Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres - Short-Wave Solar Radiation in the Earth's Atmosphere - page 123

In all these cases, the data were obtained for 5 viewing angles (0 ◦ ,10 ◦ ,15 ◦ ,
45 ◦ ,70 ◦ ) and for 5 azimuth angles to control the cloudiness homogeneity. One
set of measurements took about 10 minutes in the Arctic experiments. The
measurements were accomplished at midday, when the solar zenith angle was
changing weakly during the 10-minute period. The transmitted radiance for
different azimuth angles and for the one viewing angle varying in the range of
the measurement error was averaged in the data processing.
During the Arctic experiment the observations of the downwelling and
upwelling irradiance were accomplished and ground albedo A was obtained
in Radionov et al. (1981). Different types of snow cover were studied (fresh
snow, wet snow and so on), and in all cases the spectral dependence of ground
albedo A was weak. On the 13th August 1979, the ground surface was covered
with wet snow and ground albedo A was about 0.6. On the 8th October 1979,
the ground surface was covered with fresh snow and ground albedo A was
about 0.9.
In addition, the observation of direct solar radiation was carried out in
the clear sky during the Arctic experiment of 1979. It gave the opportunity of
calibrating the instrument in units of solar incident flux π S at the top of the

τ atmosphere necessary for the retrieval of optical thickness . The experiment

on 12th April 1996 was accomplished in a similar manner excluding the mea-
surement of direct solar radiation in the clear sky, hence the instrument was
not calibrated and optical thickness τ could not have been obtained. Figure 3.22

illustrates the spectral irradiances for cosines 1.0, 0.985, 0.966, 0.707, 0.340.

Fig. 3.22. Results of the transmitted radiance observation (relative units) for overcast sky on
12th April 1996