Evaluation of ERS-1 scatterometer winds with ocean buoy observations

Wind vectors observed by the ERS-1 AMI scatterometer are compared with wind and wave ocean buoy observations. Effects of oceanographic and atmospheric parameters on the scatterometry are also assessed by using the buoy data. Three ERS-1 AMI wind data products such as the JPL Value-Added Products, the ESA Fast Delivery Products and the IFREMER Off-Line Scatterometer Wind Products, were collocated with buoys located offshore and in deep water from the NDBC network, the TOGA-TAO arrays and the JMA buoys. Time difference and spatial separation between the ERS-1 and buoy observations were limited to less than 60 min and 25 km, respectively. Wind speed, air temperature and humidity measured by the buoys were adjusted to values corresponding to a height of 10 m. Wind speeds comparisons displayed rms differences 2 m s^-1 or less for each data product. The IFREMER winds gave the best agreement among the three data products. Systematic biases depending on the wind speeds are found for the JPL and ESA winds. The JPL winds underestimate at low wind ranges and overestimate at high winds. For the wind directions, large deviations are primarily found at low wind speeds which may be due to less accurate performance of the ambiguity removal algorithms. The comparison of the wind direction for the JPL and IFREMER improves significantly when wind speeds less than 5 m s^-1 are excluded. Dependencies of the wind speed residuals on oceanic and atmospheric parameters observed by the buoys exhibited negative correlations with the sea surface temperature and positive correlations with the significant wave height