American Meteorological Society 14 th Conference on Interactions of the Sea and Atmosphere 30 January – 2 February 2006, Atlanta , Georgia 8.2 USE OF REAL-TIME HIGH FREQUENCY RADAR OBSERVATIONS TO ESTIMATE WINDS THAT CAN BE USED AS PART OF ON-LINE OBJECTIVE ANALYSES IN CALIFORNIA COASTAL REGIONS Jessica Drake and John F. Vesecky Electrical Engineering Dept., University of California at Santa Cruz, 1156 High St., Santa Cruz CA 95064 831-459-4099, fax 831-459-4829, vesecky@soe.ucsc.edu Francis L. Ludwig Environmental Fluid Mechanics Laboratory, Stanford University, Stanford, CA 94305-4020 650-725-5948, fax 650-725-9720, fludwig@stanford.edu Douglas Sinton Meteorology Dept., San Jose State University, One Washington Square, San Jose, CA 95192-0104 408-924-5181, fax 408-924-5191, sinton@met.sjsu.edu Jeffrey D. Paduan Dept. of Oceanography, Naval Postgraduate School, 833 Dyer Rd., Monterey, CA 93943 831-656-3350, fax 831-656-2712, paduan@nps.edu I. Introduction Winds in coastal areas are both important and difficult to measure. In the San Francisco and Monterey Bay areas, several systems offer data via real-time web access. These include high frequency (HF, decameter wavelength) ground-wave radars at multiple frequencies and on shore and offshore (buoy) anemometers. This paper contains a brief review of our empirical technique for measuring ocean winds with surface-wave HF radar, and reports how we integrate those winds into a system for objective analysis of routine meteorological information and finally produce a wind field map over the San Francisco and Monterey Bay areas – land and sea. HF radar has established itself as a useful tool for observing near surface currents in the coastal ocean. Radar observations of ocean currents are not directly related to winds, but the shear in surface currents results from wind stress at the surface. Current shear can be estimated from radar measurements at multiple frequencies (Meadows, 2002), making it reasonable to consider estimating winds from radar data. The M0 and M1 buoys (see Fig, 1) can provide independent data for building and testing empirical wind estimation methods, The buoys are maintained by the Monterey Bay Aquarium Research Institute. (MBARI) Onshore anemometer data obtained from many sources around San Francisco Bay are archived by the U. S. Geological Survey (USGS) and San Jose State University (SJSU). These archives also include several sites around Monterey Bay that can be used with an objective analysis computer program to estimate winds over both the San Francisco and Monterey Bays. The inclusion of wind estimates from radar observations adds important information for the analyses (Vesecky et al., 2005). The objective analysis of all these merged data is the final product that is published in real-time via the WWW. 2. Methods 2.1 Measurement of ocean surface currents and winds with high-frequency radar High frequency, ground-wave radar is useful for observing near surface currents in the coastal ocean (e.g. Barrick et al. 1985). The radars detect currents because constructive interference gives returns almost exclusively from a single (Bragg resonant) ocean wavelength equal to half the radar wavelength. Oversimplifying, the radar deduces radial current components from the difference between the radar return’s Doppler shift and the expected Doppler shift due to the theoretical gravity wave speed (in the absence of surface current) for the observed ocean wavelength. Effective depth of the current measurement depends on the radar wavelength, with longer waves feeling the current to greater depths. Theoretical (Stewart and Joy, 1974) and empirical (Teague et al. 2001) relationships have been developed between effective current measurement depth and wavelength. Multiple radars observing the same area of the ocean, with some straightforward trigonometry, provide estimates of the two dimensional current Corresponding Author Address: Jessica A. Drake, Electrical Engineering Dept., University of California at Santa Cruz, 1156 High St., Santa Cruz CA 95064, Jessica.Drake@gmail.com