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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

by J. F. Gayet, G. Mioche, L. Bugliaro, A. Protat, A. Minikin, M. Wirth, A. Dörnbrack, V. Shcherbakov, B. Mayer, A. Garnier, C. Gourbeyre show all authors
Atmospheric Chemistry and Physics ()
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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\ncloud (11 080 m/{Ã}?{Â}?{Â}?58 C). The airborne measurements were\nperformed with the DLR Falcon aircraft specially equipped\nwith a unique set of instruments for the extensive in situ\ncloud measurements of microphysical and optical properties\n(Polar Nephelometer, FSSP-300, Cloud Particle Imager and\nPMS 2-D-C) and nadir looking remote sensing observations\n(DLR WALES Lidar). Quasi-simultaneous space observations\nfrom MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR\nand CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the\nDLR Falcon depict very well convective cells which overshoot\nby up to 600m the tropopause level. Unusual high\nvalues of the concentration of small ice particles, extinction,\nice water content (up to 70 cm{Ã}?{Â}?{Â}?3, 30 km{Ã}?{Â}?{Â}?1 and 0.5 gm{Ã}?{Â}?{Â}?3,\nrespectively) are experienced. The mean effective diameter\nand the maximum particle size are 43 {Ã}?{Â}?m and about 300 {Ã}?{Â}?m,\nrespectively. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The SEVIRI retrieved parameters confirm the occurrence of small\nice crystals at the top of the convective cell. Smooth and\nfeatureless phase functions with asymmetry factors of 0.776\nindicate fairly uniform optical properties. Due to small ice\ncrystals the power-law relationship between ice water content\n(IWC) and radar reflectivity appears to be very different\nfrom those usually found in cirrus and anvil clouds. For\na given equivalent reflectivity factor, IWCs are significantly\nlarger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth\nof the overshooting cell, RASTA reflectivity profiles scaled\ninto ice water content show that retrieved IWC up to 1 gm{Ã}?{Â}?{Â}?3\nmay be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 gm{Ã}?{Â}?{Â}?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\nin a continental deep convective systems which are known\ngenerally to generate intense electric fields causing efficient\nice particle aggregation processes. Vigorous updrafts could\nlift supercooled droplets which are frozen extremely rapidly\nby homogeneous nucleation near the {Ã}?{Â}?{Â}?37 C level, producing\ntherefore high concentrations of very small ice particles\nat upper altitudes. They are sufficient to deplete the water\nvapour and suppress further nucleation as confirmed by\nhumidity measurements. These observations address scientific\nissues related to the microphysical properties and structure\nof deep convective clouds and confirm that particles\nsmaller than 50 {Ã}?{Â}?m may control the radiative properties in\nconvective-related clouds. These unusual observations may\nalso provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on\ncommercial aircraft during flights through areas of high ice\nwater content. However, large uncertainties of the measured\nand derived parameters limit our observations.

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