THE NEAR-SURFACE LAYER OF THE OCEAN
For rough seas, the angle of incidence of direct solar rays varies with the
slope of the waves. Difficulty again arises when the sun is low on the
horizon, giving rise to shadow zones and multiple reflections. For solar
elevation greater than about 25
o surface roughness slightly increases the
ocean surface reflectance. For lower angles, surface roughness results in a
sharp reduction of the reflectance (Figure 1-7).
The formula for albedo (1.56) contains the ratio of upward to downward
irradiance, I u /I d . Its magnitude and spectral distribution vary considerably as
the absorption and scattering properties of the water change. In extremely
clear water the spectral ratio of the upward to downward irradiance can
reach 10% for O = 0.4 m wavelength, while in strongly scattering water
this ratio can also reach 10% for Obetween 0.55 to 0.56 Pm. Ratio I b /I d
(remember that I u and I d are the integrals over all wavelengths) has a
maximum of only 2% (when the water is very clear or strongly scattering)
though it does not exceed 0.5% in typical situations. The upwelling light
from the sea determines ocean color, which contains useful information
about the upper ocean waters. The ocean color can be remotely sensed from
satellites (see Section 7.1.6).
Experimental values of the albedo are very close to those shown above
for the reflectance. Under clear skies, when solar elevations [ exceed 20
o -
25
o
, the sea state plays a relatively minor role, the albedo of the sea can be
roughly estimated from the Laevastu (1960) formula:
300 /
A
[ ,
(1.58)
where [is in degrees, and A is in %.
For angles 25
o < [< 50
o , empirical formula
250 /
A
[
(1.59)
is more accurate than (1.58).
Payne (1972) parameterized sea surface albedo values from field data as
a function of atmospheric transmittance (defined as the ratio of downward
irradiance incident at the sea surface to irradiance at the top of the
atmosphere) and solar zenith angle. Subsequent field study of Katsaros et al.
(1985) and simulation with a radiation model by Ohlmann et al. (2000a)
demonstrated good agreement with the Payne et al. (1972) parameterization
for solar elevation angles [ >25
o .
For solar elevations [ <25
o , the values obtained by different authors,
however, differ greatly as a result of the strong influence of the sea state.
This presents a serious problem when calculating the rate of solar energy
absorption by the ocean in latitudes higher than 50
o or 60
o . For these
28
P
For rough seas, the angle of incidence of direct solar rays varies with the
slope of the waves. Difficulty again arises when the sun is low on the
horizon, giving rise to shadow zones and multiple reflections. For solar
elevation greater than about 25
o surface roughness slightly increases the
ocean surface reflectance. For lower angles, surface roughness results in a
sharp reduction of the reflectance (Figure 1-7).
The formula for albedo (1.56) contains the ratio of upward to downward
irradiance, I u /I d . Its magnitude and spectral distribution vary considerably as
the absorption and scattering properties of the water change. In extremely
clear water the spectral ratio of the upward to downward irradiance can
reach 10% for O = 0.4 m wavelength, while in strongly scattering water
this ratio can also reach 10% for Obetween 0.55 to 0.56 Pm. Ratio I b /I d
(remember that I u and I d are the integrals over all wavelengths) has a
maximum of only 2% (when the water is very clear or strongly scattering)
though it does not exceed 0.5% in typical situations. The upwelling light
from the sea determines ocean color, which contains useful information
about the upper ocean waters. The ocean color can be remotely sensed from
satellites (see Section 7.1.6).
Experimental values of the albedo are very close to those shown above
for the reflectance. Under clear skies, when solar elevations [ exceed 20
o -
25
o
, the sea state plays a relatively minor role, the albedo of the sea can be
roughly estimated from the Laevastu (1960) formula:
300 /
A
[ ,
(1.58)
where [is in degrees, and A is in %.
For angles 25
o < [< 50
o , empirical formula
250 /
A
[
(1.59)
is more accurate than (1.58).
Payne (1972) parameterized sea surface albedo values from field data as
a function of atmospheric transmittance (defined as the ratio of downward
irradiance incident at the sea surface to irradiance at the top of the
atmosphere) and solar zenith angle. Subsequent field study of Katsaros et al.
(1985) and simulation with a radiation model by Ohlmann et al. (2000a)
demonstrated good agreement with the Payne et al. (1972) parameterization
for solar elevation angles [ >25
o .
For solar elevations [ <25
o , the values obtained by different authors,
however, differ greatly as a result of the strong influence of the sea state.
This presents a serious problem when calculating the rate of solar energy
absorption by the ocean in latitudes higher than 50
o or 60
o . For these
28
P