Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 126)

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C 1. Final Report of the Working Group on Solar Diffuse Shading Geometry
Prepared by: G. Major and A. Ohmura
C 1.1 Terms of reference
The Baseline Surface Radiation N etwork (BSRN) is a subprogram of the W orld Clim ate Research
Program m e (W CRP). The Fifth W orkshop of BSRN form ed a W orking Group on Solar Diffuse Shading
Geom etry: " This group was tasked to determ ine geom etric specifications for shading-disc diam eters
and separations which are appropriate for all com binantions of instrum entation typically used to m easure
diffuse solar irradiance within BSRN accuracy requirem ents for low-to-m oderate aerosol conditions,
and which allow for the sum of m easured diffuse and direct solar irradiance to m eet the accuracy
requirem ents for global solar irradiance, If this is not possible for som e com binations of instrum ents
in current use, the working group should recom m end geom etries which would m ost closely allow these
accuracy requirem ents to be fulfilled, as well as new instrum ent dim ensions that would be needed
to allow the BSRN requirem ents to be fully satisfied. The group should also address sim ilar questions
of geom etry for m easurem ent of direct solar irradiance (pyrheliom eters). It will report its findings and
recom m endastions at the next BSRN workshop in 2000. Mem bers of this group were identified as
A. Ohm ura (Chair) and G. Major." (Report of the Fifth BSRN…)
The Sixth W orkshop of BSRN extended the activity of the working group: "The m eeting agreed to the
continuation of studies by an Ad-hoc W orking Group on Solar Diffuse Shading Geom etry, com prised
of A. Ohm ura (Chairm an) and G. Major. The group is to report back on the m agnitude of potential errors
in diffuse and direct solar irradiance observations due to the actual fields of view used in typical BSRN
instrum antation. This assesssm ent is to include the effect of various levels of spectral aerosol optical
depth in the atm osphere." (Report of the Sixth BSRN…)
The Seventh W orkshop of BSRN accepted the results presented by the m em bers of the and term inated
the activity of the working group.
C 1.2 Activities of the W orking Group
(1) On the Melbourne BSRN W orkshop A. Ohm ura presented his theoretical consideration of calculation
of the circum solar radiation contained in the direct solar radiation m easurem ent or excluded from
the diffuse solar radiation m easurem ents (Annex 3 to this W G report (C 2.4)).
(2) On the Melbourne BSRN Workshop G. Major presented a work done at the Hungarian Meteorological
Service that contains:
(i)
Basic geom etrical data of som e diffusom eters used on BSRN Stations
(ii) Ratios of diffuse radiation values m easured in Budapest using 3 different shading spheres
with CM pyranom eters
(iii) a suggetsed standard geom etry of diffusom eters (Annex 1 to this W G report (C2.2)).
(3) Taking into account the recom m endation of the Melbourne W orkshop, G. Major and M. Putsay
calculated the "optim al" geom etrical param eters of diffusom eters for different pyranom eter-
pyrheliom eter pairs and sent the results to the International Pyrheliom eter Com parison in Davos,
2000 Septem ber (Annex 2 to this W G Report (C 2.2)).
(4) To understand better the behavior of diffusom eters Dr. Joseph Michalsky organized a com parison
of diffusom eters that was held in Billings, Oklahom a, USA, between 24 Septem ber and 10 October
2001. On this Intensive Observation Period 14 diffusom eters participated. The shading device
for all the instrum ents was Sci-Tec solar tracker with its standard shading sphere (50.8 m m in
diam eter). The distance between the pyranom eter sensor and the center of the shading sphere
(arm length) varied between 50 and 51.5 cm. While the shading device could be regarded as identical
for all diffusom eters, the diam eter of the pyranom eters sensing surface varied between 4 m m and
32 m m , so the geom etrical difference cam e from the pyranom eters. G. Major estim ated the possible
difference between diffuse m esurem ents due to the different geom etry and found that it would
not exceed 2.5 W m . More details in: Michalsky et al 2002.
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(5) On the Regina BSRN W orkshop A. Ohm ura presented num erical results of his theoretical approach
for Absolute Cavity Radiom eters and CH-1 pyrheliom eter as well as for diffusom eters used in
or suggested for BSRN Stations (Annex 3 to this W G Report).The theoretical background behind
the m ethods used by Ohm ura and Major is the sam e, the difference com es from the different order