HF radar has become an important tool for mapping surface currents in the coastal ocean. It is well known that HF radars are capable of measuring wind direction by using the relative strength of the echoes from the approaching and receding ocean waves at the Bragg resonant wavelengths. Here we examine the ability of multifrequency HF radar to measure wind speed as well as direction. In this study we use data collected over Monterey Bay, California in the late summer of 2000. At that time there were two buoys in the radar's observational area that were capable of measuring wind speed and direction - one near the Bay mouth and one nearer the shore. We investigate the relationship between the wind speed and the near surface currents and Bragg line ratios as measured by two multifrequency HF radars near Santa Cruz and Moss Landing, California. These radars operated at 4.8, 6.8, 13.4 and 21.8 MHz, measuring currents at effective depths of 2.5, 1.8, 0.9 and 0.6 in respectively. The method of partial least squares is used with results for speed of a standard error of prediction (SEP) of approximate to 1m/s, a bias of approximate to 0.5 m/s and an R-2 of approximate to 0.8. For direction the SEP approximate to 40degrees, the bias approximate to 4degrees and R-2 of approximate to 0.45. These preliminary results suggest that wind speed as well as direction can be estimated from multifrequency HF radar data.

Monterey Bay provides an interesting location for studying air-sea interaction under a cyclic, land-sea breeze, circulation in the atmosphere above the bay. The strong diuranal cycle allows investigation of the response of near-surface currents to large, periodic wind fluctuations. The observations used in this analysis were collected in the summers of 1997 and 2000. During both summers an air-sea measurement buoy M1 was active in the mouth of Monterey Bay and in 2000 a specially instrumented 'flux buoy', FB, was also deployed in the east-central portion of Monterey Bay. Two multifrequency coastal radars (MCRs) operating at 4.8, 6.8, 13.4 and 21.8 MHz were sited at Moss Landing and Santa Cruz, California, collecting current maps at hourly intervals. We present observations and analyses in two areas: the center of the bay mouth (near the M1 mooring) and 8 km off the northeast shore of the bay (near the flux buoy FB mooring). These observations occurred during periods of strong land-sea breeze circulation in June of 1997 and Aug./Sept. of 2000. At the two buoy sites we study correlation between the surface wind vector and the current vectors at effective depths of about 0.5,, 1, 2 and 3 in as sensed by the different radar frequencies. We find the strongest correlation between the wind and the currents nearest the surface and that the surface currents are rotated about 40/spl deg/ clockwise (as seen from above) with respect to the wind vector, i.e. in the same sense as the Ekman spiral. We also study the spatial coherence of the surface current response to the surface winds.

The authors present three aspects of current HF radar research. First, they examine the consistency of measurements by HF ground wave radars with different designs, but operating on the same physical principles. This is done using data from the commercially available SeaSonde (Codar Ocean Systems) and from the Multifrequency Coastal Radar (MCR), which is a research system. Data from the two systems are compared for co-located units at Santa Cruz and Moss Landing CA on Monterey Bay. They conclude that the two systems make current-vector-field measurements that are consistent to an accuracy of better than 10 cm/s and that the data from two such systems can be integrated to form reliable composite current maps. Second, they present results from an air-sea interaction investigation using the MCR systems on Monterey Bay during 1997 and 2000. We show that near surface currents are correlated with the wind with correlation coefficients 0.6 and are rotated 35 to 45/spl deg/ with respect to the wind in the sense of the Ekman spiral. They also show results of near shore observations on Lake Michigan during the EEGLE campaign of 2000. These measurements show the capability of HF radars to operate over fresh water.