Spectral variability of downwelling irradiance in water induced by wave focusing

Abstract

If the water surface is not perfectly flat, the downwelling irradiance in water is highly variable due to focusing and defocusing of the sun and sky light. The time scales and intensity variations caused by this wave focusing effect are well-studied. Much less is known about the induced spectral variability. We have developed a spectral model which describes the wave focusing effect by two parameters that can be determined for each irradiance measurement, and applied it to a large data set from three German lakes to study the spectral variability in the depth range from 0 to 5 m. It was found that the variance of spectral irradiance can be described quantitatively by a "geometry factor" which has a smooth spectral shape in the range 400 to 700 nm. For our data set the average variability across that range was 5 %; for 9 % of the data the spectral changes were > 15 %.