Assessment of WERA long-range HF-radar performance from the user's perspective

Abstract

Since April 2006, long range (8.3MHz) WERA HF radars have been operated on the Southeastern United States coastline, as part of the U.S. Integrated Ocean Observing System (IOOS) and in particular the national HF Radar network. These radars measure currents operationally, and waves and winds experimentally across the wide continental shelf of Georgia (GA) and South Carolina (SC). Half-hourly data at 3km horizontal resolution are acquired to a range of approximately 200 km, providing measurements across the wide continental shelf and into the adjacent Gulf Stream at the shelf edge. Radar performance in range and quality is discussed. Ease in siting of these space and cable intensive systems along populated coastlines, and the feasibility of their operation by non-radar specialists is also briefly discussed. Long term in situ measurements of currents, waves and winds concurrent with the long-term radar measurements were available. These measurements were also acquired as part of the U.S. national coastal ocean observatory network, under the evolving auspices of SABSOON, SEACOOS, and SECOORA. Wind, wave and ADCP measurements from several instrumented Navy towers at the 27, 33, and 44 m isobaths are available, and winds and wave information also exist at the 18m isobath from an NDBC buoy. Comparisons between radar-derived estimates and the in situ measurements are examined for a variety of parameters, including (near) surface currents, significant wave heights, directional wave spectra, and wind direction and speed. Radar estimates of surface velocity compare quite well with in situ ADCP near surface current data, with complex vector correlation magnitude of 0.95, and phase angle of -0.2 degrees. The negative sign is consistent with an expected counterclockwise rotation with depth between the radar surface current estimates and the subsurface upper water column ADCP measurements. Tidal amplitudes, which are large and predominantly semidiurnal on the GA/SC coast are extremely well reproduced by the radar estimates. Radar significant wave height estimates from manufacturer-supplied software are much noisier than measurements from in situ pressure sensors or wave buoys, but capture several-hour low-passed variability fairly well. The spectra from which the radar significant wave heights are estimated have been examined, also exhibiting higher variability than indicated by the in situ estimates. Wind direction estimates from radar using manufacturer-supplied software were evaluated for a range of wind speeds and direction (fetch limited or not) and by differentiating between conditions where the predominant water waves satisfied the long-wave assumption or not. For non-fetch limited wind speeds in excess of the Bragg wave propagation speed, and wave fields for which the long-wave assumption is relevant, correlations between radar and in situ anemometer wind directions were good, at approximately 0.7. HF-radar directional wave spectra estimates are also derived with aftermarket software from SeaView Sensing Ltd, and are compared to in situ estimates from a five beam ADCP. These novel in situ measurements use the ADCP slant beam horizontal velocities combined with vertical velocities from the vertically oriented 5th beam to measure wave orbital velocities and derive directional wave spectra. Wave buoy accelerometer estimates of directional wave spectra are also available for comparison. Radar significant wave heights and wind direction from SeaView software are also being examined.