Measurements of upper ocean surface current shear with high‐frequency radar

Abstract

Estimates of the variation with depth of both the speed and direction of near‐surface ocean currents within the top meter have been made using time series data collected from a dual‐frequency, high‐frequency radar located on the California coast, south of Monterey. Long‐term averages derived from this data set over 3 different years consistently revealed an expected reduction in the speed of the near‐surface current as a function of depth, consistent with a logarithmic vertical current profile. An unexpected and previously unmeasured clockwise rotation with increasing depth of the near‐surface current was also observed. Hypotheses are suggested for the apparent rotation.

Skill Assessment of HF Radar–Derived Products for Lagrangian Simulations in the Bay of Biscay

“…The differences observed between HF radar data and moored ADCP velocities in stratified conditions for a longer time series (2009-11) were similar and higher than those observed in well-mixed periods. As already discussed in Solabarrieta et al (2014), when analyzing the comparisons between the drifters used here and the HF radar data, it has to be kept in mind that the effective averaging depth for surface current measurements by HF radars has been estimated as 5%-16% of the wavelength of the backscattering surface waves (Barrick 1977;Fernández et al 1996;Stewart and Joy 1974). For the Basque Country system (4.5 MHz), the measurements made are expected to integrate currents vertically within the first ;2 m of the water column.…”

HF Radar Instruments, Past to Present

“…Recent results from an experiment conducted at Duck. NC in 1994 show wind direction maps obtained with OSCR (Fernandez et al, 1997).…”

“…Ha (1979) used the multifrequency Stanford radar with a highly directional transmitting antenna to measure currents along its boresight and compared his measurements with drifting spar buoys. The same radar was used with a phased-array receiving antenna at Granite Canyon, south of Monterey, to study the effects of upwelling along the California coast (Fernandez, 1993;Shkedy et al, 1995: Fernandez et al, 1996. Maresca et al (1980) examined tidal currents in the San Francisco Bay.…”

Surface waves on currents with arbitrary vertical shear

“…To treat profiles with arbitrary current depth-dependence, various approximation techniques have been developed, typically involving expansions in a small parameter representing the magnitude of the current velocity relative to the phase velocity of the waves 2,6-8 , or the departure from a velocity potential solution 9 . These methods have been used for many practical calculations such as inferring the background current from phase velocity measurements [2][3][4][5] , yet complications occur when applying them to problems involving the entire wave-spectrum as their accuracy is difficult to predict a priori and can suffer in certain wavelength regimes. Many problems such as the above-mentioned ship waves and ring waves are conveniently solved in Fourier space, whereupon integration over all horizontal wave vectors is performed, and a fast dispersion calculation method giving the same approximation accuracy over the entire wave-spectrum at little extra cost is thus desired.…”

“…Furthermore, measurements of wave frequencies at known wavelengths (e.g. using high-frequency radar) can be used to infer the underlying current profile [2][3][4][5] , relevant for predicting storm surges and understanding the mechanisms of climate change 5 . Many studies and models in these areas have used simple velocity profiles such as depth-uniform or linear depth dependence, largely due to mathematical tractability as analytical solutions exist only for a select few of these current profiles 1 .…”