Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 107

zenith angle a 1 ,..., a 5 , b 1 ,..., b 5 from (3.6). These parameters were supposed
to be used during the correspondent correction of the irradiance spectra with
other observation schemes (not soundings). However, the analysis indicated
that the calibration curve D turns out to be highly dependent on the experiment
series (i. e. linked with the laboratory calibration), which their use impossi-
ble for other experiments. The accomplished estimations affirm the standard
deviations of calibration curve D tobeequalto2-3%inaverage,i.e.thecalibra-
tion accuracy has been successfully improved by applying the above-described
approach. However, even this error is too high, and it creates difficulties in ap-
plying the modern complex approaches of observational result interpretation,
as will be shown in Chap. 5.

3.4
Results of Solar Radiance Observation.
Spectral Reflection Characteristics of Ground Surface

The main aim of the accomplished airborne observations of the solar radiance
in the atmosphere was studying spectral reflectance properties of the surfaces.
As has been shown in Sect. 1.4 the reflected characteristics of the surface
described with function R ( ϕ )aredefinedfromtherelationbetween

the income and reflected radiation with (1.73). The simplest characteristic of
the surface, the albedo is defined as a ratio of the upwelling to downwelling
solar irradiance (1.72) (see the footnote on page 33) (Sivukhin 1980).
Nevertheless, taking into account the insignificant yield of the multiple
scattered radiation to downwelling irradiance in the clear atmosphere, the ob-
served reflection characteristics are assumed to correspond to the theoretical
ones. However, the relationship between the observed reflection characteris-
tics and ratio of direct and scattered radiation in the downwelling irradiance
(Vasilyev O 1986) is particularly essential during comparison of the results
obtained in the clear and cloudy atmosphere.
Owing to the diffused reflection (Sect. 1.4) function of four arguments

µ ϕ µ , , ,

ϕ ) it is impossible to measure for the solar radiation field because
the radiance really measured from direction (

µ ϕ µ ,

R ( , ,

µ ϕ )dependsonthewholefield

of the income radiation [look at the definition of the reflection operator in
(1.74)]. Therefore, the maximally informative characteristic of the reflection
available fromthe observation is a spectral brightness coefficient (SBC) defined
as follows:
r (

0 ,

ϑ ϕ = ϑ ϕ | , ) I ( , ) I 0 , (3.9)

ϑ ϑ = ϕ where is the viewing angle (direction 0isthenadir), is the viewing

ϕ = ϑ ϕ

azimuth ( )isthesolarradiance

reflected from the surface and I 0 istheradiancereflectedfromtheabsolutely
white orthotropic surface.
The direct measurements of value I 0 were technically impossible during the
flight so following the scheme of the SBC observations was used. Radiance
I 0 was measured using the same instrument and simultaneously downwelling

0 corresponds to the Sun's direction), I ( ,