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| Titel |
Parameterization of atmospheric longwave emissivity in a mountainous site for all sky conditions |
| VerfasserIn |
J. Herrero, M. J. Polo |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 16, no. 9 ; Nr. 16, no. 9 (2012-09-05), S.3139-3147 |
| Datensatznummer |
250013459
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| Publikation (Nr.) |
 copernicus.org/hess-16-3139-2012.pdf |
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| Zusammenfassung |
| Longwave radiation is an important component of the energy balance of the
Earth's surface. The downward component, emitted by the clouds and aerosols
in the atmosphere, is rarely measured, and is still not well understood. In
mountainous areas, direct observations are even scarcer and the fitting of
existing models is often subjected to local parameterization in order to
surplus the particular physics of the atmospheric profiles. The influence of
clouds makes it even harder to estimate for all sky conditions. This work
presents a long-time continuous dataset of high-resolution longwave
radiation measured in a weather station at a height of 2500 m a.s.l. in
Sierra Nevada, Spain, together with the parameterization of the apparent
atmospheric emissivity for clear and cloudy skies resulting from three
different schemes. We evaluate the schemes of Brutsaert, and Crawford and
Duchon with locally adjusted coefficients and compare them with a completely
parametric expression adjusted for these data that takes into account three
possible significant atmospheric states related to the cloud cover: clear,
completely covered, and partly covered skies. All the parametric expressions
are related to the screen-level values of temperature, relative humidity and
solar radiation, which can be frequently found in standard weather stations.
Unobserved cloudiness measurements needed for Brutsaert scheme for cloudy
sky are also parameterized from screen-level measurements. The calibration
performed for a 6-yr period at the study site resulted in satisfactory
estimations of emissivity for all the analyzed schemes thanks to the local
fitting of the parameterizations, with the best achievement found for the
completely parametric expression. Further validation of the expressions in
two alternative sites showed that the greater accuracy of the latter can
also be found in very close sites, while a better performance of the
Brutsaert scheme, with a more physical background and the successful
parameterization of the clouds effect, is found in nearby sites outside the
initial mountain range. The results show the feasibility for the local
calibration of expressions to estimate instantaneous atmospheric emissivity
for all sky conditions only using surface data, either with a completely
parametric scheme if longwave data are available, or through obtaining
of locally fitted coefficients for Brutsaert and derived schemes.
Nevertheless, the best performance of the first approach would be at the
expense of a reduced local applicability. |
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