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| Titel |
Vertical profiles of droplet effective radius in shallow convective clouds |
| VerfasserIn |
S. Zhang, H. Xue, G. Feingold |
| 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 ; 11, no. 10 ; Nr. 11, no. 10 (2011-05-17), S.4633-4644 |
| Datensatznummer |
250009743
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| Publikation (Nr.) |
 copernicus.org/acp-11-4633-2011.pdf |
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| Zusammenfassung |
| Conventional satellite retrievals can only provide information on cloud-top
droplet effective radius (re). Given the fact that cloud ensembles in a
satellite snapshot have different cloud-top heights, Rosenfeld and Lensky (1998)
used the cloud-top height and the corresponding cloud-top re from
the cloud ensembles in the snapshot to construct a profile of re
representative of that in the individual clouds. This study investigates the
robustness of this approach in shallow convective clouds based on results
from large-eddy simulations (LES) for clean (aerosol mixing ratio Na =
25 mg−1), intermediate (Na = 100 mg−1), and polluted
(Na = 2000 mg−1) conditions. The cloud-top height and the cloud-top
re from the modeled cloud ensembles are used to form a constructed
re profile, which is then compared to the in-cloud re profiles. For
the polluted and intermediate cases where precipitation is negligible, the
constructed re profiles represent the in-cloud re profiles fairly
well with a low bias (about 10 %). The method used in Rosenfeld and Lensky
(1998) is therefore validated for nonprecipitating shallow cumulus clouds.
For the clean, drizzling case, the in-cloud re can be very large and
highly variable, and quantitative profiling based on cloud-top re is
less useful. The differences in re profiles between clean and polluted
conditions derived in this manner are however, distinct. This study also
investigates the subadiabatic characteristics of the simulated cumulus
clouds to reveal the effect of mixing on re and its evolution. Results
indicate that as polluted and moderately polluted clouds develop into their
decaying stage, the subadiabatic fraction fad becomes smaller,
representing a higher degree of mixing, and re becomes smaller (~10 %)
and more variable. However, for the clean case, smaller fad
corresponds to larger re (and larger re variability), reflecting
the additional influence of droplet collision-coalescence and sedimentation
on re. Finally, profiles of the vertically inhomogeneous clouds as
simulated by the LES and those of the vertically homogeneous clouds are used
as input to a radiative transfer model to study the effect of cloud vertical
inhomogeneity on shortwave radiative forcing. For clouds that have the same
liquid water path, re of a vertically homogeneous cloud must be about
76–90 % of the cloud-top re of the vertically inhomogeneous cloud in
order for the two clouds to have the same shortwave radiative forcing. |
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