Directivity of wind vectors derived from the ERS-1/AMI scatterometer

Directional distribution of the wind vectors observed by the ERS-1 active microwave instrument (AMI) scatterometer over the global oceans is investigated to examine effects of errors in the geophysical model function used to retrieve the wind vectors from backscattering measurements. Three scatterometer data products, together with wind data derived from two operational meteorological analysis models, are analyzed and compared with each other for a period of 2 months. Frequency distributions of the wind directions relative to the midbeam looking direction are calculated in bins of 10 degrees. It is found that scatterometer-derived wind vectors may exhibit systematic directional preference relative to the antenna beams in comparison with those of the model winds. This artificial directivity is considered to be caused by imperfections in the model function used to retrieve the wind vectors. Dependencies of the directional distributions on incidence angle and wind speed are also discussed for each data products. The distribution of wind directions is very sensitive to the errors in the model function and can be used as an effective measure of self-consistency to validate the model function and retrieved wind vectors.