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Fig. 7.15
Depth-dependence
of the ratio of downwelling
irradiance (q
d
) to scalar
irradiance (q
0
) in the surface
layer of Estonian lakes Harku
and Maardu in,
a
winter with
ice cover and
b
summer (Arst
et al. 2006)
that below the surface of ice-free water (Arst et al. 2006). In contrast to ice-free situations,
beneath ice scalar irradiance decreases with increasing depth more rapidly than planar
irradiance, which means that diffuse attenuation coef
cient is larger for the scalar irra-
diance than for the planar irradiance (Fig.
7.15
).
Re
ectance is the ratio of
upwelling irradiance to downwelling irradiance just above the surface, and transmittance
is the ratio of downwelling irradiance below the ice to downwelling irradiance just above
the ice. They are illustrated for different lakes in Finland and Estonia in Fig.
7.16
.Itis
seen that in the melting season they both are quite different from earlier winter situations.
Re
fl
ectance and transmittance are spectral quantities. Re
fl
ectance is rather even across the optical band, and in a few humic lakes the absorption
of short-wave radiation is strong.
fl
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