Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 153)

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Using the data of Figure D 2.1
where the constant has to be determ ined 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 com ing 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 determ ined.
D 2.3 Measurements
The National Renewable Energy Laboratory kindly forwarded the circum solar m easurem ents m ade
at Boardm an, OR (latitude 45,7 N) in April and May of 1977. During the daytim e the instrum ent scanned
o
the solar disk and the circum solar sky (up to 3.2 degree) in every 10 m inutes. The resolution is 1.5`
in the solar disk and 4.5` in the outer parts. W hen 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 m orning or evening m easurem ents. An exam ple
is seen in Figure D 2.4 altogether with the least turbid cloudless cases found in the sam ple for both
the low and high Sun. W hile the circum solar 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 circum solar functions for the edge scattering altogether with the case of the cloudless
high Sun. Again: the clear parts of the cloudy cases show m uch higher radiance than the absolutely
clear atm ospheric colum n.
Looking at Figures D 2.4 and D 2.5 one has to rem em ber that real clouds differ m uch from the above
described m odel one.
D 2.4 The applied radiance distributions
Considering only the above described m easurem ents and m odel, the following radiance distribution
functions have been selected for further calculation:
-- 60 degrees solar elevation, m ountain 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 com bined 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 exam ple 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 circum solar 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.