Calvin C. Teague

A comparison of multifrequency HF radar and ADCP measurements of near-surface currents during COPE-3

A high-frequency multifrequency coastal radar operating at four frequencies between 4.8 and 21.8 MHz was used as part of the third Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and November, 1997. The radar system surveyed the open ocean east of the coast and just south of the mouth of Chesapeake Bay from two sites separated by about 20 km. Measurements were taken once an hour, and the eastward and northward components of ocean currents were estimated at four depths ranging from about 0.5 m to 2.5 m below the surface for each location on a 2 by 2 km grid. Direction of arrival of the signals was estimated using the MUSIC algorithm. The radar measurements were compared to currents measured by several moored acoustic Doppler current profilers (ADCPs) with range bins 2-14 m below the water surface. The vertical structure of the current was examined by utilizing four different radar wavelengths, which respond to ocean currents at different depths, and by using several ADCP range bins separated by 1-m intervals. The radar and ADCP current estimates were highly correlated and showed similar depth behavior, and there was significant correlation between radar current estimates at different wavelengths and wind speed.

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

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.

Multifrequency HF radar observations of currents and current shears

Techniques have been developed for using high-frequency (HF) surface-wave radar to measure ocean currents and vertical current shears in the upper 1 or 2 m of the ocean surface. An HF radar can precisely measure the phase velocity and direction of propagation of ocean waves whose wavelength is one.half the radar wavelength. In the absence of a current, the speed of the waves is given by the still-water dispersion relation. An underlying current will modify this speed. The radar measures the actual phase velocity through a Doppler shift, and the wavelength of the ocean wave is known through the first-order Bragg scattering relation, so a difference between observed and theoretical stillwater phase velocity can be calculated. In addition, longer ocean waves are affected by currents at deeper depths than are shorter ocean waves. By measuring the phase velocity at several different wavelengths, it is possible to measure a vertical current shear in the top 1 or 2 m of the ocean surface. This is a measurement that is very difficult to make by any other means. A portable coherent pulsed-Doppler HF radar system was developed at Stanford and used in several experiments, both on land on the California coast and on board a ship during the Joint Air-Sca Interaction (JASIN) experiment. The land-based experiments demonstrated that a current could be measured by an HF radar, and that its value agreed well with that measured by in-situ drifting spar buoys. In addition, there was evidence of a vertical current shear, both from the radar measurements and from the buoy measurements. The JASIN experiment was an attempt to apply these techniques to the measurement of surface current and current shear in the open ocean. The radar system was installed on board a ship, along with a receiving antenna consisting of a steerable phased array of eight wide-band loops. The steerable antenna was quite rugged and performed as expected. It produced antenna patterns consistent with the physical aperture of the array. The wind velocity during the JASIN experiment was quite low, so wind- and wave-generated currents were quite small. Nevertheless, there is some evidence of a current shear. Its magnitude is small and near the resolution limit of the radar, but it appears to be somewhat higher than estimates based on either the wind or wave conditions alone, but less than the estimates based on the sum of the two components.

Surface current measurements by HF radar in freshwater lakes

HF radar has become an increasingly important tool for mapping surface currents in the coastal ocean. However, the limited range, due to much higher propagation loss and smaller wave heights (relative to the saltwater ocean), has discouraged HF radar use over fresh water, Nevertheless, the potential usefulness of HF radar in measuring circulation patterns in freshwater lakes has stimulated pilot experiments to explore HF radar capabilities over fresh water. The Episodic Events Great Lakes Experiment (EEGLE), which studied the impact of intermittent strong wind events on the resuspension of pollutants from lake-bottom sediments, provided an excellent venue for a pilot experiment. A Multifrequency Coastal HF Radar (MCR) was deployed for 10 days at two sites on the shore of Lake Michigan near St. Joseph, MI. Similarly, a single-frequency CODAR SeaSonde instrument was deployed on the California shore of Lake Tahoe. These two experiments showed that when sufficiently strong surface winds (2 about 7 m/s) exist for an hour or more, a single HE radar can be effective in measuring the radial component of surface currents out to ranges of 10-15 km. We also show the effectiveness of using HF radar in concert with acoustic Doppler current profilers (ADCPs) for measuring a radial component of the current profile to depths as shallow as 50 cm and thus potentially extending the vertical coverage of an ADCP array.

Detecting upwelling along the central coast of California during an El Niño year using HF-radar

Abstract Surface currents in the vicinity of Granite Canyon, California (36°25.9′N, 121°55.0′W), were measured hourly using HF-radar in 1990–1992. The 1990 data revealed the M 2 and S 2 semi-diurnal tidal constituents. These high frequency components were removed from 6-month records taken during part of the upwelling season of 1991 and 1992. Daily and weekly variations in current speed and direction were generally similar in 1991 and 1992 even though 1992 was an El Nino year. Correlation analysis revealed that in both years daily and weekly variation in currents were mostly explained by corresponding changes in the alongshore component of the wind stress, indicating the effects of coastal upwelling. Variation in sea surface temperatures adjacent to the coast were correlated with the currents generated by coastal upwelling in 1991, but not in 1992. These observations are consistent with the hypothesis that during an El Nino event the water is upwelled to the surface from above a depressed thermocline. In 1992, at Granite Canyon, normal coastal upwelling was superimposed upon the El Nino event of that year.

Bistatic-Radar Observation of Long-Period, Directional Ocean-Wave Spectra with Loran A

Bistatic-radar scattering from medium- to long-wavelength (80 to 200 meters) ocean waves has been observed with the use of loran A (1.85 megahertz) transmissions and a receiver located 280 kilometers away. The received echoes have been converted into a time-delay, Doppler-frequency map in which the effects of anisotropies in the ocean-wave spectra are clearly shown. The distribution of the echoes in delay-Doppler space is consistent with Bragg scattering from trains of dispersed ocean waves.

Second-Order Scattering from the Sea: Ten-Meter Radar Observations of the Doppler Continuum

Ten-meter radar observations of the sea have been used to study second-order interactions between waves in electromagnetic scattering from the sea. Techniques of coherent, pulsed radar provide echo frequency spectra from several range intervals. The echo spectra are resolved with an analysis window of a few millihertz. These spectra show a clear second-order echo continuum which appears as sidebands about the first-order Bragg scattering lines. Up to one-half of the total echo power has been observed in these sidebands. The principal characteristics of these sidebands vary with time, apparently in response to the sea state. The form of the echo spectra is consistent with the results of perturbation theory computations based on Rices method.

Canal and river tests of a RiverSonde streamflow measurement system

Results of field tests of a RiverSonde streamflow radar are compared with in-situ current measurements at a canal and a river in central California during June, 2000. Typical water velocity in the middle of the canal was about 0.45 m s/sup -1/ and 0.30 ms/sup -1/ at the edges. Velocity in the river was about 20% lower with similar cross-channel variation. Differences between the RiverSonde and in-situ velocities were 6-18% of the mean flow, with similar differences among the various in-situ velocities. In addition to the surface velocities, the total volume flow was estimated based on the in-situ depth measurements. Volume flow for the canal was about 37 m/sup 3/ s/sup -1/ and for the river was about 64 m/sup 3/ s/sup -1/, with differences between the various radar and in-situ techniques of less than 10%.

Initial river test of a monostatic RiverSonde streamflow measurement system

A field experiment was conducted on May 7-8, 2002 using a CODAR RiverSonde UHF radar system at Vernalis, California on the San Joaquin River. The monostatic radar configuration on one bank of the river, with the antennas looking both upriver and downriver, provided very high-quality data. Estimates of both along-river and cross-river surface current were generated using several models, including one based on normal-mode analysis. Along-river surface velocities ranged from about 0.6 m/s at the river banks to about 1.0 m/s near the middle of the river. Average cross-river surface velocities were 0.02 m/s or less.

Bistatic-Radar Techniques for Observing Long-Wavelength Directional Ocean-Wave Spectra

Bistatic-radar Bragg scattering of medium-to long-wavelength radio waves by ocean waves can be used as a means for observing directional ocean-wave spectra. Only moderate antenna directivity is required; areal and directional resolution are provided by high-resolution delay-Doppler processing of the radar echoes. Directional characteristics of long-wavelength (80-200 m) ocean waves have been observed using LORAN A transmission (1.85 MHz) and a receiver located 280 km from the transmitter. The received signals have been converted into a time-delay Doppler-frequency map and into a plot of normalized radar cross section ?0, as a function of directional ocean-wavenumber.