HF radar comparisons with moored estimates of current speed and direction: Expected differences and implications

Abstract

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

Remote sensing of ocean surface currents: a review of what is being observed and what is being assimilated

“…For example, it has been suggested that HF radar currents include either the entire wave-induced Stokes drift (Graber et al, 1997), part of it or none of it (Röhrs and Christensen, 2015). In their work, Röhrs and Christensen (2015) compare HF radar currents with two types of surface drifters: seven iSphere drifters without drogue (found to be driven by the Eulerian current and the Stokes drift at the surface) and seven CODE-type drifters (following the ocean current at 1 m depth).…”

The Influence of Fetch on the Response of Surface Currents to Wind Studied by HF Ocean Surface Radar

“…Their results indicate that the surface currents measured by the HF radar should respond to Stokes drift from all waves with wavelengths longer than the Bragg waves. Many recent research projects were conducted with the assumption that Stokes drift is present in the HF radar surface current data (Graber and Haus 1997;Gremes-Cordero et al 2003;Chapron et al 2005;Ullman et al 2006). Ullman et al (2006) used two ranges of the Coastal Ocean Dynamics Applications Radar (CODAR) measurements with different effective depths (ϳ0.5 and ϳ2.4 m), and compared the radar data with the drifters at a depth of 0.65 m. The comparison suggests the existence of Stokes drift in the HF radar data and the importance of effective depth of radar measurements.…”

Improved Detection of the First-Order Region for Direction-Finding HF Radars Using Image Processing Techniques

“…While the total reductions in rms difference were small relative to the absolute rms differences (i.e., 5%-10%, Table 3), they still represent a potentially significant part of the increase in rms difference, or error, above the lower error bound of 5-6 cm s 21 observed for HF radars (Paduan and Washburn 2013). In terms of this lower bound, previous works have found that the direction-finding algorithm itself contributes 3-4 cm s 21 of error (Laws et al 2010) and the near-surface shear between the radar at the surface and the ADCP below can account for 2-3 cm s 21 of rms difference (Graber et al 1997). The error reduction seen here, which exists in addition to these and other sources of rms difference or error, should be considered to be one aspect of the total error budget for radial velocities.…”