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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

Gayet, J.-F. and Mioche, G. and Bugliaro, G. and Protat, L. and Minikin, A. and Wirth, M. and Dörnbrack, A. and Shcherbakov, V. and Mayer, B. and Garnier, A. and Gourbeyre, C. (2012) 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. Atmospheric Chemistry and Physics, 12, pp. 727-744. DOI: 10.5194/acp-12-727-2012.

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Official URL: http://www.atmos-chem-phys.net/12/727/2012/

Abstract

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 600m 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 gm−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 gm−3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 gm−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 chainlike 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.

Document Type:Article
Title: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
Authors:
AuthorsInstitution or Email of Authors
Gayet, J.-F.Univ. Blaise Pascal, Clermont-Ferrand, F
Mioche, G.Univ. Blaise Pascal, Clermont-Ferrand, F
Bugliaro, G.DLR
Protat, L.Univ. Pierre et Marie Curie, F
Minikin, A.DLR
Wirth, M.DLR
Dörnbrack, A.DLR
Shcherbakov, V.Univ. Blaise Pascal, Clermont-Ferrand, F
Mayer, B.DLR
Garnier, A.Univ. Pierre et Marie Curie, F
Gourbeyre, C.Univ. Blaise Pascal, Clermont-Ferrand, F
Date:2012
Journal or Publication Title:Atmospheric Chemistry and Physics
Refereed publication:Yes
In Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:12
DOI:10.5194/acp-12-727-2012
Page Range:pp. 727-744
Status:Published
Keywords:Cloud remote sensing, in-situ measurements, CIRCLE-2, convection, cloud microphysics, airtraffic hazards
HGF - Research field:Aeronautics, Space and Transport, Aeronautics, Space and Transport
HGF - Program:Aeronautics, Transport
HGF - Program Themes:ATM and Operation, Transport System
DLR - Research area:Aeronautics, Transport
DLR - Program:L AO - Air Traffic Management and Operation, V VS - Verkehrssystem
DLR - Research theme (Project):L - Climate, Weather and Environment, V - Verkehrsentwicklung und Umwelt (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Cloud Physics and Traffic Meteorology
Deposited By: Jana Freund
Deposited On:09 Nov 2011 14:24
Last Modified:26 Mar 2013 13:32

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