The ASCAT scatterometer level 2 product includes direct estimates of the global sea surface wind vector field with a nominal spatial sampling of 25 km and an accuracy of 3 m/s (in wind vector). However, since backscatter measurements of up to 70 km away from each wind vector cell (WVC) center are used in the spatial averaging, WVC’s within the first 50-75 km off the coast will be flagged because of land/ice contamination. Therefore, it is worthwhile to investigate whether more advanced methods of near-coastal spatial averaging may result in more good-quality ASCAT winds near the coast.
In contrast with the ERS scatterometer processing, ASCAT includes a radar backscatter full resolution product. The product contains the individual radar backscatter values, 256 values along each antenna beam, localised on the surface of the Earth. In such product the data are organised per position along the antenna beam and not per WVC in the swath. The sampling of individual backscatter values along-beam is of approximately 2 km for mid beams and 3 km for side beams. The full resolution backscatter values along every antenna beam represent footprints of about 10 km x 20 km of various shapes and orientations.
We propose to use L1b full resolution product and investigate replacing the Hamming filter by an alternative averaging appropriate for the coastal area to produce 25-km winds, e.g., a simple 25 by 25-km box (i.e., constant weighting function) averaging like that used for SeaWinds. The objective is, in contrast to the 25-km level 2 product, to provide sea-surface wind information down to 17.5 km off the coastline. The ASCAT level 2 product also includes a high-resolution 12.5-km sampled research wind product, which will flag wind data within the first 25-37.5 km off the coast. Within the NWP SAF Continuous Development and Operations Phase (CDOP) plans, a 12.5 km resolution scatterometer coastal product (up to 12.5 km off the coast) should be ready by 2011. The development of the 25-km coastal ASCAT Wind Data Processor (AWDP) prototype proposed here will be a good starting point for this future high resolution product. Moreover, the development may result in a much earlier implementation of coastal winds in the 25-km product.
M Portabella, A Verhoef, A Stoffelen. ASCAT coastal AWDP prototype