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Figure 189: Envisat ASAR wind field from Puerto Rico in the Caribbean Sea observed on 16
March 2007 at 02:31 UTC. The lee effect is longer than 200 km.
between. The spectral analysis showed SAR to be able to resolve spatial details down to
6 km for the 2 km wind fields. The spectral content of SAR at 25 km is, as expected, higher
than for QuikSCAT at the same resolution. The analysis indicates that SAR resolves the
mesoscale details of marine winds with higher accuracy than scatterometer and mesoscale
models (Karagali, 2012).
15.8 SAR wind fields near offshore wind farms
Large offshore wind farms have been constructed and operated since 2000. High-resolution
SAR wind fields from ERS-2 and Envisat have been retrieved and analyzed. The data was
recorded nearthe first twolarge offshorewind farms, Horns Rev-1and Nysted-1. Alsoairborne
E-SAR C- and L-band co- and cross-pol data from the Horns Rev-1 wind farm has been
retrieved and analyzed (Christiansen & Hasager, 2005; 2006). High-resolution SAR wind
fields in many cases show the wind farm wake, i.e. an area with reduced winds downwind
of the wind farms. The magnitude of the wake near the wind farms was found to be similar
to the wake effect predicted by wake models and observed from meteorological masts. More
surprisingly the study demonstrated that the wind farm wake may extend as far as 20 km
downwind of a large offshore wind farm. This is much further than predicted by current wake
models. Therefore the potential power production from wind farms in clusters may be more
affected by wakes than is assumed in general.
284 DTU Wind Energy-E-Report-0029(EN)