Experimental and modeled UV erythemal irradiance under overcast conditions: the role of cloud optical depth

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Antón, Manuel, Alados Arboledas, Lucas, Guerrero Rascado, Juan Luis, Costa, M. J., Chiu, J. C., Olmo Reyes, Francisco José
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2012
• Subjects: Solar ultraviolet radiation; Ground based measurements; B irradiance; Sky imager; Aerosol characterization; Technical note; Broken cloud; Ozone; Surface; Spain; Aerosol Robotic Network
• Online Access: http://hdl.handle.net/10481/31814; https://doi.org/10.5194/acp-12-11723-2012

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 (R2 ~ 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%. Manuel Antón thanks Ministerio de Ciencia e Innovación and Fondo Social Europeo for the award of a postdoctoral grant (Ramon y Cajal). C. Chiu was supported by the Office of Science (BER, US Department of Energy, Interagency agreement DE-SC0006001) as part of the ASR program. We also thank the AERONET team for providing instrument calibration and data processing. MODIS data were obtained from the Level 1 and Atmosphere Archive and Distribution System (LAADS, http://ladsweb.nascom.nasa.gov ). This work was partially supported by the Andalusian Regional Government through projects P08-RNM-3568 and P10-RNM-6299, the the Ministerio de Ciencia e Innovación through projects CGL2008-05939-C03-03/CLI, CGL2010-18782, CGL-2011-2992-1-C02-01 and CSD2007-00067, and by European Union through ACTRIS project (EU INFRA-2010-1.1.16-262254). This work is co-financed through FEDER (Programa Operacional Factores de Competitividade – COMPETE) and National funding through FCT – Fundaçaõ para Ciencia e a Tecnologia in the framework of project FCOMP-01-0124-FEDER-009303 (PTDC/CTE-ATM/102142/2008).