Evaluation of ASCAT Coastal Wind Product Using Nearshore Buoy Data

The new scatterometer advanced scatterometer (ASCAT) on board MetOp-A satellite provides surface wind speed and direction over global ocean. Providing accurate nearshore wind data from satellites is challenging because satellite data are unavailable very close to shore due to the contaminating effect of the land. Besides, land-sea breezes and shore topography produce small space scale and time-scale wind variations that can be smoothed by the satellite's space averaging and aliased by the satellite's twice-a-day sampling. The complexity of nearshore winds is one of the prime causes that the regions are so important. For example, over one-third of the total marine fish catch occurs within nearshore zone.The accuracy of ASCAT coastal wind product is determined through various comparisons with buoys. The nearshore buoys used in the comparisons locate in US West Coast and China Coast. As the time interval of US West Coast buoy wind is 10-minute interval and the spatial resolution of ASCAT wind product is 12.5 km, a scatterometer wind and a buoy wind measurement are considered to be collocated if the distance between the wind vector cell center and the buoy location is less than 12.5 km and if the acquisition time difference is less than 5 minutes in US West Coast. As the time interval of China Coast buoy wind is 1 hour, the acquisition time difference is less than 30 minutes in China Coast. The buoy winds at a given anemometer height are converted to 10 m neutral winds in order to enable a good comparison with the 10 m scatteromter winds. The time ranges of wind data used for comparison from US West Coast buoys and China Coast buoys are the whole year of 2012 and the first half year of 2012 individually.It shows that the accuracy of the wind speed of ASCAT product is high and the accuracy of the wind direction of ASCAT product is influenced by several factors, such as the distance from coast, wind speed and wind direction. The overall wind speed correlation coefficient between buoy data and ASCAT product is 0.94, and wind speed correlation coefficients between each buoy and ASCAT product are all above 0.9. The overall wind direction correlation coefficient between buoy and ASCAT product is 0.71, and the wind direction correlation coefficient between the nearest buoy and ASCAT product is only 0.55. Processing the satellite data by discarding observations recorded in light winds (below 3 m·s^-1) can improve the accuracy of the ASCAT wind products by reducing the mean bias of wind direction from 21.89°to 11.83°, and the wind direction correlation coefficient increased from 0.71 to 0.84. In addition, the accuracy of the wind flow from land is low, while the accuracy of the wind flow from sea is higher. In most China nearshore regions, the applicability of ASCAT coastal wind product is good, but in Bohai Sea, the effect of topography on ASCAT coastal wind product is apparent.