Clear-sky radiation models and aerosol effects

Abstract

This chapter offers a description of the main factors that affect the transmission of solar radiation through the cloudless atmosphere, and the corresponding modeling approaches. The limitations of broadband modeling are discussed, and methodological improvements are described. A detailed discussion of the various inputs required by different clear-sky radiation models, and how to obtain such data, is provided so that the reader can operate these models with appropriate inputs, depending on the application and geographical coverage. In particular, the benefits of using atmospheric data provided by recent reanalyses are described. The impact of aerosol attenuation on the different irradiance components is discussed, with a focus on the aerosol optical depth. Its methods of measurement, properties, reduction methods, accuracy, and spatiotemporal variability are described. The error propagation between aerosol data and the predicted irradiance is quantified, and examples are provided. Seven models of the literature are selected for further discussion and validation. This validation is performed using high-quality radiometric data from Tamanrasset, Algeria, and is done in two different ways: an ideal validation based on the best possible (locally measured) aerosol information and a practical method (generalizable anywhere) based on reanalysis data. A sensible degradation of performance is obvious when using the second approach. Finally, some likely or desirable future developments in the field are described.