Earthdata Forum

Hello,
When I open MODIS ocean color level-2 data on seadas, there are two options for getting chlorophyll-a concentration.One is named as chlor_a that uses OCI algorithm another is chl_ocx that uses OC3 algorithm.
I went through a document on ocean colour website (https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/) that says, default chlorophyll-a retrieving algorithm uses combination of OCX algorithm and colour index algorithm.
Chlorophyll-a concentration obtained using Chlor_a option on seadas, is derived using only colour index algorithm or combination of color index and ocx algorithm?  Kindly answer my question.

Manzar

The chlor_a data field in our L2 files are a blend of the CI algorithm for low chlorophyll retrievals and the OCx  algorithm for higher retrievals.  The documentation you sited is correct.

don

Thank you for the reply. I got my answer.

What bands of MODIS aqua are used for CI algorithm. I went through several documents but could not get this answer. Please reply

Did you look here:  https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/

Section 2 ...

...but in case of TL;DR...:wink: they're  443, 547, and 671

Sean

Thank you for your reply.

Yes I read that page. It says that bands closest to 443, 555 and 670nm are used. I found it little confusing as MODIS aqua does have 555 band but there is no band 671. However, 667 and 678 bands are available which are close to 670 (I downloaded .nc level 2 data from ocean colour web and opened it on seadas).

Yeah, I suppose that can be confusing...especially when I mistype 667 as 671 (got VIIRS on the brain) ;)
MODIS *has* 555, but for the ocean color world it's green band is 547 (also listed elsewhere as 551 - the spec doesn't match reality).
The 555nm band is a "land" band, with a lower SNR and broader bandpass than the "ocean" 547nm band.  The 678nm band is
the fluorescence channel - and while close to 670 it's farther from it than 667.  For the ocean products, look at the ocean band set :)

Sean

Thank you Sean.