Evidence for ice particles in the tropical stratosphere from in-situ measurements M. de Reus1,2,*, S. Borrmann1,2, A. Bansemer3, A. J. Heymsfield3, R. Weigel2, C. Schiller4, V. Mitev5, W. Frey2, D. Kunkel1,2, A. Kürten6, J. Curtius6, N. M. Sitnikov7, A. Ulanovsky7, and F. Ravegnani8 1Max Planck Institute for Chemistry, Particle Chemistry Department, Mainz, Germany 2Institute for Atmospheric Physics, Mainz University, Germany 3National Center for Atmospheric Research, Boulder, USA 4Institute of Chemistry and Dynamics of the Geosphere, Research Centre Jülich, Germany 5Swiss Centre for Electronics and Microtechnology, Neuchâtel, Switzerland 6Institute for Atmospheric and Environmental Sciences, Goethe University of Frankfurt, Germany 7Central Aerological Observatory, Dolgoprudny, Moskow Region, Russia 8Institute of Atmospheric Sciences and Climate, Bologna, Italy *now at: Elementar Analysensysteme GmbH, Hanau, Germany
Abstract. In-situ ice crystal size distribution measurements are presented within the
tropical troposphere and lower stratosphere. The measurements were performed
using a combination of a Forward Scattering Spectrometer Probe (FSSP-100)
and a Cloud Imaging Probe (CIP), which were installed on the Russian high
altitude research aircraft M55 "Geophysica" during the SCOUT-O3
campaign in Darwin, Australia. One of the objectives of the campaign was to
characterise the Hector convective system, which appears on an almost daily
basis during the pre-monsoon season over the Tiwi Islands, north of Darwin.
In total 90 encounters with ice clouds, between 10 and 19 km altitude were
selected from the dataset and were analysed. Six of these encounters were
observed in the lower stratosphere, up to 1.4 km above the local tropopause.
Concurrent lidar measurements on board "Geophysica" indicate that these
ice clouds were a result of overshooting convection. Large ice crystals,
with a maximum dimension up to 400 μm, were observed in the
stratosphere. The stratospheric ice clouds included an ice water content
ranging from 7.7×10−5 to 8.5×10−4 g m−3 and were observed
at ambient relative humidities (with respect to ice) between 75 and 157%.
Three modal lognormal size distributions were fitted to the average
size distributions for different potential temperature intervals, showing
that the shape of the size distribution of the stratospheric ice clouds are
similar to those observed in the upper troposphere.
In the tropical troposphere the effective radius of the ice cloud particles
decreases from 100 μm at about 10 km altitude, to 3 μm at the
tropopause, while the ice water content decreases from 0.04 to 10−5 g m−3.
No clear trend in the number concentration was observed with
altitude, due to the thin and inhomogeneous characteristics of the observed
The ice water content calculated from the observed ice crystal size
distribution is compared to the ice water content derived from two
hygrometer instruments. This independent measurement of the ice water
content agrees within the combined uncertainty of the instruments for ice
water contents exceeding 3×10−4g m−3.
Stratospheric residence times, calculated based on gravitational settling,
and evaporation rates show that the ice crystals observed in the
stratosphere over the Hector storm system had a high potential of
humidifying the stratosphere locally.
Utilizing total aerosol number concentration measurements from a four
channel condensation particle counter during two separate campaigns, it can
be shown that the fraction of ice particles to the number of aerosol
particles remaining ranges from 1:300 to 1:30 000 for tropical upper
tropospheric ice clouds with ambient temperatures below −75°C.
Citation: de Reus, M., Borrmann, S., Bansemer, A., Heymsfield, A. J., Weigel, R., Schiller, C., Mitev, V., Frey, W., Kunkel, D., Kürten, A., Curtius, J., Sitnikov, N. M., Ulanovsky, A., and Ravegnani, F.: Evidence for ice particles in the tropical stratosphere from in-situ measurements, Atmos. Chem. Phys., 9, 6775-6792, doi:10.5194/acp-9-6775-2009, 2009.