ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-12-11723-2012 Experimental and modeled UV erythemal irradiance under overcast conditions: the role of cloud optical depth Antón M. 2 1 Alados-Arboledas L. https://orcid.org/0000-0003-3576-7167 4 3 Guerrero-Rascado J. L. 4 3 2 Costa M. J. https://orcid.org/0000-0003-2981-2232 2 C Chiu J. https://orcid.org/0000-0002-8951-6913 5 Olmo F. J. 4 3 Departamento de Física, Universidad de Extremadura, Badajoz, Spain Geophysics Centre of Evora, University of Evora, Evora, Portugal Departamento de Física Aplicada, Universidad de Granada, Granada, Spain Centro Andaluz de Medio Ambiente (CEAMA), Universidad de Granada, Granada, Spain Department of Meteorology, University of Reading, Reading, UK 10 12 2012 12 23 11723 11732 Copyright: © 2012 M. Antón et al. 2012 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/12/11723/2012/acp-12-11723-2012.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/12/11723/2012/acp-12-11723-2012.pdf

This paper evaluates the relationship between the cloud modification factor (CMF) in the ultraviolet erythemal range and the cloud optical depth (COD) retrieved from the Aerosol Robotic Network (AERONET) "cloud mode" algorithm under overcast cloudy conditions (confirmed with sky images) at Granada, Spain, mainly for non-precipitating, overcast and relatively homogenous water clouds. Empirical CMF showed a clear exponential dependence on experimental COD values, decreasing approximately from 0.7 for COD = 10 to 0.25 for COD = 50. In addition, these COD measurements were used as input in the LibRadtran radiative transfer code allowing the simulation of CMF values for the selected overcast cases. The modeled CMF exhibited a dependence on COD similar to the empirical CMF, but modeled values present a strong underestimation with respect to the empirical factors (mean bias of 22%). To explain this high bias, an exhaustive comparison between modeled and experimental UV erythemal irradiance (UVER) data was performed. The comparison revealed that the radiative transfer simulations were 8% higher than the observations for clear-sky conditions. The rest of the bias (~14%) may be attributed to the substantial underestimation of modeled UVER with respect to experimental UVER under overcast conditions, although the correlation between both dataset was high (<i>R</i><sup>2</sup> ~ 0.93). A sensitive test showed that the main reason responsible for that underestimation is the experimental AERONET COD used as input in the simulations, which has been retrieved from zenith radiances in the visible range. In this sense, effective COD in the erythemal interval were derived from an iteration procedure based on searching the best match between modeled and experimental UVER values for each selected overcast case. These effective COD values were smaller than AERONET COD data in about 80% of the overcast cases with a mean relative difference of 22%.

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