Earthdata Forum

Dear Sir,

Would like know what is the difference between MODIS-A 'FLH' and 'nFLH' product and why we should use the 'nFLH' product? Though the algorithm that was develpoed is for 'FLH' as mentioned in MODIS, ATBD-22.

we have already gone through the linke ' https://oceancolor.gsfc.nasa.gov/atbd/nflh/'.

We would like to know the basis for nFLH.

The reference mentioned below has not provided the details of the above.

Behrenfeld, M. J., Westberry, T. K., Boss, E. S., et al. (2009). Satellite-detected fluorescence reveals global physiology of ocean phytoplankton. Biogeosciences 6, 779-795.http://dx.doi.org/10.5194/bgd-5-4235-2008

Hence, request you to kindly clarify the doubt.

Best Regards

The terminology is a bit muddled, due to multiple names for the same product.  The OCSSW source code is your best reference when you want to know exactly how a product is computed.     If you have the OCSSW Processing System the place to start for such questions is $OCSSWROOT/share/common/product.xml, where you can find the hint: <!-- fsat is nflh -->.  With this, you can compare Appendix A of the paper with the actual source code, $OCSSWROOT/ocssw-src/src/l2gen/fluorescence.c:


[...]
/*---------------------------------------------------------------------*/
/* fsat_modis - normalized fluorescence line height for each pixel     */
/*             fsat output is in radiance units (mW/cm^2/um/sr)        */
/*---------------------------------------------------------------------*/

/**
* flh_modis - computes normalized fluorescence line height for MODIS
* computation based on nLw measurements (thus the normalized moniker)
* @param l2rec
* @param flh
*/
void fsat_modis(l2str *l2rec, float flh[]) {
[...]
            /**
             * fsat (Behrenfeld et al.[2009] equation A2)
             */
            flh[ip] = nLw2 - (70. / 81.) * nLw1 - (11. / 81.) * nLw3;

            // base = nLw3 + (nLw1 - nLw3) * ((748.0 - 678.0) / (748.0 - 667.0));
            // flh[ip] = nLw2 - base - input->flh_offset;

            /**
             * bias correction as per Behrenfeld et al.
             */
            flh[ip] -= bias;


If you don't have access to Linux or MacOS you can install git and follow the manual installation instructions to download the bundle with the OCSSW sources and use git to extract the source files.