Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 25)

Philipona, R ., E.G . D utton, T . Stoffel, J. M ichalsky, I.R eda, A. Stifter, P. W endling, N . W ood, S.A. C lough,
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E. J. Mlawer, G . Anderson, H .E. R evercomb and T .R . Shippert, 2001: Atmospheric longwave irradiance
uncertainty: Pyrgeometers compared to an absoulte sky-s canning radiometer, atmospheric emitted
radiance interferom eter, and radiative transfer m odel calculations. J. G eophys. R es., 106 (D 22) 28129 -
28141.
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A recent com parison indicates that the uncertainty associated with infrared m easurem ents 8
approxim ate the BSRN target values when the instrum ent responsivities used are those of the
independent calibration laboratories. W hen using field calibrations, one-hour m ean irradiances
are found to com pare to approxim ately 1 W m for night conditions and 2 W m during daylight
-2 -2
hours. These differences are attributable to different m ethods used in the determ ination of the flux
from the instrum ent outputs. The m ost significant finding of the com parison is the need to develop
an absolute calibration m ethod that will elim inate the large uncertainties associated with
laboratory calibrations.
Param eters to be acquired are: outputs of pyrgeom eter therm opile; instrum ent body tem peratures
and dom e tem peratures of the pyrgeom eter for instrum ents requiring such m easurem ents. Only
the values associated with the calculated irradiance (m ean, m axim um , m inim um and standard
deviation) are required by BSRN archive. All other raw data should be archived at the centre
responsible for the m easurem ents.
2.2.1.6 Upwelling Infrared Radiation
T his m easurem ent, required at BSRN stations undertaking the expanded program m e, will be
done with the sam e type of ventilated pyrgeom eter used for observing the downward infrared
irradiance. It is suggested that a horizontal shadow band be installed to protect the instrum ent
do m e from heating due to direct solar radiation when the sun is at low solar elevation. The angle
sustained will be less than 5° (i.e., covering nadir angles 85° to 90°). The instrum ent should be
m ounted on a tower with a m inim um height of 30 m to provide a representative m easurem ent of
the surrounding area. The actual height of the downfacing sensor should be reported to the
archive.
Param eters to be acquired are: outputs of pyrgeom eter therm opile, and instrum ent body and
dom e tem peratures for pyrgeom eters requiring such m easurem ents. Only the values associated
with the calculated irradiance (m ean, m axim um , m inim um and standard deviation) are required by
BSRN archive. All other raw data should be archived at the centre responsible for the
m easurem ents.
2.2.2 Accuracy of Meteorological Measurem ents
The requirem ent of m eteorological observations at or near BSRN sites infers that certain levels of
confidence m ust be placed in the m easurem ents. Most of these observations are m ade at
stations that are part of national observing networks where changing instrum ents to m eet BSRN
needs are difficult or even im possible. In such cases, the site scientist should determ ine the
uncertainty of each of these observations either directly, or by consulting the appropriate experts,
and provide it to the BSRN Archive.
W here instrum entation is obtained specifically for the m easurem ent of m eteorological variables at
a BSRN site, research quality instrum entation should be obtained. Table 2.2 presents a guideline
on the uncertainty and resolution for som e typical m easurem ent fields. Experts in the appropriate
m easurem ent fields fam iliar with the clim atology of the BSRN station should be consulted on
specific instrum ents.
2.3 Accuracy of Data Acquisition Equipment
2.3.1 Tim e
Tim e is critical with respect to the frequency of the m easurem ent sequence and the absolute tim e
of the observation. This requires that the clocks for all observations be m aintained to within ±1%
of the averaging period used for the m ost frequent m easurem ents. For a one m inute average this
equates to a tim e accuracy of 0.6 seconds. Because of the difficulty in m anually setting a clock to