Wind Statistics Offshore based on Satellite Images

Ocean wind maps from satellites are routinely processed both at Risø DTU and CLS based on the European Space Agency Envisat ASAR data. At Risø the a priori wind direction is taken from the atmospheric model NOGAPS (Navel Operational Global Atmospheric Prediction System) provided by the U.S. Navy’s Master Environmental Library. At CLS the a priori wind direction is taken from the ECMWF (European Centre of Medium-range Weather Forecasting). It is also possible to use other sources of wind direction e.g. the satellite-based ASCAT wind directions as demonstrated by CLS. The wind direction has to known a priori as the SAR system only has one view of the surface and both wind speed and direction has to be retrieved. As alternative to modeled wind direction, ground-based wind direction may be used for a specific site, and this will be tested in a later phase of the Norsewind project when sufficient ground-based observations become available. At present preliminary results are obtained using the routine methods. The first step in the process is to retrieve raw SAR data, calibrate the images and use a priori wind direction as input to the geophysical model function. From this process the wind speed maps are produced. The wind maps are geo-referenced. The second process is the analysis of a series of geo-referenced SAR-based wind maps. Previous research has shown that a relatively large number of images are needed for achieving certain accuracies on mean wind speed, Weibull A and k (scale and shape parameters) and energy density, and that the filtering and statistical treatment should be done with care. For this purpose, Risø DTU during recent years has developed the S-WAsP program. This will be applied for the relevant offshore areas of the Norsewind project. This includes the Baltic, North and Irish Seas. Results comparing satellite scatterometer winds to offshore meteorological observations have shown good results, and more comparisons are planned in this respect during the Norsewind project.