Determination of Parameters of the Atmosphere and the Surface in a Clear Atmosphere - Short-Wave Solar Radiation in the Earth's Atmosphere - page 176

bright surfaces (sand, snow) and the problem of the aerosol phase function
approximation is not so important for the present instance.
As the width of the instrumental function of the K-3 spectrometer is 6 nm,
in addition to the evident spectral dependence of the molecular absorption
within this interval there is the spectral dependence of the volume coefficients
of the molecular scattering, and of the aerosol scattering and absorption. For
example, the difference between the values of the volume coefficient andmolec-
ular scattering in interval 6 nm reaches 10% in the UV region. The estimation
of the influence of ignoring this dependence on the uncertainty of the solar
irradiance calculation shows that the spectral dependence of the molecular
scattering needs to be accounted for and the aerosol dependence could be
neglected. The latter provides essentially for faster work of the algorithm.
The general estimation of the simplifications and approximations intro-
duced to the applied algorithm is demonstrated in Table 5.2. Note that the
uncertainties caused by the molecular absorption and aerosol scattering have
been calculated for the corresponding extremal models, and hence, could be
considered as the limit ones. The uncertainties presented in Table 5.2 are the
systematic ones, so their mutual compensation could occur. Thus, the com-
parisonbetweentheresultsofthecalculationswith“turningon”(applied
algorithm) and “turning off” (etalon algorithm) all possible simplifications
has been accomplished to estimate the total accuracy of the direct problem

Tab l e 5 . 2 . Estimation of the uncertainty of the irradiance calculations

Source of the uncertainty Uncertainty (%) Comments

λ i Considering the absorption by five

gases only

0.1-0.5 At separate wavelengths

λ i

Considering the molecular
absorption in the selected spectral
intervals only

0.3 At separate wavelengths

λ i Tabulating the cross-sections of the

molecular absorption

0.7-2.0 At separate wavelengths

Ignoring the polarization 1.5 Decreasing from the UV to the

VD and NIR spectral regions

Maximal for F ↑ above the water

Parameterization of the aerosol
phase function

1.0-2.0

Ignoring the anisotropy of the
reflection from the surface

1.0-2.0 Maximal for the sand surface

Ignoring the spectral dependence
of the aerosol characteristics
within the interval of the
instrumental function width

0.5