SOLIS Clear Sky Model
The SOLIS clear sky model [3] uses the radiative transfer model libRadtran [4] to calculate input parameters for a fitting function called the modified Lambert-Beer (MLB) relation. For this, only two radiative transfer calculations are needed for a given atmospheric state to get the irradiance for a full day. Since SOLIS can provide spectrally resolved irradiance data, it can be used for different applications. Beside improved information for the planning of solar energy systems, the calculation of photosynthetic active radiation, UV index, and illuminance is possible.
We use climatologies with monthly averages of AOD [5] and water vapour content [6] as input parameters for SOLIS and get the direct and global irradiance as output.
We propose a new model [7] to calculate direct irradiance as a function of the clear sky index k* and the direct irradiance at clear sky conditions bciear. An appropriate clear sky model is the basis for this approach, see sec. 2.2.
For cloud events, described by k* < (1 - c(6)) (whith с(в) is a fit function), an exponentially rising parametrisation b(k*) for the direct irradiance was found empirically. It is given by
b(k*) = bdear k*P
with P as a fit parameter.
Situations where k* - 1 becomes smaller than c(6), are defined as clear sky situations and the parametrisation results in
b(k*) = bciear + (k*-1) a where a is a fit parameter.
If the clear sky index becomes greater than (1 + c(6)) we assume a special cloudy situation. In this case the global irradiance becomes more than Iciear in consequence of an increasing diffuse irradiance by reflection on clouds. The direct irradiance is then parameterised by
b(k*) = bdear / k*. (7)
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In Fig. 3 the beam fraction as a function of clearsky index is given for satellite and ground derived values.
