Rayleigh optical thickness in msl12_sensor_info.dat

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fsteinmetz
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Rayleigh optical thickness in msl12_sensor_info.dat

by fsteinmetz » Wed Apr 25, 2018 4:58 pm America/New_York

Dear SeaDAS team,

I am relying on Rayleigh optical thickness values provided in msl12_sensor_info.dat, as Tau_r (for VIIRS, SeaWiFS and MODIS).
However after recalculating these values using the SRF as provided in https://oceancolor.gsfc.nasa.gov/docs/rsr/ and using [Bodhaine, 99], and I am finding large discrepancies, for example for VIIRS from +2% at 410nm, to nearly -2% at 671nm - with differences of similar order of magnitudes for MODIS and SeaWiFS.
Could you clarify how these Tau_r values have been generated, and how they are used ?

Thanks,
François

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OB.DAAC - SeanBailey
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Rayleigh optical thickness in msl12_sensor_info.dat

by OB.DAAC - SeanBailey » Mon Apr 30, 2018 2:38 pm America/New_York

François,

The method we used is described here: https://oceancolor.gsfc.nasa.gov/reprocessing/r2009/sources/

To make sure I could replicate the numbers following the description given, I whipped up a quick python script...and turned it into an example which I've added to the above webpage.

BTW, this is NOT the actual code we used (that was in IDL), but it was nice to see I could match the numbers independently :grin:

I hope this helps.

Sean

fsteinmetz
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Rayleigh optical thickness in msl12_sensor_info.dat

by fsteinmetz » Wed May 02, 2018 10:18 am America/New_York

Hi Sean,

Many thanks for your detailed answer !
Now I understand why I have found different values. The first thing I did when looking at the spectral response (semilogy(rsr.index, rsr.M1)) was to filter out the values lower than 2% (rsr.M1[rsr.M1 < 0.02] = 0).
So apparently I should not do that, but I am a bit worried by the look of the RSR and by the uncertainty that these out-of-band responses (>1% around 800nm for M1) introduce in the ROD calculation.

François

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Rayleigh optical thickness in msl12_sensor_info.dat

by OB.DAAC - SeanBailey » Thu May 03, 2018 8:07 am America/New_York

François,

Yes, it can be a little disconcerting to see  significant out-of-band responses - but that is what was measured and needs to be included.

Regards,
Sean

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