Earthdata Forum

Dear SeaDAS team,

I have some questions regarding the Rayleigh scattering lookup tables used in your atmospheric correction algorithms. I noticed that earlier versions used the lookup tables generated by Gordon and Wang (1994), but more recent versions seem to use tables generated by the Ahmad-Fraser Code (Ahmad and Fraser, 1982), updated in 2018.

I would appreciate clarification on the following points:
1) After comparing the two sets of lookup tables, I observed noticeable differences, especially at smaller solar zenith angles. Could you please explain the reason for this discrepancy?
2) What are the specific differences between the two lookup tables, and why was the change made from the Gordon & Wang tables to the Ahmad-Fraser ones?
3) Is the original Gordon and Wang (1994) lookup table still available for download? If so, could you please let me know where I can access it?

Thank you in advance for your help!

Best regards,
Diana Zhao

Diana,

The switch to using the Ahmad-Fraser code for the RT table generation was done for the R2009 reprocessing, so while it seems recent to me, too, it was almost 16 years ago. In fact, since then, we've switched again to code developed by Pengwang Zhai* (only applied to PACE OCI right now, but will be used for all missions with the next multi-mission reprocessing). There were a number of reasons for the switch from the original Gordon and Wang models, including:

• A-F is a vector RT code, GW is a scalar code (this explains much of the differences you see)

• ensure consistency in the Rayleigh tables between sensors

• ensure consistency with the A-F aerosol models that were also implemented with R2009

• Zia Ahmad joined our group and we had direct control over the code used so could add support for more sensors - something we could not do with the GW code

I still have access to the SeaDAS distributions going back to SeaDAS v4.5 (Feb 2004, so 20 year old stuff...). The GW tables were last distributed with SeaDAS v5.4 (June 2009). They're not gone, just not readily available to the world. It has been on my todo list to make as many of the old SeaDAS distributions available as I can, but with NASA's web modernization effort and the uncertainties with where this content will ultimately reside, I have not made much progress. I have placed a single GW Aqua MODIS Rayleigh table at the following link for comparison purposes:
https://oceancolor.gsfc.nasa.gov/fileshare/sean_bailey/seadas/rayleigh_modisa_443_iqu.hdf

Note for the future...the oceancolor domain is to be migrated to earthdata.nasa.gov, so the above link will only be available until that time...

Regards,
Sean

*Zhai, P., Hu, Y., Chowdhary, J., Trepte, C. R., Lucker, P. L., Josset, D. B. (2010). A vector radiative transfer model for coupled atmosphere and ocean systems with a rough interface. Journal of Quantitative Spectroscopy and Radiative Transfer, 111(7), 1025-1040.
Zhai, P., Gao, M., Franz, B. A., Werdell, P. J., Ibrahim, A., Hu, Y., Chowdhary, J. (2022). A Radiative Transfer Simulator for PACE: Theory and Applications. Frontiers in Remote Sensing