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Figure C 4.1. Measured sky functions and their approximations by ones

calculated for model atmosphere containing rural aerosol and haze particles too

(courtesy of Major, 2001).

C 4. Annex 3 to Diffuse Geometry WG Report: Examination of shading mechanisms for diffuse sky

irradiance measurement for use in the BSRN

Prepared by: Atsumu Ohmura, Institute for Atmosphere and Climatic Science, Swiss Federal Institute

of Technology (ETH)

C 4.1 Introduction

The uncertainty of the effect of the shading mechanism for diffuse sky irradiance observation compelled

the BSRN to form a working group, which will examine the effect of the shading on the irradiance

measurement and recommend the most suited mechanism for the BSRN standard. The core question

is how a mechanism can be designed to observe the same irradiance in diffuse radiation which is also

measured by the standard pyrheliometers under variety of atmospheric conditions. Major (1994) sought

for the best geometrical structure for the diffusometer conceived around the Kipp CM-11 pyranometer

and concluded that it is possible to keep the maximum difference in diffuse radiation received by the

pyrheliometer and diffusometers within 0.8 W m . This study shows, however, a consistently larger

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diffuse radiation blacked by the Kipp shading mechanism. From the fundamental difference in the

geometrical structures between pyrheliometers and diffusometers based on pyranometers with a shading

mechanism, exact symmetry is not possible. It is therefore desirable to find the best geometrical

combination which provides the unbiased shading with smallest possible maximum deviations. As

a pyrheliometer, the generic form of the seven absolute cavity radiometers of the World Standard Group

is considered. Various sky functions as recently compiled by Major et al.(2000) are reproduced in Figure

C 4.1. As the sky function, the interpolated radiance among the observations was taken, which

approximates the mean observed irradiance and yields total irradiance of 1000 W m for the angle

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between 1º and 4º measured from the centre of the sun.

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