Coupling of COAMPS and WAVEWATCH with Improved Wave Physics

Abstract: The Model Coupling Executable Library (MCEL), developed at the University of Southern Mississippi's Center of Higher Learning, has been successfully used to couple the Coupled Ocean/Atmospheric Mesoscale Prediction System (COAMPS) and the ocean wave model WAVEWATCH. An example of its application is shown for Hurricane Gordon, showing that two-way coupling results affects boundary layer physics differently than one-way coupling --- in this case, resulting in larger z_o and, consequently, larger surface fluxes and a more intense hurricane. However, since analyzing MCEL is difficult because the wave physics is inaccurate, improvements to the wave algorithms are also part of the deliverables. A new analytical expression for the wind/wave growth factor has been derived based on normal modes analysis and rapid distortion theory valid for all wave regimes except for tropical cyclone conditions. This new algorithm is validated against a numerical simulation of the Reynolds-stress transport equations and matches well. In contrast, other wave growth expression used in ocean models like the WAve Model (WAM) and WAVEWATCH do not produce the same results, with larger wave growth values peaking at smaller wave age values. These differences are attributed to the application of curve fitting by the other algorithms, while the new formulation is an analytical expression derived from first principles and includes factors missing in previous schemes such as turbulent interaction. If the Reynolds-stress transport equations solutions are reasonably accurate, it indicates that all the previous wave growth schemes, including WAVEWATCH, have serious deficiencies. Another unique result from this work includes a second analytical wave growth formulation valid for tropical cyclone conditions.