Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador descarregar pdf (Página 131)

119

Figure C 2.2. Compared log measured and calculated circumsolar functions.

Letters A, B, C, D, E, F symbolizes different aerosol models.

Summarizing the above mentioned results it should be stated, that the circumsolar radiation is a very

variable parameter and it is almost independent of other characteristics (direct or diffuse radiation,

optical depth) of the solar radiation field.

C 2.2.3 Circumsolar diffuse irradiances

Let us suppose that the diffusometers listed in Table C 2.1 are at the same place and measure the

diffuse radiation. Let us select a situation when the

- solar elevation is 45 degrees

- the direct radiation is 700 W m

-2

- the diffuse radiation is 160 W m

-2

- the circumsolar sky function is the mean of measurements (seen in Figs.C 2.2 or C 2.3).

Using the penumbra functions (Fig. C 2.1) of diffusometers, the calculated irradiance of their sensors

seen in Table C 2.2a. For a more complete picture, in Table C 2.2b irradiances calculated for some

pyrheliometers are shown. Since their slope angles are less than that of diffusometers, therefore the

effect of circumsolar radiation on their output is less. More details regarding the pyrheliometers can

be found in Major (1995).

Four diffusometers from the ten “measure” identical value. It seems practical to declare these four

instruments as “standard group” and to relate the output of the others to that of the standard group.

Moreover, the standard group measures the highest value, so all the others have deficit compared

to the standard. In Table C 2.3 the values of deficit are shown for the diffusometers that are not in the

standard group. Since the really measured deficit of Hungary III to Hungary I is collected for a long

time and this way its variability with the conditions could be looked for, the deficits are normalized to

that of HunIII. It is expected that the normalized deficit would not change with changing solar elevation

and atmospheric conditions.

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Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador Página 131