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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 ()


During the CIRCLE-2 experiment carried out over Western Europe in May\n2007, combined in situ and remote sensing observations allowed to\ndescribe microphysical and optical properties near-top of an\novershooting convective cloud (11 080 m/-58 degrees C). The airborne\nmeasurements were performed with the DLR Falcon aircraft specially\nequipped with a unique set of instruments for the extensive in situ\ncloud measurements of microphysical and optical properties (Polar\nNephelometer, FSSP-300, Cloud Particle Imager and PMS 2-D-C) and nadir\nlooking remote sensing observations (DLR WALES Lidar).\nQuasi-simultaneous space observations from MSG/SEVIRI,\nCALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA\nradar reflectivity from the French Falcon aircraft flying above the DLR\nFalcon depict very well convective cells which overshoot by up to 600 m\nthe tropopause level. Unusual high values of the concentration of small\nice particles, extinction, ice water content (up to 70 cm(-3), 30 km(-1)\nand 0.5 g m(-3), respectively) are experienced. The mean effective\ndiameter and the maximum particle size are 43 mu m and about 300 mu m,\nrespectively. This very dense cloud causes a strong attenuation of the\nWALES and CALIOP lidar returns. The SE-VIRI retrieved parameters confirm\nthe occurrence of small ice crystals at the top of the convective cell.\nSmooth and featureless phase functions with asymmetry factors of 0.776\nindicate fairly uniform optical properties. Due to small ice crystals\nthe power-law relationship between ice water content (IWC) and radar\nreflectivity appears to be very different from those usually found in\ncirrus and anvil clouds. For a given equivalent reflectivity factor,\nIWCs are significantly larger for the overshooting cell than for the\ncirrus. Assuming the same prevalent microphysical properties over the\ndepth of the overshooting cell, RASTA reflectivity profiles scaled into\nice water content show that retrieved IWC up to 1 gm(-3) may be observed\nnear the cloud top. Extrapolating the relationship for stronger\nconvective clouds with similar ice particles, IWC up to 5 gm(-3) could\nbe experienced with reflectivity factors no larger than about 20 dBZ.\nThis means that for similar situations, indication of rather weak radar\necho does not necessarily warn the occurrence of high ice water content\ncarried by small ice crystals. All along the cloud penetration the shape\nof the ice crystals is dominated by chain-like aggregates of frozen\ndroplets. Our results confirm previous observations that the chains of\nice crystals are found in a continental deep convective systems which\nare known generally to generate intense electric fields causing\nefficient ice particle aggregation processes. Vigorous updrafts could\nlift supercooled droplets which are frozen extremely rapidly by\nhomogeneous nucleation near the -37 degrees C level, producing therefore\nhigh concentrations of very small ice particles at upper altitudes. They\nare sufficient to deplete the water vapour and suppress further\nnucleation as confirmed by humidity measurements. These observations\naddress scientific issues related to the microphysical properties and\nstructure of deep convective clouds and confirm that particles smaller\nthan 50 mu m may control the radiative properties in convective-related\nclouds. These unusual observations may also provide some possible\ninsights regarding engineering issues related to the failure of jet\nengines commonly used on commercial aircraft during flights through\nareas of high ice water content.\nHowever, arge uncertainties of the measured and derived parameters limit\nour observations.

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