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 ra dars produced a large surface wind direction data set covering much of the North Atlantic and Pacific oceans. We compare wind directions der ived from the sea echo Bragg line ratio with concurrent in situ measur ements made by ships and buoys and with the National Meteorological Ce nter (NMC) Global Data Assimilation System model. Using 1900 such comp arisons, we derive an empirical model for the dependence of surface wi nd direction on the Bragg line ratio of HF radar sea echoes. The rms d ifference between the empirical model and the combined NMC and in situ data is about 33-degrees. When only very high quality radar data are used, the rms difference between the radar and buoy measurements alone is 24-degrees. The pattern of surface streamlines derived from 1 day' s radar measurements over the northeastern Pacific Ocean closely match es that derived from the NMC model.