Threshold Definition

[cassandra21]

Hi everyone,

Currently, I am working with monthly statistics derived from L2map MODIS-Aqua satellite parameters.
When conducting a monthly statistical analysis on the maximum values in the Cape Verde region, specifically POC, I found that around 70% of the registered values align with the maximum threshold set by NASA for L3 images. If the threshold were to be increased, what would be the outcome? Are the thresholds established to prevent external contamination in the data?

Thank you :)

[OB General Science - guoqingw]

Hello,

The POC algorithm is mainly for open ocean or Case 1 waters, and it relies on the fact that variation in Rrs(λ) at blue wavelengths are primarily driven by phytoplankton pigments and covarying matters. For coastal or optically complex case 2 waters, the empirical relationship used in the algorithm may not always hold. For the threshold, you may want to check the corresponding Rrs(λ) ratios for your study area.

The following is the algorithm description for your reference:
https://oceancolor.gsfc.nasa.gov/resources/atbd/poc/;

And the original publication:
https://bg.copernicus.org/articles/5/171/2008/bg-5-171-2008.pdf

Hope that helps.
Guoqing

[cassandra21]

Hi!

Silly question: if POC algorithm is mainly for open ocean or case 1 waters, we can assume that the same applies for Kd490 and Chl a, correct?

[OB General Science - guoqingw]

They are global algorithms, based on similar assumptions, and use the blue-green band ratios. Chlorophyll algorithm is a little complicated with the blend of a band difference and band ratio algorithms.

[cassandra21]

Hi there!

I need some help with the last answer I got. For the Kd490 and POC products, to a certain extent you would expect similar results as these two algorithms have similar assumptions on their basis and both use the blue/green ratio, right? I am also getting similar results between these two and Chl a and I was expecting this to be somewhat correlated with the algorithm itself rather than just the type of water (case-1). So it would be wrong to assume that the existing similarities are also driven by the algorithm? Should I state that as I am working with Case-1 waters, Chl a is the main constituent influencing the light and therefore the Kd_490 results, plus the fact that the POC represents the organic matter available in the water as this is a product of synthesis and early degradation of planktonic primary production?

Thanks for your help!

[OB General Science - guoqingw]

For case-1 water, where phytoplankton is the primary constituent influencing the optical properties, the algorithms for Chla, Kd490, and POC were developed based on this assumption. If we’re discussing similarities among NASA standard products for these variables, the algorithms’ shared foundation is one factor. However, from a biological oceanography perspective, the main reason for their similarity lies in phytoplankton's role as the dominant component in these waters. Ultimately, in situ data would provide the most reliable validation and insights into the true relationships among these variables.

Hope this helps,
Guoqing