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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 Network (BSRN) is a subprogram of the World Climate Research

Programme (WCRP). The Fifth Workshop of BSRN formed a Working Group on Solar Diffuse Shading

Geometry: " This group was tasked to determine geometric specifications for shading-disc diameters

and separations which are appropriate for all combinantions of instrumentation typically used to measure

diffuse solar irradiance within BSRN accuracy requirements for low-to-moderate aerosol conditions,

and which allow for the sum of measured diffuse and direct solar irradiance to meet the accuracy

requirements for global solar irradiance, If this is not possible for some combinations of instruments

in current use, the working group should recommend geometries which would most closely allow these

accuracy requirements to be fulfilled, as well as new instrument dimensions that would be needed

to allow the BSRN requirements to be fully satisfied. The group should also address similar questions

of geometry for measurement of direct solar irradiance (pyrheliometers). It will report its findings and

recommendastions at the next BSRN workshop in 2000. Members of this group were identified as

A. Ohmura (Chair) and G. Major." (Report of the Fifth BSRN…)

The Sixth Workshop of BSRN extended the activity of the working group: "The meeting agreed to the

continuation of studies by an Ad-hoc Working Group on Solar Diffuse Shading Geometry, comprised

of A. Ohmura (Chairman) and G. Major. The group is to report back on the magnitude of potential errors

in diffuse and direct solar irradiance observations due to the actual fields of view used in typical BSRN

instrumantation. This assesssment is to include the effect of various levels of spectral aerosol optical

depth in the atmosphere." (Report of the Sixth BSRN…)

The Seventh Workshop of BSRN accepted the results presented by the members of the and terminated

the activity of the working group.

C 1.2 Activities of the Working Group

(1) On the Melbourne BSRN Workshop A. Ohmura presented his theoretical consideration of calculation

of the circumsolar radiation contained in the direct solar radiation measurement or excluded from

the diffuse solar radiation measurements (Annex 3 to this WG 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 geometrical data of some diffusometers used on BSRN Stations

(ii) Ratios of diffuse radiation values measured in Budapest using 3 different shading spheres

with CM pyranometers

(iii) a suggetsed standard geometry of diffusometers (Annex 1 to this W G report (C2.2)).

(3) Taking into account the recommendation of the Melbourne Workshop, G. Major and M. Putsay

calculated the "optimal" geometrical parameters of diffusometers for different pyranometer-

pyrheliometer pairs and sent the results to the International Pyrheliometer Comparison in Davos,

2000 September (Annex 2 to this WG Report (C 2.2)).

(4) To understand better the behavior of diffusometers Dr. Joseph Michalsky organized a comparison

of diffusometers that was held in Billings, Oklahoma, USA, between 24 September and 10 October

2001. On this Intensive Observation Period 14 diffusometers participated. The shading device

for all the instruments was Sci-Tec solar tracker with its standard shading sphere (50.8 mm in

diameter). The distance between the pyranometer 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 diffusometers, the diameter of the pyranometers sensing surface varied between 4 mm and

32 mm, so the geometrical difference came from the pyranometers. G. Major estimated the possible

difference between diffuse mesurements due to the different geometry 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 Workshop A. Ohmura presented numerical results of his theoretical approach

for Absolute Cavity Radiom eters and CH-1 pyrheliom eter as well as for diffusometers used in

or suggested for BSRN Stations (Annex 3 to this WG Report).The theoretical background behind

the methods used by Ohmura and Major is the same, the difference comes from the different order

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