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| Titel |
Effect of remote clouds on surface UV irradiance |
| VerfasserIn |
M. Degünther, R. Meerkötter |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 18, no. 6 ; Nr. 18, no. 6, S.679-686 |
| Datensatznummer |
250014022
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| Publikation (Nr.) |
 copernicus.org/angeo-18-679-2000.pdf |
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| Zusammenfassung |
Clouds affect local surface UV irradiance,
even if the horizontal distance from the radiation observation site amounts to
several kilometers. In order to investigate this effect, which we call remote
clouds effect, a 3-dimensional radiative transfer model is applied. Assuming the
atmosphere is subdivided into a quadratic based sector and its surrounding, we
quantify the influence of changing cloud coverage within this surrounding from
0% to 100% on surface UV irradiance at the sector center. To work out this
remote clouds influence as a function of sector base size, we made some
calculations for different sizes between 10 km × 10 km and 100 km × 100 km. It
appears that in the case of small sectors (base size < 20 km × 20 km)
the remote clouds effect is highly variable: Depending on cloud structure, solar
zenith angle and wavelength, the surface UV irradiance may be enhanced up to 15%
as well as reduced by more than 50%. In contrast, for larger sectors it is
always the case that enhancements become smaller by 5% if sector base size
exceeds 60 km × 60 km. However, these values are upper estimates of the remote
cloud effects and they are found only for special cloud structures. Since these
structures might occur but cannot be regarded as typical, different satellite
observed cloud formations (horizontal resolution about 1 km × 1 km) have also
been investigated. For these more common cloud distributions we find remote
cloud effects to be distinctly smaller than the corresponding upper estimates,
e.g., for a sector with base size of 25 km × 25 km the surface UV irradiance
error due to ignoring the actual remote clouds and replacing their influence
with periodic horizontal boundary conditions is less than 3%, whereas the upper
estimate of remote clouds effect would suggest an error close to 10%.
Key words: Atmospheric composition and structure
(transmission and scattering of radiation) - Meteorology and atmospheric
dynamics (radiative process) |
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