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| Titel |
Cluster analysis of midlatitude oceanic cloud regimes: mean properties and temperature sensitivity |
| VerfasserIn |
N. D. Gordon, J. R. Norris |
| 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 ; 10, no. 13 ; Nr. 10, no. 13 (2010-07-14), S.6435-6459 |
| Datensatznummer |
250008627
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| Publikation (Nr.) |
 copernicus.org/acp-10-6435-2010.pdf |
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| Zusammenfassung |
Clouds play an important role in the climate system by reducing the amount
of shortwave radiation reaching the surface and the amount of longwave
radiation escaping to space. Accurate simulation of clouds in computer
models remains elusive, however, pointing to a lack of understanding of the
connection between large-scale dynamics and cloud properties. This study
uses a k-means clustering algorithm to group 21 years of satellite cloud
data over midlatitude oceans into seven clusters, and demonstrates that the
cloud clusters are associated with distinct large-scale dynamical
conditions. Three clusters correspond to low-level cloud regimes with
different cloud fraction and cumuliform or stratiform characteristics, but
all occur under large-scale descent and a relatively dry free troposphere.
Three clusters correspond to vertically extensive cloud regimes with tops in
the middle or upper troposphere, and they differ according to the strength
of large-scale ascent and enhancement of tropospheric temperature and
humidity. The final cluster is associated with a lower troposphere that is
dry and an upper troposphere that is moist and experiencing weak ascent and
horizontal moist advection.
Since the present balance of reflection of shortwave and absorption of
longwave radiation by clouds could change as the atmosphere warms from
increasing anthropogenic greenhouse gases, we must also better understand
how increasing temperature modifies cloud and radiative properties. We
therefore undertake an observational analysis of how midlatitude oceanic
clouds change with temperature when dynamical processes are held constant
(i.e., partial derivative with respect to temperature). For each of the
seven cloud regimes, we examine the difference in cloud and radiative
properties between warm and cold subsets. To avoid misinterpreting a cloud
response to large-scale dynamical forcing as a cloud response to
temperature, we require horizontal and vertical temperature advection in the
warm and cold subsets to have near-median values in three layers of the
troposphere. Across all of the seven clusters, we find that cloud fraction
is smaller and cloud optical thickness is mostly larger for the warm subset.
Cloud-top pressure is higher for the three low-level cloud regimes and lower
for the cirrus regime. The net upwelling radiation flux at the top of the
atmosphere is larger for the warm subset in every cluster except cirrus, and
larger when averaged over all clusters. This implies that the direct
response of midlatitude oceanic clouds to increasing temperature acts as a
negative feedback on the climate system. Note that the cloud response to
atmospheric dynamical changes produced by global warming, which we do not
consider in this study, may differ, and the total cloud feedback may be
positive. |
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