On the retrieval of circumsolar radiation from satellite observations and weather model output

Concentrating solar technologies compete with other rapidly
developing renewable energy sources. To succeed it is vital to
lower the levelized cost of energy. There are several parameters
that can be optimized to reach this goal, but a key component is
the improvement of the resource assessment. A better prediction of
the solar resource for new facilities brings down financing costs
as financial risks are reduced. Moreover, improved solar resource
assessment allows to optimize new facilities in regard to the local
insolation conditions. This increases energy and cost efficiency.
One parameter that is becoming more and more important for the
resource assessment is the circumsolar radiation. It is caused by
forward scattering of sun light by cloud or aerosol particles.
However, measuring circumsolar radiation is demanding and only very
limited data sets are available. As a step to bridge this gap, a
method was developed in this study to determine circumsolar
radiation from readily available data on clouds and aerosol.
Specifically, the effective radius and optical thickness of cirrus
clouds were used, as well as area mass loadings of several aerosol
components. The core of the method to determine the circumsolar
radiation is a fast yet precise parameterization. It allows to
compute the circumsolar radiation by simple analytical expressions
from previously tabulated coefficients, instead of solving the
radiative transfer by time-consuming numerical simulations. The
lookup tables were generated by extensive calculations using a
specifically adjusted version of the Monte Carlo radiative transfer
model MYSTIC. To this end, MYSTIC was enhanced with a realistic
radiation source: The point source used so far was replaced by a
extended sun disk which features a wavelength dependent brightness
distribution. The evaluated aerosol area mass loadings were
obtained from the European Centre for Medium-Range Weather
Forecasts (ECMWF) as model output of the Integrated Forecast System
(IFS). To derive the cirrus cloud properties the APICS retrieval
framework was applied to Meteosat Second Generation (MSG)
measurements. During the course of this study APICS was optimized
regarding the retrieval of optically thin cirrus clouds. To this
end, a new ground albedo data set was generated on the basis of MSG
measurements which serves as a priori assumption in the retrieval.
This new data set is, in contrast the so far used one, consistent
to the other assumptions made within the retrieval. This is an
important pre-requisite for the successful retrieval of optically
thin cirrus clouds. Furthermore, APICS was operated with a new
cloud mask based on output of the COCS cirrus cloud property
retrieval algorithm. It replaces the formerly used cloud mask from
the MeCiDa cirrus detection algorithm. Thereby in the order of 70
to 80 percent more optically thin cirrus clouds can be considered,
which allow enough light to pass for operation of a typical solar
thermal utility. Considering cirrus clouds the prevailing ice
particle shape is an uncertainty factor in the cloud property
retrieval as well as in the computation of circumsolar radiation.
So far it cannot be determined from MSG but must be assumed a
priori. To allow for an uncertainty analysis concerning this
parameter APICS was extended to consider several new ice particle
shapes in the retrieval process. It was found, the nescience of the
ice particle shape leads to an uncertainty of up to 50% in the mean
circumsolar irradiance. The newly developed method for the
retrieval of circumsolar radiation was validated with ground
measurements of the circumsolar ratio (CSR) performed at the
Plataforma Solar de Almería (PSA). This showed that the statistical
distribution of the circumsolar radiation can be well characterized
with both of the two employed ``Baum'' ice particle shape
parameterizations. When comparing instantaneous values timing and
amplitude errors become evident, tough. For the circumsolar ratio
(CSR) the validation yielded a mean absolute deviation (MAD) of
0.11 for both ``Baum'' parameterizations, a bias of 4% and -11%,
respectively, and a Spearman rank correlation r_rank of 0.54 and
0.48, respectively. If measurements with sub-scale cumulus clouds
within the relevant satellite pixels were manually removed, the
agreement of instantaneous values improved. This reflects in the
MAD values of 0.08 and 0.07, respectively, and r_rank values to
0.79 and 0.76, respectively. Furthermore, it was found that for
aerosol the CSR is strongly underestimated if the IFS output is
used head on. Only after adjusting the aerosol mass loadings
reasonable values can be obtained. An underrepresentation of large
dust particles in the IFS seems most likely to be reason for this.
In the future the method developed in this study can be extended
and combined with other data sources. While ground-based reference
measurements so far only allowed the assessment of the circumsolar
radiation at few specific measurement sites, the newly developed
method makes it possible to survey arbitrary sites.