Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 141)

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Figure C 4.1. Measured sky functions and their approxim ations by ones
calculated for m odel atm osphere containing rural aerosol and haze particles too
(courtesy of Major, 2001).
C 4. Annex 3 to Diffuse Geometry W G Report: Examination of shading mechanisms for diffuse sky
irradiance measurement for use in the BSRN
Prepared by: Atsum u Ohm ura, Institute for Atm osphere and Clim atic Science, Swiss Federal Institute
of Technology (ETH)
C 4.1 Introduction
The uncertainty of the effect of the shading m echanism for diffuse sky irradiance observation com pelled
the BSR N to form a working group, which will exam ine the effect of the shading on the irradiance
m easurem ent and recom m end the m ost suited m echanism for the BSRN standard. The core question
is how a m echanism can be designed to observe the sam e irradiance in diffuse radiation which is also
m easured by the standard pyrheliom eters under variety of atm ospheric conditions. Major (1994) sought
for the best geom etrical structure for the diffusom eter conceived around the Kipp CM-11 pyranom eter
and concluded that it is possible to keep the m axim um difference in diffuse radiation received by the
pyrheliom eter and diffusom eters within 0.8 W m . This study shows, however, a consistently larger
-2
diffuse radiation blacked by the Kipp shading m echanism . From the fundam ental difference in the
geom etrical structures between pyrheliom eters and diffusom eters based on pyranometers with a shading
m echanism , exact sym m etry is not possible. It is therefore desirable to find the best geom etrical
com bination which provides the unbiased shading with sm allest possible m axim um deviations. As
a pyrheliom eter, the generic form of the seven absolute cavity radiom eters of the W orld Standard Group
is considered. Various sky functions as recently com piled by Major et al.(2000) are reproduced in Figure
C 4.1. As the sky function, the interpolated radiance am ong the observations was taken, which
approxim ates the m ean observed irradiance and yields total irradiance of 1000 W m for the angle -2
between 1º and 4º m easured from the centre of the sun.