Spectral Variability of Sky and Sun Glint in Hyperspectral Above-surface Reflectance Observations

Philipp Grotsch, CUNY City College of New York, New York, NY, United States, Robert Foster, US Naval Research Laboratory, Remote Sensing Division, Washington, United States and Alex Gilerson, The City College of New York, NOAA-CREST Optical Remote Sensing Laboratory, New York, NY, United States
Above-surface radiance observations of water need to be corrected for reflections on the surface to derive reflectance. Sky radiance in specular direction is widely used to approximate the surface-reflected radiance signal (Lr), assuming a spectrally uniform diffuse sky light distribution, and neglecting potential sun glint contamination. These assumptions, however, are not valid for wind-roughened water surfaces, for which large parts of the spectrally varied sky, including the sun disc, contribute to Lr. The three component glint model (3C) was developed to spectrally resolve Lr also for observations recorded at high wind speed and with fixed-position measurement geometries that periodically lead to significant sun glint contributions.

In this study, we used 3C to process an extensive 5-year data set of continuous above-surface hyperspectral observations from the Long Island Sound Coastal Observatory (LISCO), recorded in a fixed direction and at wind speeds ranging up to 15 m/s. Measurements were conducted in burst mode and yield a representative sample of Lr variability for each measurement cycle. Implications of commonly used averaging schemes for such measurement series are discussed, corroborating to an automated quality-control scheme.

A spectrally resolved sky radiance distribution model was then used in combination with Cox-Munk wave-slope statistics to validate the observed spectral variability in Lr with regards to wind speed, aerosol concentration, illumination/viewing geometry, and sun glint contamination.

Both observed and simulated Lr independently indicate that spectral dependencies of the sky light distribution and the contribution of sun glint may not be neglected for observations recorded at wind speeds exceeding 5 m/s. This finding is specifically relevant for coastal and inland waters that feature low reflectance values in the blue and ultra-violet spectral regions where Rayleigh-scattered sky light is most intense and variable.