On the air-sea coupling in the WAM wave model

Wind generates waves on the surface of the ocean. Modifications of the sea state influence the wind profile just above it. The current WAM wave model, Cycle 4, incorporates the coupling back of the waves to the wind by modifying the Charnock constant by a coupling parameter defined as the ratio of the wave-induced stress to the total momentum flux. Shortcomings of the numerical implementation of the coupling are discussed. This coupling module is compared to one using a fixed Charnock constant which is shown to produce equally accurate significant wave heights. An alternative air-sea coupling parameterization is put forward, replacing the present wind input source function in the WAM model. It uses recently developed physics to calculate the momentum fluxes directly, without applying a Charnock type relation. A more accurate pre-described high-frequency tail of the wave spectrum is used to calculate the wave-induced stress. An iterative parameter tuning method is presented to estimate control parameters occurring in the air-sea coupling modules. It minimizes a cost-function representing the modelling error using a finite difference approach together with the Levenberg-Marquardt optimization method. Model experiments are performed for the North Sea to tune, validate and intercompare the alternative wind-wave coupling parameterizations in the WAM model. The impact on calculated wave parameters like significant wave height is studied.