Operational Synthetic Aperture Radar High Resolution Winds in the Arctic

Operational Synthetic Aperture Radar High Resolution Winds in the Arctic Frank Monaldo On Assignment at NOAA NESDIS from Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel MD 20723 As the Arctic Ocean becomes more ice free over longer periods of the year, transit and other marine activities will become more common. Hence, there is a need for more complete monitoring of the Arctic environment. This growing need corresponds to increasing abilities for such monitoring. Over the last decade, the ability to use spaceborne, high-resolution (<500 m) synthetic aperture radar (SAR) radar cross section measurements to derived wind speed measurements to better than 2 m/s accuracy has matured. SAR wind measurements have been validated against buoy measurements, model predictions, and other remote sensing techniques. In light of this maturation, NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) and National Ice Center (NIC) are soon to commission a real-time SAR wind speed product. Currently, data are being received at NIC from the Radarsat-2 SAR for the primary purpose of sea ice location and identification. SAR winds are now routinely produced from this imagery. We anticipate that the availability of SAR wind imagery over the Arctic will be significantly augmented with the launch of Sentinel-1 this year. Other SAR satellites such a TerrasSAR-X and the COSMO SkyMed constellation offer still more opportunities. In the future, we anticipate data from the Radarsat constellation. From parts of SAR imagery without ice, it is possible to estimate the wind speed and use this as an aid in assessing local weather as well as the sea ice environment. The APL/NOAA SAR Wind Speed Retrieval System (ANSWRS) was developed with the support of and under the auspices of NOAA. It is a set of software and protocols that can processes SAR imagery in near real time (within 10 minutes of when an image is available) into wind speed data at 500-m resolution or better. Wind speed estimates can be processed to the actual SAR resolution, sometimes on the order of meters. However, radar cross section variations associated with other phenomena such as ocean surface waves complicate interpretation of data. We recommend averaging SAR data to between 250 and 500 m for wind speed retrieval. The actual wind speed values are available and stored in the NOAA-standard netCDF format. In addition, color-coded wind speed imagery are provided in PNG, KMZ, and GeoTIFF formats to aid the analysts at NIC and elsewhere in interpretation. At off nadir incidence, the radar cross section of the ocean surface is directly related to ocean surface roughness of the scale of the radar wavelength. In the case of C-band, the wavelength is about 5 cm. As wind speed increases this surface roughness increases and so does radar cross section. Cross section is the highest when the radar look direction and the wind direction are aligned and minimized when these directions are orthogonal. In order to retrieve the wind speed from SAR radar cross section measurements, ANSWRS initializes the retrieval with wind direction estimates from the NOAA Global Forecast (GFS) model. In this presentation, we explain the details of SAR wind speed retrieval, provide an historical perspective on such retrievals and summarize how ANSWRS is implemented and its limitations. The architecture of ANSWRS is designed to be extensive, so that SAR imagery can be used not only for wind speed, but from ocean wave spectra, for ship detection, oil spill detection and other uses. We anticipate that ANSWRS represents an important step in more complete Arctic monitoring.