Winds over the sea
Verifying SAR wind speed algorithms
Verification of the process models used for wind retrieval from ERS-SAR consists of two parts: (i) establishing the validity of the CMOD-4 scatterometer model, and (ii) confirming the reliability of the technique used for independently estimating the wind direction.
The verification of CMOD-4 is established in the published papers defining the model (Stoffelen and Anderson, 1993; 1997a; b).
The validity of the approach for estimating wind direction from the orientation of wind streaks has also been demonstrated in the literature (e.g. Horstmann et al., 2000).
The subsequent validation of the wind retrieval from Envisat's ASAR is based on independent estimates of wind velocity at the time of the image acquisition, obtained from buoy measurements or from atmospheric models. The expected C-band radar backscatter cross section is then calculated by using the CMOD algorithm for the given wind speed and direction. The scatter plot between the CMOD-predicted backscatter and the corresponding ASAR-measured single-look complex backscatter can be used to confirm how well the wind can be estimated from ASAR, once the SAR is independently calibrated. Figure 1 illustrates a typical scatter plot (Johnsen et al., 2002).
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Figure 1: Plot of CMOD radar backscatter intensity based on the estimated wind velocity against the corresponding observed backscatter intensity from ASAR single look complex data. |