## Chlorophyll-A algorithm question

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samsiuca
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### Chlorophyll-A algorithm question

Hello,

The algorithm product summary https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/ describes the result as "near surface concentration". How deep into the water does "near-surface" imply? An infinitesimally thin layer on the surface of the water? Or perhaps some centimetres/metres down (the Hu et. al paper referenced in the product page uses <30m deep waters for testing)?

Thanks,
- Sam

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gnwiii
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### Chlorophyll-A algorithm question

Roughly speaking, you get a weighted contribution from the underwater concentration.   The wavelengths used in chlor_a algorithms can penetrate many meters, but due to attenuation of the light on both downward and return paths, the contribution to Rrs is heavily weighted towards the surface.   You can get a rough idea of the weighting by plotting `exp(-2*Kd*z)` (using `z` for depth).  One practical consequence is that remote sensing chlor_a underestimates the contribution of deep chl maxima (doi:10.3390/rs9030210 gives to references to several papers that discuss the problem).

samsiuca
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Joined: Tue Feb 28, 2017 8:20 am America/New_York

### Chlorophyll-A algorithm question

Thanks for the quick response gnwiii!

When you say ""Kd", I assume you are referring to https://oceancolor.gsfc.nasa.gov/atbd/kd_490/? Is there a particular paper(s) or study from where the -2 coefficient comes from (as https://oceancolor.gsfc.nasa.gov/docs/technical/NASA-TM-2016-217551.pdf just writes exp(?Kd*z))?

- Sam

gnwiii
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### Chlorophyll-A algorithm question

The factor of two comes from the double length of the path down and back up.  The simple exponential  formula gives a rough and ready approximation for the contribution from various depths, so Kd_490 can be used "for purposes of illustration".

samsiuca
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Joined: Tue Feb 28, 2017 8:20 am America/New_York

### Chlorophyll-A algorithm question

Hello,

Within the file https://oceancolor.gsfc.nasa.gov/docs/ocssw/chl_8h_source.html , I see that a chlmax variable has been set to 1000.0 , which I assume is the same limit Sean Bailey mentioned in https://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?pid=25280;hl=chlor%20100. Is there a reason why this cap is in place (it seems a bit arbitrary)?

Best Regards,
- Sam S.

OB.DAAC - SeanBailey
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