34
J. Gower and S. King
relation (Gower and King 2007a) is:
if L 865 < 15 mW m
−2 sr
−1 nm
−1 ,
then FLH = L 681 − L 665 − 0.364 (L 709 − L 665 )
(2.1)
where L 865 represents Level 1 radiances (as measured at the satellite) at a wavelength
of 865 nm, and similarly for other wavelengths, and the factor 0.364 represents the
wavelength ratio (681–665)/(709–665). For MODIS, bands are different, as noted
above, and the factor becomes 0.096.
It is often assumed that the measured fluorescence signal will increase with sun
elevation, and this is compensated by normalizing FLH to what would be observed
had the sun been at zenith (NFLH), that is NFLH = FLH/cos(Z), where Z is the zenith
angle of the sun at the time of observation. In fact, we show below that normalization
is not appropriate.
We have suggested that fluorescence is proportional to chlorophyll concentration
at low concentrations, but saturates due to absorption of both stimulated and emitted
radiation at higher concentrations (Gower et al. 2004; Gower and King 2007a). For
FLH computed from Level 1 data, we propose
FLH = (0.18C/(1 + 0.2C)) − 0.24
(2.2)
as a good average relation between chlorophyll (in mg.m
−3 ) and fluorescence in
radiance units (W m
−2 nm
−1 ster
−1 ). The offset, here given as − 0.24, is variable,
depending on viewing and atmospheric conditions. The factor 0.2 is the ratio of
absorption of fluorescence by 1 mg m
− 3 of chlorophyll, to the absorption of fluorescence by water. The term appears in equation (2.2) by analogy with the relation
R = k × b/a for water reflectance (Morel and Prieur 1977). This factor expresses the
fact that FLH increases less rapidly with C, as C increases. At a value of C = 20 mg
m
− 3 the rate is reduced by a factor 5 from the value at low C. At higher values,
the combination of water and chlorophyll absorption results in a radiance peak near
700 nm for C = 30 mg m
− 3 , and near 710 nm for C = 300 mg m
− 3 , which prevents
observation of fluorescence.
2.3 Detection of Blooms and Vegetation Using MCI
MERIS provides the Maximum Chlorophyll Index (MCI), useful for detecting concentrated surface blooms and floating vegetation. MCI measures a radiance peak at
700–710 nm. The index is unique to MERIS, in that MODIS and VIIRS do not have
a band near 709 nm. The peak indicates the presence of a high surface concentration
of chlorophyll-a against a scattering background (Gitelson et al. 1992; Yacobi et al.
1995; Gower et al. 1999, 2005, 2008a, b). MCI is high in “red tide” conditions
(intense, visible, surface plankton blooms), and also when aquatic vegetation is
present leading to a “red edge” step increase in radiance. For some blooms, the
peak dominates the spectrum, giving a radiance change due to the bloom that is over
ten times greater at 709 nm than at any other wavelength. We have demonstrated
use of the MCI to detect plankton blooms (Gower et al. 2005), floating Sargassum
J. Gower and S. King
relation (Gower and King 2007a) is:
if L 865 < 15 mW m
−2 sr
−1 nm
−1 ,
then FLH = L 681 − L 665 − 0.364 (L 709 − L 665 )
(2.1)
where L 865 represents Level 1 radiances (as measured at the satellite) at a wavelength
of 865 nm, and similarly for other wavelengths, and the factor 0.364 represents the
wavelength ratio (681–665)/(709–665). For MODIS, bands are different, as noted
above, and the factor becomes 0.096.
It is often assumed that the measured fluorescence signal will increase with sun
elevation, and this is compensated by normalizing FLH to what would be observed
had the sun been at zenith (NFLH), that is NFLH = FLH/cos(Z), where Z is the zenith
angle of the sun at the time of observation. In fact, we show below that normalization
is not appropriate.
We have suggested that fluorescence is proportional to chlorophyll concentration
at low concentrations, but saturates due to absorption of both stimulated and emitted
radiation at higher concentrations (Gower et al. 2004; Gower and King 2007a). For
FLH computed from Level 1 data, we propose
FLH = (0.18C/(1 + 0.2C)) − 0.24
(2.2)
as a good average relation between chlorophyll (in mg.m
−3 ) and fluorescence in
radiance units (W m
−2 nm
−1 ster
−1 ). The offset, here given as − 0.24, is variable,
depending on viewing and atmospheric conditions. The factor 0.2 is the ratio of
absorption of fluorescence by 1 mg m
− 3 of chlorophyll, to the absorption of fluorescence by water. The term appears in equation (2.2) by analogy with the relation
R = k × b/a for water reflectance (Morel and Prieur 1977). This factor expresses the
fact that FLH increases less rapidly with C, as C increases. At a value of C = 20 mg
m
− 3 the rate is reduced by a factor 5 from the value at low C. At higher values,
the combination of water and chlorophyll absorption results in a radiance peak near
700 nm for C = 30 mg m
− 3 , and near 710 nm for C = 300 mg m
− 3 , which prevents
observation of fluorescence.
2.3 Detection of Blooms and Vegetation Using MCI
MERIS provides the Maximum Chlorophyll Index (MCI), useful for detecting concentrated surface blooms and floating vegetation. MCI measures a radiance peak at
700–710 nm. The index is unique to MERIS, in that MODIS and VIIRS do not have
a band near 709 nm. The peak indicates the presence of a high surface concentration
of chlorophyll-a against a scattering background (Gitelson et al. 1992; Yacobi et al.
1995; Gower et al. 1999, 2005, 2008a, b). MCI is high in “red tide” conditions
(intense, visible, surface plankton blooms), and also when aquatic vegetation is
present leading to a “red edge” step increase in radiance. For some blooms, the
peak dominates the spectrum, giving a radiance change due to the bloom that is over
ten times greater at 709 nm than at any other wavelength. We have demonstrated
use of the MCI to detect plankton blooms (Gower et al. 2005), floating Sargassum