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| Titel |
On the observation of unusual high concentration of small chain-like aggregate ice crystals and large ice water contents near the top of a deep convective cloud during the CIRCLE-2 experiment |
| VerfasserIn |
J.-F. Gayet, G. Mioche, L. Bugliaro, A. Protat, A. Minikin, M. Wirth, A. Dörnbrack, V. Shcherbakov, B. Mayer, A. Garnier, C. Gourbeyre |
| 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. 2 ; Nr. 12, no. 2 (2012-01-16), S.727-744 |
| Datensatznummer |
250010526
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| Publikation (Nr.) |
 copernicus.org/acp-12-727-2012.pdf |
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| Zusammenfassung |
| During the CIRCLE-2 experiment carried out over Western Europe in May 2007,
combined in situ and remote sensing observations allowed to describe
microphysical and optical properties near-top of an overshooting convective
cloud (11 080 m/−58 °C). The airborne measurements were
performed with the DLR Falcon aircraft specially equipped with a unique set
of instruments for the extensive in situ cloud measurements of microphysical
and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager
and PMS 2-D-C) and nadir looking remote sensing observations (DLR WALES
Lidar). Quasi-simultaneous space observations from MSG/SEVIRI,
CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar
reflectivity from the French Falcon aircraft flying above the DLR Falcon
depict very well convective cells which overshoot by up to 600 m the
tropopause level. Unusual high values of the concentration of small ice
particles, extinction, ice water content (up to 70 cm−3, 30 km−1
and 0.5 g m−3, respectively) are experienced. The mean effective
diameter and the maximum particle size are 43 μm and about
300 μm, respectively. This very dense cloud causes a strong
attenuation of the WALES and CALIOP lidar returns. The SEVIRI retrieved
parameters confirm the occurrence of small ice crystals at the top of the
convective cell. Smooth and featureless phase functions with asymmetry
factors of 0.776 indicate fairly uniform optical properties. Due to small ice
crystals the power-law relationship between ice water content (IWC) and
radar reflectivity appears to be very different from those usually found in
cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus.
Assuming the same prevalent microphysical properties over the depth of the
overshooting cell, RASTA reflectivity profiles scaled into ice water content
show that retrieved IWC up to 1 g m−3 may be observed near the
cloud top. Extrapolating the relationship for stronger convective clouds with
similar ice particles, IWC up to 5 g m−3 could be experienced
with reflectivity factors no larger than about 20 dBZ. This means that for
similar situations, indication of rather weak radar echo does not necessarily
warn the occurrence of high ice water content carried by small ice crystals.
All along the cloud penetration the shape of the ice crystals is dominated by
chain-like aggregates of frozen droplets. Our results confirm previous
observations that the chains of ice crystals are found in a continental deep
convective systems which are known generally to generate intense electric
fields causing efficient ice particle aggregation processes. Vigorous
updrafts could lift supercooled droplets which are frozen extremely rapidly
by homogeneous nucleation near the −37 °C level, producing
therefore high concentrations of very small ice particles at upper altitudes.
They are sufficient to deplete the water vapour and suppress further
nucleation as confirmed by humidity measurements. These observations address
scientific issues related to the microphysical properties and structure of
deep convective clouds and confirm that particles smaller than
50 μm may control the radiative properties in convective-related
clouds. These unusual observations may also provide some possible insights
regarding engineering issues related to the failure of jet engines commonly
used on commercial aircraft during flights through areas of high ice water
content. However, large uncertainties of the measured and derived parameters
limit our observations. |
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