Multifrequency HF Radar Observations of Currents and Current Shears

摘要

Techniques were developed for using high frequency radar to measure 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. In the absence of a current, the waves travel at a speed 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 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 and used in several experiments, both on land on the California coast and on board a ship during a JASIN experiment. Two different antenna systems were developed during the course of the experiments.