Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 127)

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of integration of the radiation falling on the receiver of the instrum ent: Ohm ura integrates first the
direction of the rays, Major integrates first along the surface of the radiation receiver. The num erical
solutions of the equations would result som e difference between the two type of calculation. Both
m em bers of the W G suggests Eppley 8-48 for diffuse m easuerem ents at BSRN Stations, however
their shading devices differ:
Ohm ura's calculations use a sphere with a 28 m m radius and an arm length of 641 m m .
Major's calculations use a sphere with a 30 m m radius and an arm length of 726 m m .
The circum solar radiation (W m ) excluded by these two "arrangem ents" are as follows: -2
Solar zenith angle, deg Major's shading Ohm ura's shading
0 12.8 12.9
30 10.6 11.1
60 6.0 6.3
C 1.3 Statements and recommendations
(1) T he m ethods of calculating the received/excluded circum solar radiation by
pyrheliom eters/diffusom eters used by the two m em bers of the W G have identical theoretical
background, the num erical values provided by the two m ethods differ only by few tenths of a W att
(or by few percent in relative sense) due to the num erical approxim ations of the analytical form s.
(2) As consequence of different geom etry of the pyrheliom eters the irradiances received by the
pyrheliom etric sensors used in BSRN stations m ight differ by m ore than 1 W m depending on -2
the aureole conditions.
(3) The differences of irradiances received by the sensors of the diffusom eters 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 diffusom eters 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 W G Report).
C 1.4 References
Report of the Fifth Bsrn Science and Review W orkshop (Budapest, Hungary, 18-22 May 1988) W CRP
Inform al Report No. 10/1998.
Report of the Sixth Bsrn Science and Review W orkshop (Melbourne, Australia, 1-5 May 2000) W CRP
Inform al Report No. 17/2001
Michalsky J.J. and 17 other authors, 2002: Comparison of D iffuse Shortwave Irradiance
Measurements. In: Proceedings of the Twelfth Atmospheric Radiation Measurements (ARM) Science
Team Meeting, April 8-12, 2002, St Petersburg Florida.