Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 154)

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The cloud radiances have been tuned to the measured ones, while the cloudless parts are the same

as calculated for the atmospheric column containing the named model aerosol. This latter does not

agree with the measurements.

D 2.5 The pyrheliometers

Geometrical differences can be found even amongst the newly developed pyrheliometers. The calculations

were made for 4 pyrheliometer geometry (see Fig. D 2.9). ABS represents PMO2, PMO5, Pacrad and

HF instruments. CRO3 is the Crommelynck 3L pyrheliometer that has the smallest slope angle and

the largest limit angle. The KIPP and NIP thermoelectric pyrheliometers are used for continuous recording

of the direct radiation.

D 2.6 The effect of clouds

Calculations have been made for the cloudless atmosphere and for both types of cloudy situation.

The distance of clouds from the solar centre was 1, 2 and 3 degrees respectively.

The deviations of calculated outputs of pyrheliometers between the clear and cloudy situations are

presented in Figs D2.10 - D2.13. In all cases the cloud increases the radiation entering into the instrument,

but the increase never reaches 1 percent.

D 2.7 Conclusions

The presence of clouds in the circumsolar part of the sky increases the output of pyrheliometers but

this increase does not exceed 1 percent.

It seems that the observed changes in the output of a pyrheliometer in cloudy conditions are related

mainly to changes in the transmission and scattering of the atmospheric column, the scattering and

reflection of clouds have smaller effect.

D 2.8 Reference

Noring, J.E., Grether, D.F. and Hunt A.J. 1991: Circumsolar Radiation Data: The Lawerence Berkeley

Laboratory Reduced Data Base NREL/TP-262-4429

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Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario Pagina 154