Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario Pagina 24
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Alados-Arboledas, L., J. Vida and J.I. Jiméniz, 1988: Effects of solar radiation on the performance of
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pyrgeometers with silicon domes. Jour. Atmos. Ocean. Tech., 5, 666 - 670.
Udo, S.O., 2000: Quantification of solar heating of the dome of a pyrgeometer for a tropical location: Ilorin,
Nigeria. Jour. Atmos. Ocean. Tech., 17, 995 - 1000.
Philipona, R. C. Fröhlich and Ch. Betz, 1995: Characterization of pyrgeometers and the accuracy of
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atmospheric long-wave radiation measurements. Appl. Optics, 34, 1598 - 1605.
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2.2.1.3 Global Radiation
BSRN target uncertainty is 2% (5 W m ). Although the global radiation may be determined as a
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sum of direct and diffuse irradiance, a direct measurement will be made with a ventilated
pyranometer (the same instrument type as for diffuse radiation) to provide a basis for quality
control; including instrument characterisation and calibration (see Section 8.3 - Calibration
procedures).
The same thermal offset issues associated with the measurement of diffuse irradiance must also
be considered for global irradiance. The difference in the magnitude of the irradiance signals
(global vs. diffuse) reduces the overall relative uncertainty associated with thermal offset for
global irradiance measurements.
Parameters to be acquired are: output of pyranometer thermopile; pyranometer body
temperature. Only the values associated with the calculated irradiance (mean, maximum,
minimum and standard deviation) are required by BSRN archive. All other raw data should be
archived at the centre responsible for the measurements.
2.2.1.4 Reflected Solar Radiation
This measurement, required at BSRN stations undertaking the "expanded measurement"
programme, will be done with the same type of ventilated pyranometer as for diffuse and global
radiation. It is suggested that a horizontal shadowband be used to protect the instrument dome
from reflecting direct solar radiation onto the thermopile at low solar elevation. The angle
sustained should be less than 5° (i.e., covering nadir angles 85° to 90°). With the exception of
frost on other material on the dome that would enhance the internal reflection problem, the error
due to internal reflection or the direct beam grazing the therm opile on a level instrument is
estimated to be <1 W m . The minimum height above the surface for the measurement is 30 m
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so that the observations represents the relfectance of the surrounding area. The actual height of
the downfacing centre should be reported to the archive.
Parameters to be acquired are: output of pyranometer thermopile; pyranometer body
temperature. Only the values associated with the calculated irradiance (mean, maximum,
minimum and standard deviation) are required by BSRN archive. All other raw data should be
archived at the centre responsible for the measurements.
2.2.1.5 Downwelling Infrared Radiation
BSRN target uncertainty is 5% or 10 W m , whichever is greater. Significant evidence suggests
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that a pyrgeometer with a hemispheric silicon dome is negatively impacted by solar radiation
through dome heating . The BSRN recognizing this fact determined that downward infrared
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irradiance should be measured with a shaded and ventilated pyrgeometer. Furthermore, it noted
that, if using an Eppley PIR, the battery circuit must be disconnected and the thermistor
temperatures directly measured. It was determ ined that a “modified PIR" pyrgeometer (Eppley)
with three dome temperature sensors at 45° (but without a battery circuit) was capable of
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measuring downwelling infrared radiation to the target uncertainty. Although not norm ally used in
an unshaded mode, this modified instrument is designed to measure infrared radiation in full
sunlight. Furthermore, it was recognized that a shaded and ventilated unmodified Eppley
pyrgeometer could provide nearly the same quality of measurement. Recently, other
manufacturers have begun producing pyrgeometers that may also be suitable for use in BSRN
stations.
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