Validation of a spectral correction procedure for sun and sky reflections in above-water reflectance measurements

Kurzfassung

A three-component reflectance model (3C) is applied to above-water radiometric measurements to derive remote-sensing reflectance Rrs(l). 3C provides a spectrally resolved offset D(l) to correct for residual sun and sky radiance (Rayleigh- and aerosol-scattered) reflections on the water surface that were not represented by sky radiance measurements. 3C is validated with a data set of matching above- and below-water radiometric measurements collected in the Baltic Sea, and compared against a scalar offset correction D. Correction with D(l) instead of D consistently reduced the (mean normalized root-mean-square) deviation between Rrs(l) and reference reflectances to comparable levels for clear (D: 14.3 +- 2.5 %, D(l): 8.2 +- 1.7 %), partly clouded (D: 15.4 +- 2.1 %, D(l): 6.5 +- 1.4 %), and completely overcast (D: 10.8 +- 1.7 %, D(l): 6.3 +- 1.8 %) sky conditions. The improvement was most pronounced under inhomogeneous sky conditions when measurements of sky radiance tend to be less representative of surface-reflected radiance.Accounting for both sun glint and sky reflections also relaxes constraints on measurement geometry, which was demonstrated based on a semi-continuous daytime data set recorded in an eutrophic freshwater lake in the Netherlands. Rrs(l) that were derived throughout the day varied spectrally by less than 2 % relative standard deviation. Implications on measurement protocols are discussed. An open source software library for processing reflectance measurements was developed and is made publicly available.