141
Using the data of Figure D 2.1
where the constant has to be determined to calculate absolute radiance values.
D 2.2.2 Cloud side reflectance
Figure D 2.3 shows a cloud in the right side of the Sun. It is supposed that:
(1) the radiance coming from the cloud side is proportional to the direct radiation,
(2) the radiance is constant in the viewing angle of the cloud side,
(3) the viewing angle does not depend on the solar elevation,
(4) the viewing angle is proportional to the distance from the solar centre.
Again, the absolute value has to be determined.
D 2.3 Measurements
The National Renewable Energy Laboratory kindly forwarded the circumsolar measurements made
at Boardman, OR (latitude 45,7 N) in April and May of 1977. During the daytime the instrument scanned
o
the solar disk and the circumsolar sky (up to 3.2 degree) in every 10 minutes. The resolution is 1.5`
in the solar disk and 4.5` in the outer parts. When the solar elevation is low, the scan is parallel with
the surface. At solar noon the scan occurs in the solar vertical.
This way, cases for side reflectance could be found in the morning or evening measurements. An example
is seen in Figure D 2.4 altogether with the least turbid cloudless cases found in the sample for both
the low and high Sun. While the circumsolar radiances are quite near in the clear cases, the cloudy
radiances differ significantly from them even in the clodless part of the sky.
Figure D 2.5 shows circumsolar functions for the edge scattering altogether with the case of the cloudless
high Sun. Again: the clear parts of the cloudy cases show much higher radiance than the absolutely
clear atmospheric column.
Looking at Figures D 2.4 and D 2.5 one has to rem ember that real clouds differ much from the above
described model one.
D 2.4 The applied radiance distributions
Considering only the above described measurements and model, the following radiance distribution
functions have been selected for further calculation:
-- 60 degrees solar elevation, mountain aerosol for the solar disk and clear sky radiances (high
clear case)
-- 20 degrees solar elevation, continental background aerosol for the solar disk and clear sky radiances
(low clear case)
-- the above clear cases combined with clouds at 1, 2 and 3 degrees from the solar centre for both
the edge scattering and side reflectance situations.
The radiance functions are shown in Figures D 2.6, D 2.7 and D 2.8. For example to calculate the effect
of side reflected radiation if the cloud begins at 2 deg, the clear function has been used for the solar
disk and circumsolar sky up to 2 degrees, from 2 deg to 2.4 deg the radiance seen in Fig 6, for angles
larger 2.4 deg (bottom of the cloud) zero radiance has been taken in to account.
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