Abstract. The validation of estimates of ocean surface current speed and direction from high-frequency (HF) Doppler radars can be obtained through comparisons with measurements from moored near-surface current meters, acoustic Doppler current profilers, or drifters. Expected differences between current meter (CM) and HF radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations, sampling characteristics, mooring response, and geophysical variability. We classify these sources of errors and differences as being associated exclusively with the current meter, as being associated exclusively with the HF radar, or as a result of differing methodologies in which current meters and HF radars sample the spatially and temporally varying ocean surface current vector field. In this latter context we consider three geophysical processes, namely, the Stokes drift, Ekman drift, and baroclinicity, which contribute to the differences between surface and nearsurface vector current measurements. The performance of the HF radar is evaluated on the basis of these expected differences. Vector currents were collected during the High Resolution Remote Sensing Experiment II off the coast of Cape Hatteras, North Carolina, in June 1993. The results of this analysis suggest that 40%-60% of the observed differences between near-surface CM and HF radar velocity measurements can be explained in terms of contributions from instrument noise, collocation and concurrence differences, and geophysical processes. The rms magnitude difference ranged from 11 to 20 cm s -1 at the four mooring sites. The average angular difference ranged between 15 ø and 25 ø of which about 10 ø is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams. IntroductionA well-studied remote sensing technique to observe ocean currents is the Doppler radar technique originally described by Crombie [1955]. Crombie observed that the Doppler spectrum of the sea echo consisted of two distinct peaks symmetrically positioned about the radar frequency. These peaks arise when radar pulses are backscattered from the moving ocean surface through the Bragg scattering mechanism. In particular, the radar pulses scatter at small grazing angles from resonant surface waves traveling toward or away from the radar with precisely one half the radar wavelength. The two Doppler peaks resulting from the wave components are displaced according to the phase velocity of the surface waves. Ocean surface gravity waves of a given wavelength travel at a constant velocity in deep water. Stewart and Joy [1974] showed that the small but finite displacement of the Doppler peaks from their expected positions is then related to the underlying current flow.The concept of using high-frequency (HF) radio pulses to probe the ocean surface to deduce near-surface currents has

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[1] Among the most important parameters needed to evaluate the present and future state of Antarctic sea ice cover is the ice thickness. The retrieval of ice thickness using remote sensing techniques has been hampered by the absence of a capability to remotely measure snow thickness covering the sea ice. Data sets collected with Johns Hopkins Applied Physics Laboratory's Delay-Doppler Phase Monopulse (D2P) radar and NASA's Airborne Topographic Mapper (ATM) scanning lidar during NASA's Antarctic AMSR-E Sea Ice field campaign over the Bellingshausen Sea are used to demonstrate the potential of a remote-sensing technique for retrieval of snow cover thickness from an airborne platform. The technique takes advantage of the fact that the radar is most sensitive to the snow-ice interface while the lidar responds to the highly optically reflective snow surface. The difference between the radar-and laser-determined surfaces yields an estimate of snow thickness that appears to be reasonably consistent with expected values. The technique requires careful registration of the instrument footprints. Because there was no absolute range calibration of the lidar due to predeployment scheduling difficulties, the vertical offset between the instruments was resolved by determining the difference between measurements over leads. Elsewhere over sea ice the radar-defined surface is generally below the laser-defined surface consistent with the radar defining the snow-ice interface. Over a relatively small portion of the data, we observed opposite relationship between the sensor-defined surfaces for which we discuss a plausible physical explanation.

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