Kippzonen BSRN Scientific Solar Monitoring System Uživatelský manuál stáhnout pdf (Strana 76)

Philipona, R. C.Frööh, K. Dehne, J. DeLuisi, J. Augustine, E. Dutton, D. Nelson, B. Forgan, P. Novotny, J.

Hickey, S.P. Love, S.B. Bener, B. McArthur, A. Ohmura, J.H. Seymour, J.S.Foot, M. Shiobara, F.P.J.

Valero, and A.W . Stawa, 1998: The Baseline Surface Radiation Network pyrgeometer round-robin

calibration experiment. Jour. Atmos. Ocean. Tech., 15, 687 - 696.

Philipona, R., E.G.Dutton, T. Stoffel, J. Michalsky, I. Reda, A. Stifter, P. W endling, N. Wood, S.A. Clough,

E.J. Mlawer, G. Anderson, H.E.Revercomb, and T.R. Shippert, 2001: Atmospheric longware irradiance

uncertainty: Pyrgeometers compared to an absolute sky-scanning radiometer, atmsopheric emitted

radiance interferometer, and radiative transfer model calculations. J. Geophys. Res., 106, D22, 28129 -

28141.

8.0 Radiometer Calibration

8.1 Introduction

Well defined and documented, systematic procedures must be carefully followed to ensure accurate

and reproducible instrument responsivities. Calibrations must be routine, internally consistent and traceable

if the BSRN is to provide the quality of data required for the calibration and development of satellite

algorithms and the measurement of variations in radiation fluxes that may be responsible for climate

change.

The responsivity of each solar radiometer must be traceable to the WMO World Radiometric Reference

(WRR) which has an estimated accuracy of better than 0.3% and guarantees the homogeneity of radiation

measurements 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 (WRC), Davos, Switzerland.

The WSG is externally monitored at each WMO International Pyrheliometer Comparison (IPC) against

regional standard cavity radiometers and internally checked during favourable weather conditions

throughout the year. Figure 8.1 illustrates the linkage between the solar radiometers associated with

a given BSRN observatory and the WRR.

The calibration of broadband infrared radiometers is based upon the Stephan-Boltzmann formula for

black-body radiation. While many national meteorological or metrological organizations have the apparatus

to perform black-body calibrations, a recent round-robin comparison has indicated that few laboratories

are capable of characterizing pyrgeometer response functions well enough to provide responsivities

of the quality necessary for the BSRN. More recently, a comparison of infrared irradiance measurements

indicated that while the instruments behaved similarly in the field, laboratory calibration differences

limited the comparability of measurements to approximately ±8 W m , while field calibrations reduced

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this to between 1 - 2 W m . One of the conclusions of the paper was the laboratory calibrations, with

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few exceptions, remained inadequate.

At each observatory, the station scientist is responsible for the overall maintenance and calibration

of each instrument and its associated data acquisition system. Depending upon the instrumentation

configuration these procedures may differ slightly, but must maintain the overall standard and frequency

of calibration set out within the BSRN documentation.

8.2 Pyrheliometer Calibration

8.2.1 Absolute Cavity Radiometer/Pyrheliometer Calibration

To function to the full mandate of the BSRN, each station or network of stations must have a cavity

radiometer at each site plus one other cavity radiometer. The on-site instrument (working instrument)

will be used to continuously monitor the direct beam radiation, while the second (primary instrument)

will maintain the radiometric linkage between the WRR and the instruments at the observatory. Ideally,

both instruments should have open apertures. In locations where climate does not permit, the working

instrument may have a protective window made of an appropriate material. In such cases, an appropriate

correction must be determined for the transmission properties of the window. If the spectral range

of the window is less than the solar spectrum at the surface, this correction must account for the humidity

regimes of the locale, as the infrared portion of the spectrum will be most affected by the window.

At a minimum of once every two years, the reference instrument must be compared with either the

World Standard Group (WSG) of cavity radiometers or with a cavity radiometer which participates regularly

at the International Pyrheliometer Comparisons (IPC). Where practical, the former means of reference

is preferable. The performance of all reference instruments must be monitored regularly between IPC’s

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