Directional Spectra of Short Sea Waves and Comparison with Microwave Backscatter

The microwave signature of the ocean is important to the remote sensing community because it contains information about surface wave field properties that can be used to estimate atmospheric forcing effects. Experimental studies of the transformation from the received microwave backscatter into a surface wave spectrum have been limited by our ability to adequately measure in situ ocean surface wave properties in the short gravity and gravity-capillary regions. A new instrument was developed to study the directionality of short gravity and gravity-capillary waves for the purpose of ground-truthing microwave scatterometer measurements. It was deployed as part of an experiment along with an azimuthally rotated scatterometer, a sonic anemometer, and a six-element capacitive wire wave gage were deployed at the test site. The resulting data from the wave wire buoy show that the measured frequency spectrum slope and wavenumber spectrum slope agree with various models in the literature. There is additional agreement between the measured width of the spectra collected with the wave wire buoy and the azimuthal width of the scatterometer return. However, it was found that the widths of the scattered return predicted by current wavenumber spectral models were consistently larger than those measured by either the wave wire buoy or the actual scatterometer.