HF Radar wave-spectrum assimilation using SWAN and application for the CASPER-West Experiment

Spectral wave models such as SWAN (Simulating WAves Nearshore) can provide local high-resolution wave forecast fields. These forecasts are often inadequately constrained by observational data. The assimilation of wave observations from high-frequency (HF) radar sites can result in increased forecast fidelity. In order to assimilate HF radar wave observations into SWAN, a forward model for the HF Doppler spectrum, based on first- and second-order Bragg scattering, was developed. This model uses the spatially varying wave spectra computed using the SWAN model, forecast currents from the Navy Coastal Ocean Model (NCOM), and HF radar system parameters to predict time-varying range-Doppler maps for stations of interest. Using an adjoint of the HF radar model, the error between these predictions and the corresponding HF Doppler spectrum observations can be translated into effective wave-spectrum errors for assimilation in the SWAN model.

Testing and validation are being conducted using data collected at ten HF radar sites in the Southern California Bight during the CASPER-West experiment in October 2017. Recently, the SWAN model was updated to version 41.20 to include non-breaking dissipation, observation-consistent wind input, and white capping physics, necessitating an update to the HF radar wave assimilation as well. Comparisons of the HF range-Doppler spectrum predictions to the observed data are favorable but additional testing is needed to validate the results from the improved wind-wave physics included in SWAN 41.20. Supported by Office of Naval Research Contract No. N00014-17-C-7021.