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tracking, climate studies, air-sea interactions, propagation of polluted air masses, CO2 fluxes
(Boutin et al., 2009), assimilation in Numerical Weather Prediction (NWP) models and commercial
applications, like wind energy. An overview of the progress in scatterometry applications
is available in Liu (2002).
16.2 Principle of Function
Radars operate at different sub-bands within the microwave range of the electromagnetic
spectrum (Table 24). Scatterometers are typically operational within the C and Ku subbands.
For these wavelengths and for specific incidence angles, if the surface wave spectrum
contains a component with wavelength similar to the incident radiation then the incident
radar pulse is reflected due to Bragg resonant scattering (Martin, 2004). For more details
on the relation between the sea surface and radar wavelengths see the chapter on SAR from
Hasager and Badger (2013).
Table 24: IEEE standard radar band letter definitions and frequency ranges, taken from IEEE
(2002)
A microwave radar pulse is transmitted towards the Earth’s surface and the reflected signal
is measured. The small scale ripples on the water surface that are generated by the wind
satisfythe wavelengthrequirement for Braggscattering ofthe incidentpulse. From the energy
reflected back to the instrument due to Bragg scattering, the noise signal is defined as the
instrumentnoiseandthenaturalemissivityoftheatmosphere-earthsystematthefrequencyof
the radarpulse. Subtracting the noise signalfrom the total measured reflected signal produces
the backscattered signal which is used to estimate the normalized radar cross section (NRCS)
σ0.
The fraction of the radar signal backscattered to the instrument is mainly a function of
the surface stress. But as surface stress observations are not available for the calibration and
generation of an empirical relationship, the near-surface ocean wind velocity relative to the
orientation of the instrument is used instead. The Geophysical Model Function (GMF) is
the empirical relation between the wind velocity and the normalized radar cross section σ0.
During the decades of scatterometer applications, several GMFs have been developed and are
constantly modified to improve the accuracy of the retrieved winds.
The GMFs are based on the correlation of the measured σ0 at a location with in situ,
modelled and other satellite winds. The general form of a GMF as described in Naderi et al.
(1991), is
σ0 = f (|u|,ξ,...;θ,f,pol). (351)
|u| is the wind speed, ξ the azimuth angle between the wind vector and the incident radar
pulse (noted as χ in Figure 197), θ is the incidence angle of the radar signal measured in the
vertical plane, “f” is the frequency of the radar signal and “pol” its polarization. The term
... accounts for non-wind variables such as long waves, stratification and temperature, the
effects of which are considered small (Naderi et al., 1991).
DTU Wind Energy-E-Report-0029(EN) 297