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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, Mioche G, Bugliaro L, Protat A, Minikin A, Wirth M, Dörnbrack A, Shcherbakov V, Mayer B, Garnier A, Gourbeyre C ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 2 (2012) pp. 727-744

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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/{Ã}?{Â}?{Â}?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{Ã}?{Â}?{Â}?3, 30 km{Ã}?{Â}?{Â}?1 and 0.5 gm{Ã}?{Â}?{Â}?3,
respectively) are experienced. The mean effective diameter
and the maximum particle size are 43 {Ã}?{Â}?m and about 300 {Ã}?{Â}?m,
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{Ã}?{Â}?{Â}?3
may 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
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 {Ã}?{Â}?{Â}?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 {Ã}?{Â}?m 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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