Measurements of Ocean Surface Stress Using Aircraft Scatterometers

Scatterometer model functions that directly estimate friction velocity have been developed and are being tested with radar and in situ data acquired during the Surface Wave Dynamics Experiment (SWADE) of 1991. K-u-band and C-band scatterometers were operated simultaneously for extensive intervals for each of 10 days during SWADE. The model function developed previously from the FASINEX experiment converts the K-u-band normalized radar cross-section (NRCS) measurements into friction velocity estimates. These are compared to in situ estimates of surface wind stress and direction across a wide area both on and off the Gulf Stream (for hourly intervals), which were determined from buoy and meteorological measurements during February and March 1991. This involved the combination of a local, specially derived wind field, with an ocean wave model coupled through a sea-state-dependent drag coefficient. The K-u-band estimates u* magnitude are in excellent agreement with the in situ values. The C-band scatterometer measurements were coincident with the K-u-band NRCSs, whose u* estimates are then used to calibrate the C band. The results show the C-band NRCS dependence at 20 degrees, 30 degrees, 40 degrees, and 50 degrees to be less sensitive to friction velocity than the corresponding cases for K-u band. The goal is to develop the capability of making friction velocity estimates (and surface stress) from radar cross-sectional data acquired by satellite scatterometers.