Measurements of Surface Waves Using Low-Grazing Angle High-Resolution Pulse-Doppler Radar

Abstract

Techniques for extracting surface wave characteristics from radar backscatter have been investigated and improved over the last several decades. Much of this research has focused on the use of backscatter intensity from navigational radars for characterization of wave period and direction, and has clearly demonstrated accurate measurement of these wave characteristics. However, the precise determination of significant wave height has been more problematic due to the required application of a modulation transfer function. Furthermore, low sea states generally do not provide enough backscatter intensity for evaluation of wave characteristics, and thus, navigational radar measurements are restricted to relatively high sea states. More recently, techniques using Doppler velocities as a measurement of surface waves have been an area of increasing focus and development. An advantage of this approach is that no modulation transfer function is required, and only phase information is used from the backscattered radar signal. Recent research suggests that the relationship between Doppler velocities and wave height may be more consistent than that between radar backscatter intensity and wave height. In July 2010, surface waves were measured during an experiment at the Scripps Institution of Oceanography pier. Radar measurements were performed using a high-resolution pulse-Doppler instrumentation radar at low grazing angle (∼1 deg) with a pulse repetition frequency of 800 Hz and spatial resolutions of 10–30 cm. Radar data for X- and Ku-bands using both VV and HH polarizations were collected. Concurrent buoy measurements were also performed, along with the collection of wind speed and direction data. Measured seaways consisted of small significant wave heights (glassy conditions to <1 m), and contained combinations of wind sea and swell. Doppler processing of the radar data provided estimates of surface wave orbital velocity spectra in wavenumber and frequency domains. The velocity spectra were converted to sea surface elevation spectra. Using these spectra, peak periods were computed as well as RMS wave heights, thus providing approximate significant wave heights. The methods for extracting wave spectra, peak periods, and significant wave heights are discussed, and results are compared with buoy measurements. When sufficient capillary waves existed on the sea surface, the radar and buoy measured wave spectra were in agreement, and analysis indicates that the instrumentation radar was able to detect and spectrally distinguish between wind seas and swell.