Reference solar irradiance spectra and consequences of their disparities in remote sensing of the ocean colour

Abstract. Satellite ocean colour missions require a standard extraterrestrial solar irradiance spectrum in the visible and near-infrared (NIR) for use in the process of radiometric calibration, atmospheric correction and normalization of water-leaving radiances from in-situ measurements. There are numerous solar irradiance spectra (or models) currently in use within the ocean colour community and related domains. However, these irradiance spectra, constructed from single and/or multiple measurements sets or models, have noticeable differences – ranging from about ±1% in the NIR to ±6% in the short wavelength region (ultraviolet and blue) – caused primarily by the variation in the solar activity and uncertainties in experimental data from different instruments. Such differences between the applied solar irradiance spectra may have quite important consequences in reconciliation, comparison and validation of the products resulting from different ocean colour instruments. Thus, it is prudent to examine the model-to-model differences and ascertain an appropriate solar irradiance spectrum for use in future ocean colour research and validation purposes. This study first describes the processes which generally require the application of a solar irradiance spectrum, and then investigates the eight solar irradiance spectra (widely in use within the remote sensing community) selected on the basis of the following criteria: minimum spectral range of 350–1200 nm with adequate spectral resolution, completely or mostly based on direct measurements, minimal error range, intercomparison with other experiments and update of data. The differences in these spectra in absolute terms and in the SeaWiFS and MERIS in-band irradiances and their consequences on the retrieval algorithms of chlorophyll and suspended sediment are analyzed. Based on these detailed analyses, this study puts forward the solar irradiance spectrum most appropriate for all aspects of research, calibration and validation in ocean colour remote sensing. For an improved approximation of the extraterrestrial solar spectrum in the ultraviolet-NIR domain this study also proposes a new solar constant value determined from space-borne measurements of the last three decades.