Some Considerations in The Evaluation of Seasat-A Scatterometer (SASS) Measurements

Abstract

A study is made of the geophysical algorithms relating the Seasat-A scatterometer (SASS) backscatter measurements σ° with a wind parameter. It is shown that although σ° is closely related to surface features, identification with surface layer parameters such as friction velocity u* or the roughness length z0 is difficult from both theoretical and practical considerations. It is shown how surface truth in the form of wind speeds and coincident stability can be used to derive either u* or the equivalent neutral wind uN at an arbitrary height. It is also shown that derived u* values are sensitive to contested formulations relating u* to z0, while derived uN values are not. Also, derived quantities are seen to be more sensitive to changes in positive stability than to change in unstable conditions. Examples of geophysical verification is demonstrated using values obtained from the Gulf of Alaska Seasat Experiment (GOASEX). Results of this verification show very little sensitivity to the type of wind parameter employed. It is suggested that this insensitivity is due mainly to large scatter in the SASS and surface truth data.