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| Titel |
Experimental and modeled UV erythemal irradiance under overcast conditions: the role of cloud optical depth |
| VerfasserIn |
M. Antón, L. Alados-Arboledas, J. L. Guerrero-Rascado, M. J. Costa, J. C. Chiu, F. J. Olmo |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 23 ; Nr. 12, no. 23 (2012-12-10), S.11723-11732 |
| Datensatznummer |
250011656
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| Publikation (Nr.) |
 copernicus.org/acp-12-11723-2012.pdf |
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| Zusammenfassung |
| 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%. |
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