ask some quesitons about AOT and Angstrom Exponent

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hxczjh123
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ask some quesitons about AOT and Angstrom Exponent

by hxczjh123 » Wed Jul 11, 2018 9:36 pm America/New_York

Hello everyone?

To my understand?when the bands increase?then the AOD decrease.And?the range for Angstrom Exponent is from 0 to 2.
But recently?I have used seadas to generate GOCI L2 products using NIR iterative method?Bailey?2010??and match them up with Microtops which measure AOD. But in China yellow sea?for some match-up points?the GOCI AOD products(derieved from SeaDAS l2gen)decrease from 412nm to 745nn?and AOD in 865nm suddenly is larger than 412nm to 745nm?for others match-up points?AOD(derieved from Seadas l2gen) increase from 412nm to 865nm?and AE is negative values. So?these values for AOD and AE can not be trusted?Do these values I should abondon it?

Thank you so much for your help!

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OB.DAAC - SeanBailey
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ask some quesitons about AOT and Angstrom Exponent

by OB.DAAC - SeanBailey » Fri Jul 13, 2018 11:09 am America/New_York

Angstrom can be negative.

You can trust the AOT and Angstrom retrievals if the assumptions used in the GW approach are valid.  They may not always be valid. For example:

1) The aerosol models used by l2gen are currently non- (or weakly-) absorbing, and my not be representative of the aerosols over they Yellow sea
2) Parts of the Yellow Sea can be extremely turbid, particularly close to shore.  It is quite likely that the iterative "NIR correction" may fail to converge.

Sean

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ask some quesitons about AOT and Angstrom Exponent

by hxczjh123 » Mon Jul 16, 2018 6:47 am America/New_York

Dear Sean,
Thank you for your always reply!
1. I haved download GOCI L2 products from this site. But I found there are some negative values for Rrs(660)?Rrs(680) even in open oceans.What's the reason?
2. These GOCI L2 products generate using Gordon and Wang method or your method(Bailey,2010)?  Do it consider iterative?

OB.DAAC - SeanBailey
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ask some quesitons about AOT and Angstrom Exponent

by OB.DAAC - SeanBailey » Mon Jul 16, 2018 10:39 am America/New_York

It is not unusual for the red bands to go negative in open ocean waters - water-leaving the signal is small in the red in clear waters.  Without a REALLY high SNR,  small (just above zero)  signals can legitimately be 'negative'.   Could also be the calibration. The vicarious calibration of GOCI was a one-time "best effort", since we're not funded to maintain it, it's likely not the best it could be.
Yes, the default GW with Bailey 2010 is iterative.

Sean

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ask some quesitons about AOT and Angstrom Exponent

by hxczjh123 » Tue Jul 17, 2018 10:14 pm America/New_York

Dear Sean,
Thank you so much for your help!
So, you mean the NASA GOCI L2 products(GW method with Bailey(2010)) using the same method with using l2gen----aer_opt----choose '-2- Multi-scattering with 2-band,RH-based model selection and iterative NIR correction' ?
Best Regards.

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ask some quesitons about AOT and Angstrom Exponent

by OB.DAAC - SeanBailey » Wed Jul 18, 2018 9:03 am America/New_York

yes

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ask some quesitons about AOT and Angstrom Exponent

by hxczjh123 » Fri Aug 10, 2018 4:00 am America/New_York

Dear Sean,
Thank you for your reply! Could you please explain the one-time "best effort" specifically?
Thank you so much!

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ask some quesitons about AOT and Angstrom Exponent

by OB.DAAC - SeanBailey » Fri Aug 10, 2018 10:08 am America/New_York

We defined a simple vicarious calibration (we in fact used one defined by another group) at one time without any in depth or long term evaluation.

Sean

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