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Rayleigh optical thickness in msl12_sensor_info.dat

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

Rayleigh optical thickness in msl12_sensor_info.dat

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

Rayleigh optical thickness in msl12_sensor_info.dat

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

Rayleigh optical thickness in msl12_sensor_info.dat

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