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Estimating Hourly Beam and Diffuse Solar Radiation in an Alpine Valley: A Critical Assessment of Decomposition Models

Agency for the Protection of the Environment, Autonomous Province of Trento (APPA Trento), via Mantova, 16, 38122 Trento, Italy
Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, via Mesiano, 77, 38123 Trento, Italy
Photovoltaic Systems Research Group, Institute for Renewable Energy, European Academy (EURAC) of Bolzano, Via A. Volta 13a, 39100 Bolzano, Italy
Author to whom correspondence should be addressed.
Atmosphere 2018, 9(4), 117;
Received: 5 March 2018 / Revised: 19 March 2018 / Accepted: 20 March 2018 / Published: 21 March 2018
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
PDF [15134 KB, uploaded 3 May 2018]


Accurate solar radiation estimates in Alpine areas represent a challenging task, because of the strong variability arising from orographic effects and mountain weather phenomena. These factors, together with the scarcity of observations in elevated areas, often cause large modelling uncertainties. In the present paper, estimates of hourly mean diffuse fraction values from global radiation data, provided by a number (13) of decomposition models (chosen among the most widely tested in the literature), are evaluated and compared with observations collected near the city of Bolzano, in the Adige Valley (Italian Alps). In addition, the physical factors influencing diffuse fraction values in such a complex orographic context are explored. The average accuracy of the models were found to be around 27% and 14% for diffuse and beam radiation respectively, the largest errors being observed under clear sky and partly cloudy conditions, respectively. The best performances were provided by the more complex models, i.e., those including a predictor specifically explaining the radiation components’ variability associated with scattered clouds. Yet, these models return non-negligible biases. In contrast, the local calibration of a single-equation logistical model with five predictors allows perfectly unbiased estimates, as accurate as those of the best-performing models (20% and 12% for diffuse and beam radiation, respectively), but at much smaller computational costs. View Full-Text
Keywords: solar radiation; decomposition models; hourly diffuse fraction; Alpine valley solar radiation; decomposition models; hourly diffuse fraction; Alpine valley

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Laiti, L.; Giovannini, L.; Zardi, D.; Belluardo, G.; Moser, D. Estimating Hourly Beam and Diffuse Solar Radiation in an Alpine Valley: A Critical Assessment of Decomposition Models. Atmosphere 2018, 9, 117.

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