Observation-Based Physics in Spectral Wave-Forecast Models

Major update of the physics of the third generation models will be presented. The new source terms for wind input, whitecapping dissipation, interaction of waves with adverse winds (negative input) and swell attenuation have been developed and implemented in WAVEWATCH-III, SWAN and WAM models. Physics and parameterisations for the new source functions are based on observations, which allowed us to reveal features and processes previously unknown and not accounted for. For extreme conditions, physics of the wind input and whitecapping dissipation terms exhibit additional features irrelevant or inactive at moderate weather.

In particular, the wave growth term was shown to be a nonlinear function of wave steepness (spectral density). Additionally, the wave breaking was found to enhance the wind input. Relative reduction of the wind input at strong-wind/steep-wave conditions was observed, due to full flow separation found at such circumstances. At strong wind forcing, this causes saturation of the sea drag.

Spectral distribution of the whitecapping dissipation is the most elusive function to measure. Breaking of waves, and hence such dissipation exhibits a clear threshold behaviour in terms of wave steepness (or saturation spectrum). Other novel observed features are cumulative effect away from the spectral peak (dissipation is not local in wavenumber space), directional bimodality. It was found that at moderate winds the dissipation is fully determined by the wave spectrum whereas at strong winds it is a function of the wind speed.

In absence of breaking (swell or other circumstances when the spectral density is below the threshold), other energy sink has to be invoked. It is based on observations of wave-turbulence interactions, and dependence of such interactions on wave steepness.

Interaction of the waves with adverse wind is a necessary additional term if the above-mentioned wind input function is employed, since this function only describes forcing of waves by the following wind. These dependences are calibrated by means of observations in tropical cyclones.

In order to test the source functions independently, and control the flux balance in the model, additional observation-based constraints are implemented. At each time step, the total momentum input is verified to match an independently known wind stress.

Qualitative and quantitative effects and properties of the observation-based source terms are parameterised, and the parameterisations are presented in forms suitable for spectral wave models. The new versions of the models have undergone extensive testing by means of academic tests, regional and global wave hindcast, modelling extreme conditions ranging from tropical cyclones to the marginal ice zone.