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Up Topic Special Topics / Ocean Color Reprocessing / Atmospheric correction (locked)
- By sean Date 2009-06-16 10:03
Spectral Sources

The sources and methods for producing band-averaged atmospheric and in-water absorption and scattering coefficients have been updated for traceability and consistency across all sensors. This includes Rayleigh optical thicknesses, ozone absorption coefficients, NO2 absorption coefficients, and pure-water absorption and scattering coefficients.

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Rayleigh tables

The Rayleigh tables were recomputed for all sensors using the vector radiative tranfer code of Ahmad and Fraser (AF), as recently updated by Z. Ahmad. This was done to ensure consistency in the Rayleigh tables between sensors and to ensure consistency with the new aerosol models (see below). The AF code was also used for the MODIS Atmosphere products, so consistency across disiplines is an added benefit. The AF computations used the Rayleigh optical thicknesses and spectrally-dependent depolarization factors described above.

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Aerosol models and model selection method

A new suite of aerosol models have been developed based on knowledge gained from many years of AeroNET observations. The new models are based on observed size distributions and single-scattering albedos from AeroNET sites on various coastal and island locations. Radiative transfer computations were performed with the AF code described above, and 70 models were generated spanning 10 fine-to-coarse-mode size fractions and 7 relative humidities (RH). The model selection process was also adapted to utilize RH observations to limit the number of models to be considered.

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NO2 corrections

Corrections were added to account for absorption by nitrogen dioxide (NO2), using the approach described in Ahmad et al. 2007 and a time-series of tropospheric and stratopheric NO2 retrievals as described below.

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- By norm Date 2009-06-16 21:53
I noted in the sources page, http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/sources/), the tables of band-averaged values, which is very helpful.

It grabbed me that the aw for SeaWiFS and MODIS in the "412" channel is very different. For MODIS you give a band center of 412.123 nm (14.5 nm b/w) and aw of 0.0203 m^-1. For SeaWiFS the band center is 413.306 nm (20.124 nm b/w) and aw is ~ 0.005 m^-1 -- factor of four difference here. Is this because of the different bandwidths? It seems to me that's not a tremendously steep part of the aw spectrum so I'm not sure how this could play a role, but maybe I'm mistaken in that. It seems like this can't help but have an effect on whatever calculations are done, like a GSM inversion.

Cheers, Norm
- By bryan Date 2009-06-16 22:25
Norm,

As with many good questions, it's been asked before.  The explanation for the discrepancy is in the out-of-band response, as Sean explained here:

  http://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?pid=1595;hl=Rsr#pid1595

However, this has no direct impact to bio-optical inversions like GSM.  In standard processing, we correct the retrieved Rrs from full-band values to idealized 11-nm square bandpasses centered on the nominal sensor wavelengths.  These "nominal-band" Rrs values are what is provided to the bio-optical algorithms.  We therefore use nominal-band aw and bbw values in the bio-optical modeling.  For those numbers and related discussion, see:

  http://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?tid=2657#pid10301

I think the only place where we use the full-band aw and bbw is in the atmospheric correction process; in the iterative modeling of water-leaving radiance contributions to the NIR. 

Regards, Bryan
- By zchen Date 2009-06-17 09:09
Hi all,

Could you please let us know what version of SeaDAS will be used for processing?

thanks;

Zchen
- By bryan Date 2009-06-17 11:09
Once the reprocessing configuration and code are finalized, and reprocessing has started, a new version of SeaDAS will be created to reflect the latest processing code changes and default parameters. 

-- bryan
- By norm Date 2009-06-17 10:22
Thanks, Bryan. I am still surprised by the magnitude of the difference.

How come you're using Smith and Baker here and Pope and Fry for the other?

I have to spend more time on this site -- you've probably already answered that one too.
- By bryan Date 2009-06-17 11:19
We're using Smith and Baker for scattering, Pope and Fry + Kou et al. for absorption:

  http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/sources/water_coef.txt

Discussion on that point is welcome.  We expect to produce an additional suite of IOP-related products, once the standard radiance and chlorophyll products are completed.  See more discussion on IOP development and plans here:

  http://oceancolor.gsfc.nasa.gov/forum/oceancolor/board_show.pl?bid=24

-- bryan
- By norm Date 2009-06-17 11:31
OK, that is good. I misinterpreted the "!       300-380 Smith and Baker 1981" in the header of that file.
Sheesh, I am obtuse today.
- By WangFeng Date 2009-06-17 20:10
Hi sean,
When and where the new suite of aerosol model LUTs are available for SeaDAS?
- By gene Date 2009-06-18 07:31
to quote bryan's response up above:

"Once the reprocessing configuration and code are finalized, and reprocessing has started, a new version of SeaDAS will be created to reflect the latest processing code changes and default parameters.

-- bryan "
- By jti90292 Date 2010-03-22 03:16
Besides the NO2 corrections, would you please provide the algorithms and references for water vapor and oxygen corrections? Is there a CH4 correction as well? Thanks.

Regards,
Justin
Up Topic Special Topics / Ocean Color Reprocessing / Atmospheric correction (locked)



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