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| Titel |
Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations |
| VerfasserIn |
L. Costantino, F.-M. Bréon |
| 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 ; 13, no. 1 ; Nr. 13, no. 1 (2013-01-04), S.69-88 |
| Datensatznummer |
250011716
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| Publikation (Nr.) |
 copernicus.org/acp-13-69-2013.pdf |
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| Zusammenfassung |
In this study, we provide a comprehensive analysis of aerosol interaction
with warm boundary layer clouds over the South-East Atlantic. We use aerosol
and cloud parameters derived from MODIS observations, together with
co-located CALIPSO estimates of the layer altitudes, to derive statistical
relationships between aerosol concentration and cloud properties. The
CALIPSO products are used to differentiate between cases of mixed
cloud-aerosol layers from cases where the aerosol is located well-above the
cloud top. This technique allows us to obtain more reliable estimates of
the aerosol indirect effect than from simple relationships based on vertically
integrated measurements of aerosol and cloud properties. Indeed, it permits us
to somewhat distinguish the effects of aerosol and meteorology on the
clouds, although it is not possible to fully ascertain the relative
contribution of each on the derived statistics.
Consistently with the results from previous studies, our statistics clearly
show that aerosol affects cloud microphysics, decreasing the Cloud Droplet
Radius (CDR). The same data indicate a concomitant strong decrease in cloud
Liquid Water Path (LWP), which is inconsistent with the hypothesis of
aerosol inhibition of precipitation (Albrecht, 1989). We hypothesise that
the observed reduction in LWP is the consequence of dry air entrainment at
cloud top. The combined effect of CDR decrease and LWP decrease leads to
rather small sensitivity of the Cloud Optical Thickness (COT) to an increase
in aerosol concentration. The analysis of MODIS-CALIPSO coincidences
also evidences an aerosol enhancement of low cloud cover. Surprisingly, the Cloud
Fraction (CLF) response to aerosol invigoration is much stronger when
(absorbing) particles are located above cloud top than in cases of physical
interaction. This result suggests a relevant aerosol radiative effect on low
cloud occurrence: absorbing particles above the cloud top may heat the
corresponding atmosphere layer, decrease the vertical temperature gradient,
increase the low tropospheric stability and provide favourable conditions for
low cloud formation.
We also analyse the impact of anthropogenic aerosols on precipitation,
through the statistical analysis of CDR-COT co-variations. A COT value of 10
is found to be the threshold beyond which precipitation is mostly formed, in
both clean and polluted environments. For larger COT, polluted clouds show
evidence of precipitation suppression.
Results suggest the presence of two competing mechanisms governing LWP
response to aerosol invigoration: a drying effect due to aerosol enhanced
entrainment of dry air at cloud top (predominant for optically thin clouds)
and a moistening effect due to aerosol inhibition of precipitation
(predominant for optically thick clouds). |
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