SAR is an active microwave sensor capable of imaging the amount of backscattered signal per unit area - the normalized radar cross section (NRCS). NRCS depends on the size and geometry of roughness elements on the scale of the radar wavelength at the Earth surface.
Over a calm ocean surface, the returned NRCS is limited because radar pulses are reflected away from the SAR at an angle equal to the angle of incidence. As the wind picks up, roughness in the form of capillary and short-gravity waves is generated by the surface wind stress. The dominant scattering mechanism is then diffuse and known as Bragg scattering. The relation of NRCS to the local wind speed and direction and to the radar viewing geometry forms the key principle in ocean wind retrievals from SAR.
A range of geophysical model functions (GMFs) have been developed empirically for ocean wind retrievals from radar measurements. Generally, the empirical GMFs take the following form:
where σ0 is the normalized radar cross section (NRCS), U is wind speed at the height 10 m for a neutrally-stratified atmosphere, θ is the local incident angle, and is the wind direction with respect to the radar look direction. The coefficients A, B, C, and are functions of wind speed and the local incident angle.
Empirical model functions rely on the assumption that wind speed increases logarithmically with height above the sea surface. This is normally true if the atmospheric boundary layer is neutrally stratified. Stable stratification would typically lead to an underestimation and unstable stratification to an overestimation of the 10-m wind speed.
Several parameters other than the surface wind vector or wind stress can affect the sea surface roughness and thus the NRCS. For example, mineral oil or biogenic slicks have a damping effect on Bragg waves. Oceanographic processes including fronts and eddies, internal waves, long-period surface waves, and bathymetry may also alter the NRCS.
DTU Wind Energy operates a processing chain for SAR wind retrieval built around the SAR Ocean Products System (SAROPS) by NOAA Center for Satellite Applications and Research (STAR), US National Ice Center and Johns Hopkins University, Applied Physics Laboratory (Monaldo, Jackson, Pichel, & Li, 2015).
SAR scenes processed to level 1 (L1) are downloaded daily; primarily from the European Space Agency through Copernicus Open Access Hub (https://scihub.copernicus.eu/). The scenes are calibrated to give the Normalized Radar Cross Section.
Land and ice covered surfaces are eliminated through masking with data from the IMS Daily Northern Hemisphere Snow and Ice Analysis at 4-km Resolution by the U.S. National Ice Center (http://nsidc.org/data/docs/noaa/g02156_ims_snow_ice_analysis/).
Wind directions are obtained from global models. Envisat scenes from 2002-10 are processed using wind directions from the Climate Forecast System Reanalysis data set (CFSR, available at http://nomads.ncdc.noaa.gov/data.php?name=access#cfs-reanal-data). Envisat scenes from 2011-12 are processed using wind directions from the Global Forecast System (GFS) at 0.50° resolution (available at http://nomads.ncdc.noaa.gov/data/gfsanl). Sentinel-1 scenes from 2014 onwards as and the TerraSAR-X data in the archive are also processed using wind directions from GFS but at 0.25° resolution (available at ftp://ftp.ncep.noaa.gov/pub/data/nccf/com/gfs/prod/).
For each SAR wind field on this site, two types of outputs are available for download: Animage file in .png format shows a display of the wind field with a standard color scaling. A NetCDF (.nc) file holds the wind speed data together with various ancillary data used for the wind processing and metadata describing the product .