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| Titel |
Observations of ice multiplication in a weakly convective cell embedded in supercooled mid-level stratus |
| VerfasserIn |
J. Crosier, K. N. Bower, T. W. Choularton, C. D. Westbrook, P. J. Connolly, Z. Q. Cui, I. P. Crawford, G. L. Capes, H. Coe, J. R. Dorsey, P. I. Williams, A. J. Illingworth, M. W. Gallagher, A. M. Blyth |
| 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. 1 ; Nr. 11, no. 1 (2011-01-13), S.257-273 |
| Datensatznummer |
250009070
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| Publikation (Nr.) |
 copernicus.org/acp-11-257-2011.pdf |
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| Zusammenfassung |
Simultaneous observations of cloud microphysical properties
were obtained by in-situ aircraft measurements and ground
based Radar/Lidar. Widespread mid-level stratus cloud was
present below a temperature inversion (~5 °C
magnitude) at 3.6 km altitude. Localised convection (peak
updraft 1.5 m s−1) was observed 20 km west of the
Radar station. This was associated with convergence at
2.5 km altitude. The convection was unable to penetrate the
inversion capping the mid-level stratus.
The mid-level stratus cloud was vertically thin
(~400 m), horizontally extensive (covering 100 s of km)
and persisted for more than 24 h. The cloud consisted of
supercooled water droplets and small concentrations of large
(~1 mm) stellar/plate like ice which slowly
precipitated out. This ice was nucleated at temperatures
greater than −12.2 °C and less than
−10.0 °C, (cloud top and cloud base
temperatures, respectively). No ice seeding from above the
cloud layer was observed. This ice was formed by primary
nucleation, either through the entrainment of efficient ice
nuclei from above/below cloud, or by the slow stochastic
activation of immersion freezing ice nuclei contained within
the supercooled drops. Above cloud top significant
concentrations of sub-micron aerosol were observed and
consisted of a mixture of sulphate and carbonaceous material,
a potential source of ice nuclei. Particle number concentrations
(in the size range 0.1<D<3.0 μm) were
measured above and below cloud in concentrations of ~25 cm−3.
Ice crystal concentrations in the cloud were constant at around 0.2 L−1.
It is estimated that entrainment of aerosol particles into cloud cannot
replenish the loss of ice nuclei from the cloud layer via precipitation.
Precipitation from the mid-level stratus evaporated before
reaching the surface, whereas rates of up to 1 mm h−1
were observed below the convective feature. There is strong
evidence for the Hallett-Mossop (HM) process of secondary ice
particle production leading to the formation of the
precipitation observed. This includes (1) Ice concentrations
in the convective feature were more than an order of magnitude
greater than the concentration of primary ice in the
overlaying stratus, (2) Large concentrations of small pristine
columns were observed at the ~−5 °C level
together with liquid water droplets and a few rimed ice
particles, (3) Columns were larger and increasingly rimed at
colder temperatures. Calculated ice splinter production rates
are consistent with observed concentrations if the condition
that only droplets greater than 24 μm are capable
of generating secondary ice splinters is relaxed.
This case demonstrates the importance of understanding the
formation of ice at slightly supercooled temperatures, as it
can lead to secondary ice production and the formation of
precipitation in clouds which may not otherwise be considered
as significant precipitation sources. |
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