Spectral Reflectance of Whitecaps: Instrumentation, Calibration, and Performance in Coastal Waters

Abstract

A measurement system for determining the spectral reflectance of whitecaps in the open ocean is described. The upwelling radiance is obtained from a ship by observing a small region of the water surface over time using a six-channel radiometer (410, 440, 510, 550, 670, and 860 nm) extended from the bow of the ship. Downwelling irradiance is simultaneously measured and used to provide surface reflectance. The system includes a TV camera mounted beside the radiometer that provides a visual reference of surface events. Air/water temperature and wind speed/direction 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 are emphasized so that full advantage is taken of ship time whenever whitecap events occur. The radiometer was operated at sea and examples of the spectral reflectance of different foam types (thick foam layers to thin residual patches) generated by the ship's bow in coastal waters are presented and found to vary spectrally. The presence of submerged bubbles in the foam measurement results in a lower reflectance at the longer wavelengths. For wavebands in the visible region, the spectral reflectance values tend to equalize with higher reflecting foam from thicker foam layers.

Spectral reflectance of whitecaps

“…We used the following parameterizations [Morel and Maritorena, 2001]: C p a p = 0.2[a w (l 0 ) + 0.06x 0.65 ] exp[À0.014(l À l 0 )] and C y a y = 0.06p(l), where l is given in nm, l 0 = 440 nm, and x is the chlorophyll Figure 5. Normalized spectral reflectance of whitecaps calculated from equation (24), using values of a given by Moore et al [1998] and b = 85.5 mm (theory). Experimental data are taken from Figure 10 of Moore et al [1998].…”

“…Normalized spectral reflectance of whitecaps calculated from equation (24), using values of a given by Moore et al [1998] and b = 85.5 mm (theory). Experimental data are taken from Figure 10 of Moore et al [1998].…”

“…In particular, spectral reflectance of whitecaps was measured in a broad spectral range from the visible to the infrared [Frouin et al, 1996;Moore et al, 1998Moore et al, , 2000Nicolas et al, 2001]. Theory did not follow the progress in experimental studies of optical properties of whitecaps.…”

Spectral reflectance of whitecaps: Their contribution to water‐leaving radiance

“…The whitecap reflectance measurement system is described in detail elsewhere [Moore et al, 1998]. The system consists of Once the system has been installed on a ship, only periodic measurements of the detector dark current are necessary.…”

The Influence of Thermodynamic Phase on the Retrieval of Mixed-Phase Cloud Microphysical and Optical Properties in the Visible and Near-Infrared Region

“…On a bright surface such as ice or snow where the surface reflectance is large, the signal from the cloud is mixed with the signal from surface. In this study, a surface albedo of 0.03 is used both at 0.87µm and 2.13µm [24], [12] which is a value for dark ocean surface. Sunglint is not a factor for the solar-viewing geometry defined in this study.…”