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Routine measurements of the beam irradiance at normal incidence include the irradiance originating from within the extent of the solar disc only (DNIS), whose angular extent is 0.266° ± 1.7%, and from a larger circumsolar region, called the circumsolar normal irradiance (CSNI). This study investigates whether the spectral aerosol optical properties of the AERONET stations are sufficient for an accurate modelling of the monochromatic DNIS and CSNI under cloud-free conditions in a desert environment. The data from an AERONET station in Abu Dhabi, United Arab Emirates, and the collocated Sun and Aureole Measurement instrument which offers reference measurements of the monochromatic profile of solar radiance were exploited. Using the AERONET data both the radiative transfer models libRadtran and SMARTS offer an accurate estimate of the monochromatic DNIS, with a relative root mean square error (RMSE) of 6% and a coefficient of determination greater than 0.96. The observed relative bias obtained with libRadtran is +2%, while that obtained with SMARTS is -1%. After testing two configurations in SMARTS and three in libRadtran for modelling the monochromatic CSNI, libRadtran exhibits the most accurate results when the AERONET aerosol phase function is presented as a two-term Henyey-Greenstein phase function. In this case libRadtran exhibited a relative RMSE and a bias of respectively 27 and -24% and a coefficient of determination of 0.882. Therefore, AERONET data may very well be used to model the monochromatic DNIS and the monochromatic CSNI. The results are promising and pave the way towards reporting the contribution of the broadband circumsolar irradiance to standard measurements of the beam irradiance. [ABSTRACT FROM AUTHOR]

The k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computationally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an accurate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is -0.4Wm-2 (-0.2 %) and -4Wm-2 (-1.3 %) and the root mean square error is 1.8Wm-2 (0.7 %) and 4.5Wm-2 (1.5 %). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 μmolm-2 s-1 (+0.8 %) and 1.9 μmolm-2 s-1 (-0.1 %) respectively, and the root mean square error is 11.4 μmolm-2 s-1 (0.9 %) and 4.0 μmolm-2 s-1 (0.3 %). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance. [ABSTRACT FROM AUTHOR]

A new fast clear-sky model called McClear was developed to estimate the downwelling shortwave direct and global irradiances received at ground level under clear skies. It is a fully physical model replacing empirical relations or simpler models used before. It exploits the recent results on aerosol properties, and total column content in water vapour and ozone produced by the MACC project (Monitoring Atmosphere Composition and Climate). It accurately reproduces the irradiance computed by the libRadtran reference radiative transfer model with a computational speed approximately 105 times greater by adopting the abaci, or look-up table, approach combined with interpolation functions. It is therefore suited for geostationary satellite retrievals or numerical weather prediction schemes with many pixels or grid points, respectively. McClear irradiances were compared to 1 min measurements made in clear-sky conditions at several stations within the Baseline Surface Radiation Network in various climates. The bias for global irradiance comprises between -6 and 25Wm-2. The RMSE ranges from 20Wm-2 (3% of the mean observed irradiance) to 36Wm-2 (5 %) and the correlation coefficient ranges between 0.95 and 0.99. The bias for the direct irradiance comprises between -48 and +33Wm-2. The root mean square error (RMSE) ranges from 33Wm-2 (5 %) to 64Wm-2 (10 %). The correlation coefficient ranges between 0.84 and 0.98. This work demonstrates the quality of the McClear model combined with MACC products, and indirectly the quality of the aerosol properties modelled by the MACC reanalysis. [ABSTRACT FROM AUTHOR]

The article presents a research study which focuses on the Modified Lambert-Beer (MLB) parameterization and its usefulness in determining the surface solar irradiance (SSI) in Germany. Researchers found out its effectiveness in figuring the changes in SSI as well as the sun zenith angle (SZA) in a clear atmosphere. The parameterization's difference with the standard linear interpolation methods is also cited.

The article presents a research study concerning the altitudinal change parameterizations in determining the solar irradiance in France. It mentions its significance in assessing the sun's irradiance depending on the level of the altitude. Researchers found out the parameterizations' time saving benefits in terms of computation. The accuracy of the methods is also cited.