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| Titel |
Analysis of the decrease in the tropical mean outgoing shortwave radiation at the top of atmosphere for the period 1984-2000 |
| VerfasserIn |
A. Fotiadi, N. Hatzianastassiou, C. Matsoukas, K. G. Pavlakis, E. Drakakis, D. Hatzidimitriou, I. Vardavas |
| 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 ; 5, no. 6 ; Nr. 5, no. 6 (2005-07-11), S.1721-1730 |
| Datensatznummer |
250002916
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| Publikation (Nr.) |
 copernicus.org/acp-5-1721-2005.pdf |
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| Zusammenfassung |
| A decadal-scale trend in the tropical radiative energy budget has been
observed recently by satellites, which however is not reproduced by climate
models. In the present study, we have computed the outgoing shortwave
radiation (OSR) at the top of atmosphere (TOA) at 2.5°
longitude-latitude resolution and on a mean monthly basis for the 17-year
period 1984-2000, by using a deterministic solar radiative transfer model
and cloud climatological data from the International Satellite Cloud
Climatology Project (ISCCP) D2 database. Anomaly time series for the mean
monthly pixel-level OSR fluxes, as well as for the key physical parameters,
were constructed. A significant decreasing trend in OSR anomalies, starting
mainly from the late 1980s, was found in tropical and subtropical regions
(30° S-30° N), indicating a decadal increase in solar planetary
heating equal to 1.9±0.3Wm-2/decade, reproducing well the
features recorded by satellite observations, in contrast to climate model
results. This increase in solar planetary heating, however, is accompanied
by a similar increase in planetary cooling, due to increased outgoing
longwave radiation, so that there is no change in net radiation. The model
computed OSR trend is in good agreement with the corresponding linear
decadal decrease of 2.5±0.4Wm-2/decade in tropical mean OSR
anomalies derived from ERBE S-10N non-scanner data (edition 2). An attempt
was made to identify the physical processes responsible for the decreasing
trend in tropical mean OSR. A detailed correlation analysis using
pixel-level anomalies of model computed OSR flux and ISCCP cloud cover over
the entire tropical and subtropical region (30° S-30° N), gave a
correlation coefficient of 0.79, indicating that decreasing cloud cover is
the main reason for the tropical OSR trend. According to the ISCCP-D2 data
derived from the combined visible/infrared (VIS/IR) analysis, the tropical
cloud cover has decreased by 6.6±0.2% per decade, in relative terms.
A detailed analysis of the inter-annual and long-term variability of the
various parameters determining the OSR at TOA, has shown that the most
important contribution to the observed OSR trend comes from a decrease in
low-level cloud cover over the period 1984-2000, followed by decreases in
middle and high-level cloud cover. Note, however, that there still remain
some uncertainties associated with the existence and magnitude of trends in
ISCCP-D2 cloud amounts. Opposite but small trends are introduced by
increases in cloud scattering optical depth of low and middle clouds. |
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