Donald E. Barrick

HF Radar Sea-echo from Shallow Water

HF radar systems are widely and routinely used for the measurement of ocean surface currents and waves. Analysis methods presently in use are based on the assumption of infinite water depth, and may therefore be inadequate close to shore where the radar echo is strongest. In this paper, we treat the situation when the radar echo is returned from ocean waves that interact with the ocean floor. Simulations are described which demonstrate the effect of shallow water on radar sea-echo. These are used to investigate limits on the existing theory and to define water depths at which shallow-water effects become significant. The second-order spectral energy increases relative to the first-order as the water depth decreases, resulting in spectral saturation when the waveheight exceeds a limit defined by the radar transmit frequency. This effect is particularly marked for lower radar transmit frequencies. The saturation limit on waveheight is less for shallow water. Shallow water affects second-order spectra (which gives wave information) far more than first-order (which gives information on current velocities), the latter being significantly affected only for the lowest radar transmit frequencies for extremely shallow water. We describe analysis of radar echo from shallow water measured by a Rutgers University HF radar system to give ocean wave spectral estimates. Radar-derived wave height, period and direction are compared with simultaneous shallow-water in-situ measurements.

When are HF-radar observed wave heights modulated by periodic tidal and inertial currents?

Two recent sets of HF radar observations in the Atlantic by SeaSondes have observed unexpected periodic modulations of significant wave height at the M2 tidal and inertial frequencies. These are examined and related to the underlying, strong currents. They are interpreted in terms of kinematic advection influences that change the first-order wave characteristics due to frequency dispersion.

Coastal Tsunami Warning with Deployed HF Radar Systems

We describe the evolution of coastal HF radar observations of tsunamis, first proposed in 1979 and developed after the 2004 Indonesia and 2011 Japan tsunamis to allow routine monitoring to detect an approaching tsunami. Oceanographic tsunami theory is summarized, both for the fundamental equations of motion and in the ray optics and Greens Law approximations; the latter can be applied when water depths are slowly varying. Observations of the current velocities caused by the 2011 Japan tsunami off the Japanese, the US West, and Chilean coasts are described and examples are shown. These observations led to the development of an empirical tsunami detection method, which is outlined. Examples of offline tsunami detections are given and detection times are compared with arrival times at neighboring tide gauges. We describe the observation and offline detection of the June 2013 meteotsunami off the New Jersey coast using coastal radar systems and tide gauges. Methods to model and simulate tsunami velocities are described and videos of the resulting velocity/height maps are given. We describe preliminary methods for evaluating the suitability of radar sites for tsunami detection using simulated tsunami velocities. Factors affecting tsunami detectability are discussed and methods are described for the alleviation of false alarms.

Estimation of HF radar mixed-media path loss using the millington method

In order to predict the performance of a coastal high-frequency surface-wave radar (HFSWR) when the antenna must be located some distance back from the ocean, measurements of the path loss over beach sand at 13 and 25 MHz were made at several locations on beaches near Rotterdam, Netherlands in September 2014. The measured mixed-media path loss was compared with estimates made using a procedure developed by Millington in 1949 and were found to agree within 1-2 dB over a total range of 30 dB, even though the distances of a few hundred meters were much shorter than the several kilometers usually employed by the Millington method. Using this method along with the measured loss, it is possible to predict the degradation in achievable range for a coastal HF radar when the path to the sea includes a significant segment of low-conductivity soil, such as dry sand. Example calculations are included for 13 and 25 MHz.

Extended-range RiverSonde operation on the Hudson River

A RiverSonde was operated during June–August 2010 along the Hudson River in New Jersey at a location about 140 m from the waters edge with the antenna about 40 m above the water level. With this configuration, usable signals were obtained all the way across the river, out to a range of 1400 m from the radar. This was considerably greater than the 300 m which had been observed in previous experiments. Initial data processing shows that the along-channel velocity had the expected tidal signature with a maximum value of approximately 1 m/s and was nearly in phase with the stage measured about 5 km downstream at a NOAA gaging station.