New algorithm for MODIS chlorophyll fluorescence height retrieval: performance and comparison with the current product

Abstract

Our previous studies showed that the Fluorescence Line Height (FLH) product, which uses 3 NIR bands at 667, 678, and 746 nm on the MODerate-resolution Imaging Spectroradiometer (MODIS) sensor, and similar bands on MERIS sensor, is not reliable in coastal waters because of a peak in the elastic reflectance spectra which occurs due to the confluence of chlorophyll and water absorption spectra and which overlaps spectrally the chlorophyll fluorescence. This combination of two overlapping peaks makes fluorescence signal retrieval inaccurate. As a consequence, the present FLH algorithm significantly underestimates fluorescence magnitudes in coastal waters. To overcome this problem, we introduce a new and more accurate approach for the retrieval of FLH in turbid waters by the MODIS sensor, which exploits the correlation between the blue-green and red bands reflectance ratios. We show that by making use of the combined remote sensing reflectance's (Rrs) at 488nm, 547nm, 667nm and 678nm we can retrieve fluorescence accurately in case 2 waters even for low fluorescence quantum yield when fluorescence magnitudes are low. The derivation and validation of our algorithm was performed using extensive synthetic datasets which cover a large variability of parameters typical of coastal waters: with CDOM absorption at 400nm 0-2 m^-1, mineral concentration 0-5g/m³ and chlorophyll concentration of 0.5-100 mg/m³. In addition, we applied this proposed algorithm to MODIS satellite data and compared it with the traditional FLH algorithm.