Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 76)

Philipona, R . C .Frööh, K. D ehne, J. D eLuisi, J. Augustine, E. D utton, D . N elson, B. Forgan, P. N ovotny, J.
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H ickey, S.P. Love, S.B. Bener, B. McA rthur, A. O hmura, J.H . Seymour, J.S.Foot, M. Shiobara, F.P.J.
Valero, and A.W . S tawa, 1998: T he B aseline S urface R adiation N etwork pyrgeom eter round-robin
calibration experiment. Jour. Atm os. O cean. Tech., 15, 687 - 696.
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 longware irradiance
uncertainty: Pyrgeometers compared to an absolute sky-s canning radiometer, atmsopheric emitted
radiance interferom eter, and radiative transfer m odel calculations. J. G eophys. R es., 106, D 22, 28129 -
28141.
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8.0 Radiometer Calibration
8.1 Introduction
W ell defined and docum ented, system atic procedures m ust be carefully followed to ensure accurate
and reproducible instrum ent responsivities. Calibrations m ust be routine, internally consistent and traceable
if the BSRN is to provide the quality of data required for the calibration and developm ent of satellite
algorithm s and the m easurem ent of variations in radiation fluxes that m ay be responsible for clim ate
change.
The responsivity of each solar radiom eter m ust be traceable to the WMO W orld Radiom etric Reference
(W RR) which has an estim ated accuracy of better than 0.3% and guarantees the hom ogeneity of radiation
m easurem ents to better than 0.1%. This reference is realized by a group of seven absolute cavity
radiometers, the World Standard Group, housed at the World Radiation Centre (W RC), Davos, Switzerland.
The W SG is externally m onitored at each W MO International Pyrheliom eter Com parison (IPC) against
regional standard cavity radiom eters and internally checked during favourable weather conditions
throughout the year. Figure 8.1 illustrates the linkage between the solar radiom eters associated with
a given BSRN observatory and the W RR.
The calibration of broadband infrared radiom eters is based upon the Stephan-Boltzm ann form ula for
black-body radiation. While many national m eteorological or m etrological organizations have the apparatus
to perform black-body calibrations, a recent round-robin com parison has indicated that few laboratories
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are capable of characterizing pyrgeom eter response functions well enough to provide responsivities
of the quality necessary for the BSRN. More recently, a com parison of infrared irradiance m easurem ents
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indicated that while the instrum ents behaved sim ilarly in the field, laboratory calibration differences
lim ited the com parability of m easurem ents to approxim ately ±8 W m , while field calibrations reduced
-2
this to between 1 - 2 W m . One of the conclusions of the paper was the laboratory calibrations, with -2
few exceptions, rem ained inadequate.
At each observatory, the station scientist is responsible for the overall m aintenance and calibration
of each instrum ent and its associated data acquisition system . Depending upon the instrum entation
configuration these procedures m ay differ slightly, but m ust m aintain the overall standard and frequency
of calibration set out within the BSRN docum entation.
8.2 Pyrheliometer Calibration
8.2.1 Absolute Cavity Radiom eter/Pyrheliom eter Calibration
To function to the full m andate of the BSRN, each station or network of stations m ust have a cavity
radiom eter at each site plus one other cavity radiom eter. The on-site instrum ent (working instrum ent)
will be used to continuously m onitor the direct beam radiation, while the second (prim ary instrum ent)
will m aintain the radiom etric linkage between the W RR and the instrum ents at the observatory. Ideally,
both instrum ents should have open apertures. In locations where clim ate does not perm it, the working
instrum ent m ay have a protective window m ade of an appropriate m aterial. In such cases, an appropriate
correction m ust be determ ined for the transm ission properties of the window. If the spectral range
of the window is less than the solar spectrum at the surface, this correction m ust account for the hum idity
regim es of the locale, as the infrared portion of the spectrum will be m ost affected by the window.
At a m inim um of once every two years, the reference instrum ent m ust be com pared with either the
W orld Standard Group (W SG) of cavity radiom eters or with a cavity radiom eter which participates regularly
at the International Pyrheliom eter Com parisons (IPC). W here practical, the form er m eans of reference
is preferable. The performance of all reference instrum ents m ust be m onitored regularly between IPC’s