Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 95

Wewouldliketopointoutthatthethreemainsourcesofthesystematic
uncertainties of the obtained results are: the uncertainty of extraterrestrial
solar flux F 0 ;thenon-adequacyof(3.7)fordependenceofthesolarirradi-
ance upon the pressure and solar zenith angle; and the atmospheric parameter
variations during the observation. The first uncertainty is rather large (about
several percents) according to the estimations of Makarova et al. (1991). How-
ever, if the same magnitude of F 0 as in (3.7) is used for further interpretation,
this uncertainty will not influence the results. The second systematic uncer-
tainty, as has been shown in Vasilyev O et al. (1987) for the old system of
the equation, which is less exact than (3.7), does not exceed the random er-
ror of the observations and could be neglected. To an even greater degree,
this conclusion may be applied to the more exact equation system (3.7). Fi-
nally, consider the third uncertainty. The solution of (3.7) is mean-weighted
values over all observed spectra from the essence of the LST. Hence, they
could be attributed to the atmospheric and surface parameters averaged over
time and space. The spectra measured during the detailed descent give the
maximal yield (just because there are more of these spectra than other ones)
during the averaging. The detailed descent continues a bit longer than one
hour (Fig. 3.3a) during the sounding that coincides with the time of a bal-
loonflight.Thespacescaleoftheairborneobservationsisabout30km that
is also analogical to the horizontal distance of a balloon route. Thus, it is
safe to say that the airborne data are not worse than any radio sounding data
from the point of the space and time averaging of the atmospheric parame-
ters.
3.3
Results of Irradiance Observation
The examples of the observational results and calculations according to the
above-described technique are presented here for a clear and an overcast sky.
The typical profiles of the downwelling and upwelling spectral irradiances
are demonstrated in Figs. 3.6-3.8 and in Tables A.1-A.3 of Appendix A. The
figures illustrate the vertical profiles of the downwelling (the upper group of
the curves) and upwelling (the lower group) irradiance - 6 curves in every
group from 500mbar to 1000mbar through 100mbar from the upper curve to
thelowerone.Theseresultswereobtainedfromthesoundingdataabovethree
kinds of surface: sand, snow and water. It is important to point out that the
uncertainty of the results is rather significant at the boundaries of the spectral
regions, where the sensitivity of the photomultiplier is weak.
The analysis of the observational results indicates the decreasing of both
upwelling and downwelling irradiances with the increasing of the atmospheric
pressure in all cases. This behavior is evident for the downwelling irradiance:
solar radiation decreases owing to the radiation extinction in the atmosphere.
For the upwelling irradiance this effect points to the predominance of scattering
processes over absorption processes in the short wavelength range, i. e. the