasonably well believed that the drag coefficient increases linearly with wind speed for low to moderate winds and levels off or decreases at high winds, e.g., as experienced in a strong tropical cyclone. In the latter case, the rapidly changing wind direction associated with the tropical cyclone vortex implies that a significant difference exists between the orientation of the 10 meter wind velocity and the direction of wave propagation around the storm. This has been used to justify relationships between the wind speed and drag coefficient that vary depending on the azimuthal location around the storm relative to the forward translation velocity.

Advances in computing and wave modeling have made it feasible to concurrently model both wave conditions and storm surge, thus providing the opportunity to include sea state explicitly in determining surface stress for storm surge calculations. We have recently implemented a sea state dependent surface stress algorithm from Reichl et al (2014) in the coupled ADCIRC – SWAN coastal circulation and storm surge – wave model. This talk will present initial results and implications for storm surge modeling from this new surface stress parameterization.