Research Seminar: Jie Gao

Surface wind stress is the primary force that drives the upper-ocean circulation and formation of the surface wind waves, especially under Tropical Cyclones (TCs). Current parameterization of the surface wind stress in the ADCIRC storm surge model utilizes empirical formulas of the surface drag coefficient C_d as a function of the 10 m height wind speed U₁₀ only, in absence of considering the sea state (e.g., the Garratt’s 1977 formula, and the Powell’s storm sector-based formula). The behavior of C_d in terms of U₁₀ varied widely from study to study, but in storm surge modeling it is generally believed that C_d increases linearly with wind speed at low to moderate winds and levels off or even decreases at high winds.

Many sea state-dependent formulas of surface stress (or equivalently the C_d or the aerodynamic roughness length z₀) have been proposed based on field and laboratory studies, as the influence of waves on momentum flux at the air-sea interface has long been recognized. In this study, we propose to implement and examine five prevailing sea state-dependent formulas in the coupled ADCIRC + SWAN model to examine the behavior of C_d under high wind conditions and tailor them for shallow water application, which include the wave directional spreading-dependent C_d formula from Holthuijsen et al. (2012), the wave age-dependent z₀ formula from Drennan et al. (2003), the wave steepness-dependent z₀ formula from Taylor and Yelland (2001), and two wave spectrum-dependent stress formulas from Reichl et al. (2014): the RHG modified from Moon et al. (2004) and the DCCM from Donelan et al. (2012). The primary goal is to address the uncertainties in estimating surface wind stress using different methods, and propose a more physically-based, generalized method suitable for near-shore shallow water application, where mechanisms of wind-wave interaction are more profoundly complex than deep water region.