Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 131)

119
Figure C 2.2. Com pared log m easured and calculated circum solar functions.
Letters A, B, C, D, E, F sym bolizes different aerosol m odels.
Sum m arizing the above m entioned results it should be stated, that the circum solar radiation is a very
variable param eter and it is alm ost independent of other characteristics (direct or diffuse radiation,
optical depth) of the solar radiation field.
C 2.2.3 Circum solar diffuse irradiances
Let us suppose that the diffusom eters listed in Table C 2.1 are at the sam e place and m easure the
diffuse radiation. Let us select a situation when the
- solar elevation is 45 degrees
- the direct radiation is 700 W m -2
- the diffuse radiation is 160 W m -2
- the circum solar sky function is the m ean of m easurem ents (seen in Figs.C 2.2 or C 2.3).
Using the penum bra functions (Fig. C 2.1) of diffusom eters, the calculated irradiance of their sensors
seen in Table C 2.2a. For a m ore com plete picture, in Table C 2.2b irradiances calculated for som e
pyrheliom eters are shown. Since their slope angles are less than that of diffusom eters, therefore the
effect of circum solar radiation on their output is less. More details regarding the pyrheliom eters can
be found in Major (1995).
Four diffusom eters from the ten “m easure” identical value. It seem s practical to declare these four
instrum ents as “standard group” and to relate the output of the others to that of the standard group.
Moreover, the standard group m easures the highest value, so all the others have deficit com pared
to the standard. In Table C 2.3 the values of deficit are shown for the diffusom eters that are not in the
standard group. Since the really m easured deficit of Hungary III to Hungary I is collected for a long
tim e and this way its variability with the conditions could be looked for, the deficits are norm alized to
that of HunIII. It is expected that the norm alized deficit would not change with changing solar elevation
and atm ospheric conditions.