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SURLITOP experiment in West Brittany, France: results and validation
byVéronique COCHIN2, Nicolas THOMAS1, Vincent MARIETTE1, Klaus-Werner GURGEL3
This paper presents sea surface currents, wind direction and significant wave height mapping obtained by HF ground wave radar during the SURLITOP experiment and comparisons made between these data and in situ measurements from mooring sensors (ADCP, swell buoy) or onshore station. Comparisons with sea currents integrated on the water column simulated by numerical model MARS 2D (© IFREMER) have also been done.
HF WERA (WEllen RAdar) system was deployed in West Brittany (Iroise Sea, France) for the program SURLITOP (SURveillance LITtorale OPérationnelle), financed with half by the French Ministry for Research, coordinated by Actimar, included different partners such as the University of Hamburg Remote Sensing Group, BOOST Technologies, LSEET (Laboratoire de Sondages Electromagnétiques de l´Environnement Terrestre), the French Hydrographic Office (SHOM) and IFREMER. For this experiment, we used two HF Doppler radars working in simultaneous FMCW mode (one radar with a chirp going up and the other radar with a chirp going down), with 100kHz bandwidth, scanned between 12.190MHz and 12.565MHz (2 minutes scan before each data acquisition). In this operational mode, the radar yields a spatially (radial resolution: 1.5km, beam width: 8.5° using 16 antennas in beam forming, integration depth: 0.96m), and temporally (integration time: 9 minutes, 2048 chirps of 0.26 secondes) averaged measurement on a 2 by 2km grid, every 12 minutes sample intervals. Surface currents maps were sent on the Internet site of Actimar in quasi real time (around 25 minutes after the beginning of each measurement) throughout the period (3 months in August-November 2005).
Sea surface currents, wind directions and significant wave height were derived from HF WERA measurements using algorithms developed by University of Hamburg. Radial current comparisons were made between each radar and ADCP measurements and numerical model data (both rotated on the corresponding radar direction), for 5 different locations. RMS differences were found between 0.11 m/s and 0.18m/s, with a correlation coefficient more than 0.92. In some cases, no correlation was found, due to the presence of rocks or island in the direction of the radar (no strong currents measurable by the ADCP, which has small spatial integration, compared with the radar). Comparisons between wind directions extracted from radar measurements and from METEO FRANCE data in Ushant (onshore station) give good correlation coefficient, 0.92, with a RMS difference of 32 degrees. Comparisons between significant wave heights extracted from radar measurements and swell buoy give a correlation coefficient of 0.88, with a RMS difference of 0.60m. Iroise Sea is a strong tidal currents area, with eddies and strong bathymetric gradients. The radar system is able to identify relatively intense current gradients in the zone, due to the presence of strong bathymetric gradients and the formation of tidal thermal fronts. The radar system is also able to highlight spatio-temporal variability of the sea states, mainly induced by semi diurnal variation of relative surface current velocity compared to wind speed and direction. We observe a semi diurnal variation for the significant wave height, more important during a period of spring tide and when the wind direction is directly opposed to that of the tidal current. In the North West of Ushant, we show that the maximum of the significant wave height coincides with the maximum negative gradient of the current on the wind direction. In this case, the difference in the measured amplitude for the significant wave height is about 4 meters. Thus requires to be validated by in situ observations in these particular areas.
Radiowave Oceanography, The 6th International Workshop, Hamburg, Germany, May 15-18, 2006