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of integration of the radiation falling on the receiver of the instrument: Ohmura integrates first the
direction of the rays, Major integrates first along the surface of the radiation receiver. The numerical
solutions of the equations would result some difference between the two type of calculation. Both
members of the WG suggests Eppley 8-48 for diffuse measuerements at BSRN Stations, however
their shading devices differ:
Ohmura's calculations use a sphere with a 28 mm radius and an arm length of 641 mm.
Major's calculations use a sphere with a 30 mm radius and an arm length of 726 mm.
The circumsolar radiation (W m ) excluded by these two "arrangements" are as follows:
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Solar zenith angle, deg Major's shading Ohmura's shading
0 12.8 12.9
30 10.6 11.1
60 6.0 6.3
C 1.3 Statements and recommendations
(1) The m ethods of calculating the received/excluded circumsolar radiation by
pyrheliometers/diffusometers used by the two members of the WG have identical theoretical
background, the numerical values provided by the two methods differ only by few tenths of a Watt
(or by few percent in relative sense) due to the numerical approximations of the analytical forms.
(2) As consequence of different geometry of the pyrheliometers the irradiances received by the
pyrheliometric sensors used in BSRN stations might differ by more than 1 W m depending on
-2
the aureole conditions.
(3) The differences of irradiances received by the sensors of the diffusometers used in BSRN stations
can not exceed the 5 W m in any conditions.
-2
(4) The differences of irradiances received by the sensors of the diffusometers used in BSRN stations
could significantly be decreased by changing the size of the shading device or the arm length or
both of them (see the annexes to this WG Report).
C 1.4 References
Report of the Fifth Bsrn Science and Review Workshop (Budapest, Hungary, 18-22 May 1988) WCRP
Informal Report No. 10/1998.
Report of the Sixth Bsrn Science and Review Workshop (Melbourne, Australia, 1-5 May 2000) WCRP
Informal Report No. 17/2001
Michalsky J.J. and 17 other authors, 2002: Comparison of Diffuse Shortwave Irradiance
Measurements. In: Proceedings of the Twelfth Atmospheric Radiation Measurements (ARM) Science
Team Meeting, April 8-12, 2002, St Petersburg Florida.
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