Directional distribution of wind vectors observed by ERS-1/AMI scatterometer

The frequency distribution of the wind directions observed by the ERS-1/AMI scatterometer is investigated to examine the effects of errors in the model function used to retrieve the wind vectors from backscattering measurements. It is found that scatterometer-derived wind vectors have some directivity relative to the antenna beams. This directivity is related to errors in the model function used to retrieve the wind vectors. As already pointed out by previous calibration/validation studies, the ESA pre-launch CMOD-2 model function is not accurate. The errors in the CMOD-2 cause an irregular distribution distorted with reference to the antenna beams. The CMOD-FD (Freilich/Dunbar) model function is much improved compared with the CMOD-2, but still has some directivity relative to the beams. Similar directivity is also found in the SASS-2 wind data