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| Titel |
Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud |
| VerfasserIn |
F. Ditas, R. A. Shaw, H. Siebert, M. Simmel, B. Wehner, A. Wiedensohler |
| 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 ; 12, no. 5 ; Nr. 12, no. 5 (2012-03-05), S.2459-2468 |
| Datensatznummer |
250010855
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| Publikation (Nr.) |
 copernicus.org/acp-12-2459-2012.pdf |
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| Zusammenfassung |
| This work presents a unique combination of aerosol, cloud microphysical,
thermodynamic and turbulence variables to characterize supersaturation
fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is
based on observations with the helicopter-borne measurement platform ACTOS
and a detailed cloud microphysical parcel model following three different
approaches: (1) From the comparison of aerosol number size distributions
inside and below the SC layer, the number of activated particles is
calculated as 435±87 cm−3 and compares well with the observed
median droplet number concentration of
Nd = 464 cm−3. Furthermore, a 50% activation
diameter of Dp50≈115 nm was derived, which was linked to a
critical supersaturation Scrit of 0.16% via Köhler theory.
From the shape of the fraction of activated particles, we estimated a
standard deviation of supersaturation fluctuations of σS' = 0.09%.
(2) These estimates are compared to more direct thermodynamic
observations at cloud base. Therefore, supersaturation fluctuations (S')
are calculated based on highly-resolved thermodynamic data showing a standard
deviation of S' ranging within 0.1%≤σS'≤0.3 %.
(3) The sensitivity of the supersaturation on observed vertical wind velocity
fluctuations is investigated with the help of a detailed cloud microphysical
model. These results show highest fluctuations of S' with σS'=0.1%
at cloud base and a decreasing σS' with increasing
liquid water content and droplet number concentration. All three approaches
are independent of each other and vary only within a factor of about two. |
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