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Ann. Geophys., 25, 1235-1252, 2007
www.ann-geophys.net/25/1235/2007/
© European Geosciences Union 2007
Reference solar irradiance spectra and consequences of their disparities in remote sensing of the ocean colour
P. Shanmugam and Y. H. Ahn
Ocean Satellite Research Group, Korea Ocean Research and Development Institute, Ansan P.O. Box 29, Seoul 425-600, Korea
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.
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