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15.6 SAR wind retrieval at DTU Wind Energy
ANSWRS version 2.0 is currently being used for near real-time wind field retrieval at DTU
WindEnergy.Tomeettherequirementsofprocessinginnearreal-time,theANSWRSsoftware
produceshigh-resolution(¡1km) windspeed fieldsinitializedusingwinddirectionsdetermined
by the NOGAPS model and interpolated in time and space to match the satellite data.
NOGAPS data is available at 6-hour intervals mapped to a 1 ◦ latitude/longitude grid. At
present, DTU Wind Energy has a collection of around 15,000 Envisat scenes from 2002 to
2012. The study areas include the northern European seas, the coast of Iceland, parts of the
coast of Greenland, parts of the Mediterranean Sea, parts of the Atlantic Sea along France,
Spain and Portugal, parts of the Indian Ocean near India, parts of the Chinese coast, parts of
the Persian Gulf, and Lake Erie among other sites. For most sites our aim was to map wind
resources thus many images are retrieved and processed.
Comparison results for ANSWRS using NOGAPS wind directions have for the estimated
wind speeds in the Gulf of Alaska and the US East coast yielded agreement with buoy
measurements to within ±1.76 ms −1 standard deviation (Monaldo et al., 2001) and with
QuikSCAT wind speeds to within ±1.25 ms −1 standard deviation (Monaldo et al., 2004). In
theBalticSeacomparisontoin-situwindspeedanddirectionobservedatmeteorologicalmasts
show a root mean square error of ±1.17 ms −1 , bias of −0.25 ms −1 , standard deviation of
1.88 ms −1 for wind speed and correlation coefficient of R 2 = 0.78. Wind directions compared
to mast observations show a root mean square error of 6.3 ◦ with a bias of 7.8 ◦ , standard
deviation of 20.1 ◦ and R 2 = 0.95 (Hasager et al., 2011a). Comparable results were obtained
in the North Sea (Badger et al., 2010a).
15.7 Mesoscale wind phenomena from SAR
The high spatial resolution of SAR allows fine structures in ocean surface winds to be quantified.
Each scene is recorded during a few seconds and so the SAR wind field is a snapshot
of the conditions at the time of observation. The all-weather and day and night observing
capabilityof SAR allowagreat varietyofmarine atmospheric conditionsto be observed across
the oceans of the Earth. A few are presented here. More may be found in Badger et al. 2008
and Beal et al. 2005 with descriptions of the meteorological background. SAR wind maps
may be viewed online at web-sites:
• http://fermi.jhuapl.edu/sar/stormwatch/web_wind/ (USA and Canada),
• http://polarlow.met.no/polar_lows/ (polar lows in the Nordic Seas),
• http://soprano.cls.fr (Europe),
• http://galathea.dtu.dk/google/kmz/images/Wind/?C=M;O=D(fewfromtheworld).
Belowwe present fourselected wind maps from Greenland, Puerto Rico, Icelandand Gibraltar.
The Envisat ASAR wind field in Figure 188 is from the east coast of Greenland observed
28 August 2006 at 12:47 UTC. During this time the Danish Galathea-3 circum-global ship
expedition took place in the region. The winds were from the north. Katabatic flow is observed
from the Kangerdlugssuaq Valley at 68 ◦ N, 31 ◦ W and penetrating 100 km across the
Denmark Strait. The wind speed is around 12 ms −1 in the otherwise calm coastal sea. Further
offshore a strong flow from the north more than 15 ms −1 is found. The frontal gradient is
sharp.
The lee effect of the tropical island of Puerto Rico in the Caribbean Sea observed from
Envisat ASAR on 16 March 2007 at 02:31 UTC is shown in Figure 189. The winds are
moderate 7 to 12 ms −1 from the northeast. The lee is longer than 200 km.
On 24 September 2005 at 11:30 UTC the wind was flowing from the northwest in Iceland,
see Figure 190. The mountainous island modulates the flow and along the eastern and
southern coastline a pattern of lee effects and speed up from valleys is seen.
282 DTU Wind Energy-E-Report-0029(EN)