New algorithms for microwave measurements of ocean winds: Applications to Seasat and the special sensor microwave imager

Improved second generation normalized radar cross section (NRCS) and brightness temperature (T B ) models and associated wind retrieval algorithms are derived for the Seasat microwave radiometer SMMR and scatterometer SASS. The derivation of the NRCS model is based on the assumption of a Rayleigh distribution of wind speeds, and no in situ anemometer measurements are used. Furthermore, the NRCS model derivation is designed to preclude, as much as possible, systematic errors in the polarization and incidence angle relationships. A constant power law NRCS model is used, except for nadir observations. The nadir NRCS for winds above 15 m/s falls off faster with increasing wind speed than is predicted by a constant power law relationship. The T B model derivation consists of finding the wind‐induced emissivity coefficients, modifying the 37‐GHz atmospheric absorption coefficients and removing biases in the T B observations. The T B biases are found to be stable except for the 18‐GHz channels, which experience large, time‐dependent biases. The NRCS and T B models are incorporated into new wind retrieval algorithms, which are used to process the SASS and SMMR 3‐month data sets. Small residual systematic errors in the SASS winds (±0.5 m/s or less) are found. A histogram of the SASS winds closely resembles a Rayleigh distribution. The SASS winds are compared with 1623 National Data Buoy Office (NDBO) buoy observations, and a 1.6‐m/s rms discrepancy, with a −0.1‐m/s bias, is found. The SASS and SMMR winds are compared on a 150‐km cell‐by‐cell basis, giving 123,000 wind comparisons for the 3‐month period. The comparisons are done using eight different combinations of three SMMR channels. Good agreement is found between the SASS and SMMR winds, except for two of the channel combinations that show little, if any, skill in retrieving wind. Over the SASS primary off‐nadir swath, the SMMR and SASS wind agreement ranges from 1.3 to 2.2 m/s, depending on the channel combination. For the SMMR versus SASS nadir wind comparisons, the agreement slightly degrades. The SMMR winds appear to be more noisy than the SASS winds for winds below 3 m/s. These results indicate that the Special Sensor Microwave Imager (SSM/I), to be launched in 1986, will have the capability to measure the near‐surface wind speed to an accuracy of about 2 m/s.