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| Titel |
Robust relations between CCN and the vertical evolution of cloud drop size distribution in deep convective clouds |
| VerfasserIn |
E. Freud, D. Rosenfeld, M. O. Andreae, A. A. Costa, P. Artaxo |
| 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 ; 8, no. 6 ; Nr. 8, no. 6 (2008-03-18), S.1661-1675 |
| Datensatznummer |
250005931
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| Publikation (Nr.) |
 copernicus.org/acp-8-1661-2008.pdf |
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| Zusammenfassung |
In-situ measurements in convective clouds (up to the freezing level) over the
Amazon basin show that smoke from deforestation fires prevents clouds from
precipitating until they acquire a vertical development of at least 4 km,
compared to only 1–2 km in clean clouds. The average cloud depth required
for the onset of warm rain increased by ~350 m for each additional 100
cloud condensation nuclei per cm3 at a super-saturation of 0.5%
(CCN0.5%). In polluted clouds, the diameter of modal liquid water
content grows much slower with cloud depth (at least by a factor of ~2),
due to the large number of droplets that compete for available water
and to the suppressed coalescence processes. Contrary to what other studies
have suggested, we did not observe this effect to reach saturation at 3000
or more accumulation mode particles per cm3. The CCN0.5%
concentration was found to be a very good predictor for the cloud depth
required for the onset of warm precipitation and other microphysical
factors, leaving only a secondary role for the updraft velocities in
determining the cloud drop size distributions.
The effective radius of the cloud droplets (re) was found to be a quite
robust parameter for a given environment and cloud depth, showing only a
small effect of partial droplet evaporation from the cloud's mixing with its
drier environment. This supports one of the basic assumptions of satellite
analysis of cloud microphysical processes: the ability to look at different
cloud top heights in the same region and regard their re as if they had
been measured inside one well developed cloud. The dependence of re on
the adiabatic fraction decreased higher in the clouds, especially for
cleaner conditions, and disappeared at re≥~10 μm. We
propose that droplet coalescence, which is at its peak when warm rain is
formed in the cloud at re=~10 μm, continues to be
significant during the cloud's mixing with the entrained air, cancelling out
the decrease in re due to evaporation. |
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