Dependence of 25-MHz HF Radar Working Range on Near-Surface Conductivity, Sea State, and Tides

Abstract

AbstractA 1.6-yr time series of radial current velocity from a 25-MHz high-frequency radar system located near a coastal river plume is analyzed to determine how the working range varies in response to changing near-surface conductivity, sea state, and tides. Working range is defined as the distance to the farthest radial velocity solution along a fixed bearing. A comparison to spatially resolved near-surface conductivity measurements from an instrumented ferry shows that fluctuations in conductivity had the largest impact of the three factors considered. The working range increases nearly linearly with increasing conductivity, almost doubling from 19.4 km at 0.9 S m−1 to 37.4 km at 3.5 S m−1, which yields a slope of 7.0 km per S m−1. The next largest factor was sea state, which was investigated using measured winds. The working range increases linearly at a rate of 1 km per m s−1 of wind speed over the range of 0.5–6.5 m s−1, but it decreases weakly for wind speeds higher than 7.5 m s−1. Finally, a power spectrum of the working range time series reveals variability at tidal frequencies. Tides cause about 3 km of range variation; however, the mechanism(s) underlying this are not known explicitly. Evidence for both sea level height and the interaction of tidal currents with sea state are presented.