Comparisons of ASCAT Wind Vectors and Buoy Wind Data in China's Coastal Waters

The new scatterometer Advanced SCATterometer (ASCAT) on board MetOp-A and MetOp-B satellites provides surface wind speed and direction over global ocean. Providing accurate nearshore wind data from satellites is challenging. First, data are unavailable very close to shore because any land within the satellite's viewing area contaminates the return signal. Second, land-sea breezes and shore topography produces 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 reasons 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 nearshore buoys. The nearshore buoys used in our comparisons locate in China's coastal waters. As the spatial resolution of ASCAT wind product is 12.5km,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 12.5km. As the time interval of China coastal buoy wind is 1 hour, the acquisition time difference is less than 30 minutes in China coastal waters. The buoy winds at a given anemometer height have been converted to 10-m neutral winds in order to enable a good comparison with the 10-m scatteromter winds. The time range of wind data used for comparison from China nearshore buoys are the first half year of 2012. Comparison results show that the accuracy of the wind speed of ASCAT coastal wind 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 whole 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 whole 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. Editing the satellite data by discarding observations recorded in light winds (<3m/s) improved the accuracy of the ASCAT wind products: the mean bias of wind direction decreased from 21.89°to 11.83° ,and the wind direction correlation coefficient increased from 0.71 to 0.84. In addition, the accuracy of ASCAT wind direction is different in different wind direction: the accuracy of the wind flow from land is low, and the accuracy of the wind flow from sea is high. In most China coastal waters, ASCAT coastal wind is highly correlated with buoy wind, but in the inland sea°ŞBohai sea, the effect of topography on ASCAT coastal wind product is apparent.