Multifrequency HF radar observations of currents and current shears

C. Teague
1986 IEEE Journal of Oceanic Engineering  
During the period of January1975 to January 1982,Tchniques were developed for using high-frequency (HF: 3-3 MHz) radar to measure ocean currents and vertical current shears in the upper one or two metres of the ocean surface. The measurement of current velocity with an HF radar depends on the ability of the radar to precisely measure the phase velocity and direction of propagation of ocean waves whose wavelength is one-half the radar wavelength. Ip the absence of a current, the waves travel at
more » ... speed given by the still-water dispersion relation. An uiderlying 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 still-water 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 one or two metres 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* Se u4and used in several experiments, both on land on the California coast and on board a ship during the JASIN experiment. Two different antenna systems were developed during the course of the experiments. The land-based experiments used a balloonsupported vertical half-rhombic transm ting antenna and a single wideband receiving loop. The shipborne experiment used a multi-element omnidirectional transmitting antenna with a steerable phased array c nsisting of eight wideband loops for the receiver. Land-based experiments conducted at Peseadero, California 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 equipment and data processing techiques were developed during the Pescadero iii experiments. 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 successfully installed on board a ship with tolerable interference to the other ship-*board activities. 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 about 50% higher than what would be expected from the wind and wave conditions present at the time of the experiments.
doi:10.1109/joe.1986.1145178 fatcat:cm3ly2ad3vhxfj6upqalpwm3ym