Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 87

other physical restrictionwas added in this case: the deviations of values F ↓ ( z i ),
F ↑ ( z i )frommeasuredresults f ↓ ( z i ), f ↑ ( z i ) at any iteration can't exceed the root-
mean-square randomuncertainty of the measurements (10%, Table 3.1). Mark
that two-three iterations were enough to obtain final values F ↓ ( z i ), F ↑ ( z i ).
Figure 3.2 illustrates an example of the considered procedure.
Obtained values of the irradiances under the overcast condition F ↓ ( z i ),
F ↑ ( z i ) were the results of the secondary processing. The root-mean-square de-
viation of the smoothed profile from the initial ones was accepted as a random
uncertainty of the result. Note that the systematic error of calibration brought
a considerable yield to the total uncertainty (Table 3.1), however the irradi-
ances were considered as non-dimension combinations for further processing
and interpretation, hence it was possible to ignore the calibration uncertainty.
Note that the solar zenith angle varies negligibly (1−2 ◦ )owingtothefastac-
complishment of the experiment, and during processing, the single value of
the solar zenith angle was attributed to all spectra of the experiment.
The comparison of the measured irradiances with the extraterrestrial solar
spectruminthecaseofaclearatmosphereisofspecialinterest.Beer'sLaw
is the simplest ground of this approach if for example the optical thickness
of the atmosphere is retrieved from the observational data. It is impossible
to measure the solar extraterrestrial flux directly from the aircraft, thus the
yield of the systematic uncertainty is essential during observations in a clear
atmosphere.
The values of spectral radiative flux divergence are rather small in clear
sky, and the random uncertainties of the results of the irradiance observations
corresponding to the aircraft factors are extremely large. Thus, the main prob-
lem of experiment planning and data processing was the minimization of the
randomuncertainty of the results and correction of the systematic uncertainty
during instrument calibration.
Increasing themeasurement accuracy of the spectrometer is important itself
but the measurement uncertainty onboard the aircraft due to flight factors,
atmospheric conditions, and surface heterogeneity does not depend on an
instrument and can reachhigh values. Therefore, the onlymethodof getting the
highly accurate experimental results is applying the most adequate approaches
to the statistical data processing. It would be necessary to register several
spectra at every level if we meant to perform the statistical processing at its
simplest level - the data averaging. However, in this case, observations would
have taken a lot of time and the irradiances at different levels would have been
measured at essentially different solar zenith angles, complicating further the
interpretation.
According to the above-mentioned difficulty, a special scheme of observa-
tions called sounding was elaborated (Kondratyev and Ter-Markaryants 1976;
Vasilyev O et al. 1987). Corresponding to this scheme, two or three preliminary
ascents and descents were carried out in a range from 50m (1000mbar)to
5600m (500mbar) with registrations every 100mbar and the detailed descent
was accomplished from 5600m to 50 m at midday (during the period when
the solar zenith angle is weakly varying) with registrations every 100m (Fig.
3.3a). The registration of the numerous irradiance spectra with the minimal