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 t he normalized radar backscatter values for the C-band radars of the ac tive microwave instrument (AMI) on ERS-1 as a function of 10-m wind sp eed, azimuth angle, which is wind direction relative to radar-beam dir ection and incidence angle. The first two models, namely CMOD-1, Long (1985) [1] and CMOD-2, are analyzed, and modifications of them are use d to show by means of Monte Carlo methods that it is important to be a ble to define the backscatter model for all azimuth angles in addition to obtaining good agreement at upwind, downwind, and crosswind relati ve to the radar-beam direction. Methods are described to compare one m odel to another and to show how systematic discrepancies, which are th e result of model differences, can be found. These discrepancies are a lso expected when various models are employed to recover winds from re al backscatter data. Discrepancies between a model and an unknown ''tr ue'' model can introduce systematic biases in the recovered wind vecto rs 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 wind s. The data buoys deployed by various nations can serve as the appropr iate data because ship reports are not accurate enough.