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Change to Thuillier-based Rrs

There are two sets of solar irradiance values used in the ocean color processing: band-averaged and nominal-band.

The band-averaged values are used through-out all atmospheric correction, up to the point where full-band nLw is retrieved. In the previous SeaWiFS reprocessing, R5, the band averaged values were changed from Neckel & Labs to Thuillier 2003 (to be consistent with MODIS/Aqua processing). Unfortunately, for SeaWiFS only, we neglected to change the nominal-band solar irradiances to Thuillier.

The nominal-band solar irradiances are only used to normalize the nLw to Rrs, and only AFTER the nLw is corrected from full-band to nominal-band (10-nm square bandpass, centered on the nominal sensor wavelength). Since the nominal-band irradiances are only used to compute Rrs=nLw/Fo, and Rrs is used for computing chl, the only impact to standard products is that the retrieved chl for seawifs is still based on Neckel & Labs rather than Thuillier. Relative to Neckel & Labs, the Thuillier solar spectrum is higher in blue and lower in green, so the change to Thullier lowers the blue/green Rrs ratio and thereby raises chl. The effect on the OC3 chlorophyll algorithm varies within a range of approximately 2-8%, with the vast majority of oceanic waters changing by 3-5%. The impact to global deep-water chlorophyll relative to SeaWiFS Reprocessing 5 is shown below.

It should be undestood that this change in chlorophyll is due only to our choice in solar irradiance model. It is assumed that Thuillier 2003 is an improvement over Neckel & Labs, and the choice of Thuillier was recommended by IOCCG.

The impact to non-standard algorithms, such as the GSM01 or Carder semi-analytical IOP algorithms, will need to be evaluated by users of those algorithms. Vicarious calibration is done on the full-band nLw (against sensor bandpass convolved MOBY nLw), so there is no impact of this change in Rrs normalization to calibration.

The solar spectra are posted here.

References

Thuillier, G., M. HersÚ, P. C. Simon, D. Labs, H. Mandel, D. Gillotay, and T. Foujols, 2003, "The solar spectral irradiance from 200 to 2400 nm as measured by the SOLSPEC spectrometer from the ATLAS 1-2-3 and EURECA missions, Solar Physics, 214(1): 1-22.

Neckel, H. and D. Labs, 1984,"The Solar radiation between 3300 and 12500 ┼", Sol. Phys. 90, 205-258.


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Updated: 19 June 2009