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Moreover,SSTpartlyadjuststhe atmosphericstratificationwhichalsoaffects the backscattered
signal. Under very stable conditions, the surface winds tend to be lower due to decoupling
from the higher atmospheric levels. In such a case the surface stress is lower, thus σ0
is lower. But, the atmospheric stability also controls if there is any discrepancy between the
scatterometer derived wind and the “surface truth”, due to the ENW convention.
Scatterometers observe relative to the sea surface. Strong currents can contaminate the
wind velocity retrieval. Dickinson et al. (2001) used measurements from the Tropical Atmosphere
Ocean (TAO) buoy array stating that when the current and wind had the same
direction, the scatterometer speed was expected to be lower than in situ wind speeds.
As stated in Hoffman and Leidner (2005), light winds can pose a problem for the wind
retrieval as in such cases the ocean surface is very smooth and acts more as a reflector rather
than a scatterer. Very high winds can also be problematic as the buoy and modelled data
used to calibrate the GMFs tend to under-represent very high winds.
16.5 QuikSCAT
The SeaWinds scatterometer on board NASA’s QuikSCAT platform was the first scatterometer
to operate for many consecutive years. It provided valuable, consistent and frequent observations
of the global ocean both in terms of speed and direction. QuikSCAT was launched
in June 1999 with a design life-time of 3 years, as a quick recovery mission to fill the gap from
the loss of NSCAT. The scatterometer’s antenna failed rotating on the 23rd of November
2009, far exceeding the design life-time.
At an altitude of 803 km, QuikSCAT completed each orbit in approximately 101 minutes,
ascending in the morning and descending in the afternoon (see Figure 200 for an example).
With a wide swath of 1800 km it covered 93% of the global ocean each day. SeaWinds was
an active microwave radar operating at 13.4 GHz (Ku band), radiating microwave pulses
through a 1 m diameter antenna, and measuring the power of the signal returning back to
the instrument. The σ0 “footprint” cell had dimensions of 25·37 km and the averaging area,
the Wind Vector Cell was a square box of 25·25 km.
Figure 199: Graphical representationof the scanninggeometryof the SeaWinds scatterometer
on the QuikSCAT platform. The darker areas are covered by four looks of the antenna beams,
while the light areas by two looks. Note that there is no gap in the nadir track. Taken from
Martin (2004).
Each σ0 “footprint” cell, also called “the egg”, consisted of 12 slices. σ0 was calculated for
both the full “egg” and for each of the 8 inner slices. This meant that SeaWinds measured σ0
at a variety of dimensions, i.e. the “footprint”, the inner slices and a variety of “footprints”
composed of combinations of slices. For more information, see Martin (2004). Backscattered
signals were received from the sea, land and ice but the scattering processes over land and
ice are different than those over open ocean. Therefore, the scattering from land and ice can
contaminate the WVC and needs to be identified and removed. For this reason, a land and
300 DTU Wind Energy-E-Report-0029(EN)