Self-Consistency of Marine Surface Wind Vectors Observed by ASCAT

Marine surface wind vectors observed by the Advanced Scatterometer on the MetOp-A satellite are evaluated by assessing their self-consistency. Global statistics on wind speeds and directions were calculated from the data for a period of one year. The wind speed histograms exhibited a clear dependence on the cross-track wind vector cell (WVC) location, which corresponds to the incidence angle. This trend was reflected in the higher order statistics (standard deviation, skewness, and kurtosis) of the wind speed distribution. The histograms of the wind directions relative to the satellite flight direction clearly showed systematic errors relative to the antenna beam directions. The number density of the wind directions exhibited a systematic pattern relative to the antenna beam directions, and the pattern varied with wind speed and cross-track WVC location. These systematic errors in the wind speed and direction may affect the divergence/convergence and rotation of the wind field. The results of this study suggest the need for further refinements of the wind retrieval algorithms and the C-band geophysical model function. It was also confirmed that the evaluation technique based on the statistical distributions of scatterometer-derived vector winds is effective for identifying systematic errors in the wind speeds and directions.