Detecting CDOM Fluorescence Using High Spectrally Resolved Satellite Data: A Model Study - Towards an Interdisciplinary Approach in Earth System Science - page 109

Fig. 5 The differential part of the reference spectra calculated according to Eq. 2 in the spectral
region of Fraunhofer line H b at 486 nm calculated for different concentrations of humic and fulvic
acids and for different inelastic processes: CDOM fluorescence (left panel), VRS (middle panel),
CDOM fluorescence and VRS simultaneously (right panel)

varying concentration of fulvic and humic acids (from none to 100 g m − 3 of each)
on the differential reference spectra of CDOM and VRS. In this wavelength region
it is more likely to observe the CDOM fluorescence with satellite measurements.

However, in clear waters, the VRS signal is dominant, but is weakening with
increasing CDOM concentrations (Fig. 5 middle panel). In case of CDOM con-
centration of 1 g m − 3 fulvic and 1 g m − 3 humic acids, both CDOM

fl

fl uorescence and

filling-in. In case of CDOM being a mixture of 50 g m − 3 of
fulvic and 50 g m − 3 humic acids, VRS signal is one order of magnitude weaker than
CDOM

VRS lead to similar

fluorescence, and for 100 g m − 3 of fulvic and 100 g m − 3 of humic acids, the
VRS signal is negligible (Fig. 5 ). However, the

fl

uores-
cence itself remains almost constant through all the scenarios of different CDOM
concentrations (Fig. 5 left panel). It is due to the fact that with increasing CDOM
concentration, light is absorbed stronger and does not penetrate that deep into the
water column, hence the bulk CDOM

filling-in due to CDOM fl

fluorescence signal originates from thinner
surface layer of the ocean and is not signi

fl

cantly higher. Our results agree with the
previous studies by Pozdnyakov et al. ( 2002 ), who showed that VRS is negligible
for waters with high CDOM concentrations. Their results also indicate that the
CDOM

fluorescence signal does not increase substantially with much higher
CDOM concentrations, even though it becomes relatively more signi

fl

cant, as in
general the upwelling radiance decreases. However, contrary to their results, we
found that the VRS in clear waters is stronger than CDOM

fluorescence in waters of
high CDOM concentration. These discrepancies might be due to the fact that while
we are investigating the impact of inelastic process on the

fl

filling-in of Fraunhofer
lines at the top of the atmosphere, Pozdnyakov et al. ( 2002 ), focused on the water
volume re

ectance. Furthermore, we used the coupled ocean-atmosphere radiative
transfer model, while Pozdnyakov et al. ( 2002 ), used simple hydrooptical models,
which calculate the volume re

fl

fluorescence directly
from optical properties of water and CDOM. For scenarios of simultaneous CDOM
fl

fl ectance arising from CDOM fl

filling-in signal is in fact
getting weaker with increasing CDOM concentration (Fig. 5 right panel).
Hence,

fluorescence and VRS, we observe that the combined

fluorescence and VRS are coupled: when
there is no CDOM, the VRS signal is strong. On the other hand, when CDOM

filling-in signals due to CDOM fl