SPECTRAL REFLECTANCE OF WHITECAPS - INSTRUMENTATION, CALIBRATION, ANDPERFORMANCE IN COASTAL WATERS

A measurement system for determining the spectral reflectance of white caps in the open ocean is described. The upwelling radiance is obtaine d from a ship by observing a small region of the water surface over ri me using a six-channel radiometer (410, 440, 510, 550, 670, and 860 nm ) extended from the bow of the ship. Downwelling irradiance is simulta neously measured and used to provide surface reflectance. The system i ncludes a TV camera mounted beside the radiometer that provides a visu al reference oi surface events. Air/water temperature and wind speed/d irection are also measured along with global positioning system data. Calibration procedures and radiometric characterization of the system for operation under different sky conditions and solar zenith angles a re emphasized so that full advantage is taken of ship time whenever wh itecap events occur. The radiometer was operated at sea and examples o f the spectral reflectance of different foam types (thick foam layers to thin residual patches) generated by the: ship's bow in coastal wate rs are presented and found to var!! spectrally. The presence of submer ged bubbles in the foam measurement results in a lower reflectance at the longer wavelengths. Fbr wavebands in the visible region, the spect ral reflectance values tend to equalize with higher reflecting foam fr om thicker foam layers.