Radiometric Measurements of Whitecaps and Surface Fluxes

Surface fluxes quantify air-sea transfers of momentum, heat, and mass. Whitecap fraction W is a suitable parameter for parameterizing the enhancement of the surface fluxes by breaking waves. Different radiative properties of the whitecaps allow their detection with different measuring techniques. The high reflectance of the whitecaps in the visible portion of the electromagnetic (EM) spectrum affords capturing the whitecaps in photographs of the sea state. Applying intensity threshold I to photographic images to discriminate white water (bright pixels) against the surrounding seawater (dark pixels) yields whitecap fraction W(I). The photographic method has been the workhorse of obtaining W for decades. Developments of digital photography, automated image processing of sea state images, and robust selection of I using derivatives of the image intensity distribution have improved the extraction of W(I) values. The high emissivity of the whitecaps in the microwave portion of the EM spectrum affords detection of whitecaps as strong spikes in time series of the ocean surface brightness temperature T_B. The possibility to deploy microwave radiometers at different frequencies on ships and airplanes opens the possibility to have concurrent measurements of W with different methods and thus constrain measuring uncertainties.

Satellite-based radiometric observations yield W(T_B) on a global scale at WindSat frequencies from 6 to 37 GHz. Global estimates of sea spray production and CO₂ transfer velocity are obtained using W(T_B) data at 10 or 37 GHz. Microwave radiometry at lower frequencies (~ 1.5 GHz) combined with ubiquitously available GPS (Global Positioning System) signals can be used to study tropical cyclones. Use of millimeter-wave frequencies (40-200 GHz) will allow measurements of W at high spatial resolution in coastal and polar waters. An overview of current and upcoming possibilities of radiometric measurements of W and related surface fluxes will be presented.