An empirical relation between ocean-surface wind direction and the Bragg line ratio of HF radar sea echo spectra

Twelve days of tests with six U.S. Air Force OTH-B over-the-horizon radars produced a large surface wind direction data set covering much of the North Atlantic and Pacific oceans. We compare wind directions derived from the sea echo Bragg line ratio with concurrent in situ measurements made by ships and buoys and with the National Meteorological Center (NMC) Global Data Assimilation System model. Using 1900 such comparisons, we derive an empirical model for the dependence of surface wind direction on the Bragg line ratio of HF radar sea echoes. The rms difference between the empirical model and the combined NMC and in situ data is about 33°. When only very high quality radar data are used, the rms difference between the radar and buoy measurements alone is 24°. The pattern of surface streamlines derived from 1 day's radar measurements over the northeastern Pacific Ocean closely matches that derived from the NMC model.