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.\n\nThe 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.\n\nConsidering 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\nretrieval process. It was found, the nescience of the ice particle shape leads to an uncertainty\nof up to 50% in the mean circumsolar irradiance.\n\nThe 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 Almer\xeda (PSA).\nThis 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\nr_rank of 0.54 and 0.48, respectively. If measurements with sub-scale cumulus clouds within the\nrelevant 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.\n\nIn 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.