Comparisons of scatterometer models for the AMI on ERS-1: the possibility of systematic azimuth angle biases of wind speed and direction

There have been many different attempts to develop a model to relate the normalized radar backscatter values for the C-band radars of the active microwave instrument (AMI) on ERS-1 as a function of 10-m wind speed, azimuth angle, which is wind direction relative to radar-beam direction and incidence angle. The first two models, namely CMOD-1, Long (1985) and CMOD-2, are analyzed, and modifications of them are used to show by means of Monte Carlo methods that it is important to be able to define the backscatter model for all azimuth angles in addition to obtaining good agreement at upwind, downwind, and crosswind relative to the radar-beam direction. Methods are described to compare one model to another and to show how to systematic discrepancies, which are the result of model differences, can be found. These discrepancies are also expected when various models are employed to recover winds from real backscatter data. Discrepancies between a model and an unknown “true” model can introduce systematic biases in the recovered wind vectors as opposed to random errors, which result from sampling variability. The validation of the vector winds from scatterometer data requires a comparison of these winds with accurate conventionally measured winds. The data buoys deployed by various nations can serve as the appropriate data because ship reports are not accurate enough