Analysis of second order HF radar sea spectra recorded in storm conditions

HF radars are routinely used to study the ocean surface under all weather conditions. Knowledge of surface current and wind direction can be obtained from the first order components of the received Doppler spectrum, but to gain information on the ocean directional wave spectrum requires the analysis of the second order radar spectrum. A problem is known to arise with the interpretation of radar signals recorded during storm conditions. At Doppler frequencies outside the first order peaks, the second order spectrum tends to fall away with a lesser slope than is predicted by theory or is observed in calmer sea conditions. In this paper the authors present an analysis of this effect observed in data recorded by the WERA system during storms in late 1996. The variation of the effect with wind direction, sea state and storm evolution is shown