Determination of Sea Surface Wind Speed Using High Frequency Radar

High frequency radars have the unique capability of continuously monitoring ocean surface environments at ranges up to 300 km off the coast. In addition to providing reliable ocean surface currents and ocean waves (under slightly stricter conditions), the ability to obtain sea surface wind speeds from high frequency radar backscatter spectra is an important feature of such a radar, which will enhance its contributions to a meteorological or oceanographic observation network. However, estimation of wind speed poses a more difficult challenge because the radar doesn't measure wind directly. Wind vectors must be inferred from the wave measurements in the light of the knowledge concerning the processes of wind-wave generation. This paper initially provides a review of previous attempts for extracting wind speed. The strength and limitation of each existing method is discussed. Many of the previous methods establish empirical relationships using the dataset obtained under specific conditions. Thus, they are only viable for a certain sea state, ocean region, topography, or wind condition, and suffer from the lack of generalization and theoretical foundation. In addition, these methods often assume that the ocean wave spectrum is in equilibrium with the prevailing wind, or that the particular spectral parameters chosen best represent, in some gross way, the development of the sea. Both assumptions ignore the complex processes of wind-wave generation, e.g. developing wind seas. Based on this discussion, an enhancement of an existing inversion scheme for extracting wind speeds is presented. Closed form expressions relating the wind speed to the location of the peaks of the second-order Doppler spectrum are derived for fetch-limited seas to account for developing sea. Finally, wind speeds retrieved from experimental data are illustrated to confirm the validity of the scheme.