Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 6 2008 10.5194/acpd-8-19313-2008 http://www.atmos-chem-phys-discuss.net/8/19313/2008/ http://www.atmos-chem-phys-discuss.net/8/19313/2008/acpd-8-19313-2008.html http://www.atmos-chem-phys-discuss.net/8/19313/2008/acpd-8-19313-2008.pdf 19313 19355 2008-11-14 Evidence for ice particles in the tropical stratosphere from in-situ measurements M. de Reus reus@mpch-mainz.mpg.de S. Borrmann A. J. Heymsfield R. Weigel C. Schiller V. Mitev W. Frey D. Kunkel A. Kürten J. Curtius N. M. Sitnikov A. Ulanovsky F. Ravegnani Max Planck Institute for Chemistry, Particle Chemistry Department, Mainz, Germany Institute for Atmospheric Physics, Mainz University, Germany National Center for Atmospheric Research, Boulder, USA Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France Institute of Chemistry and Dynamics of the Geosphere, Research Centre Jülich, Germany Swiss Centre for Electronics and Microtechnology, Neuchâtel, Switzerland Div. of Engineering and Applied Science California Inst. of Technology, Pasadena, Ca., USA Institute for Atmospheric and Environmental Sciences, Goethe Univ. of Frankfurt, Germany Central Aerological Observatory, Dolgoprudny, Moskow Region, Russia Institute of Atmospheric Sciences and Climate, Bologna, Italy 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. The objective of the campaign was to characterise the outflow of 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, and were a result of overshooting convection. The ice crystals observed in the stratosphere comprise sizes up to 400 μm maximum dimension, include an ice water content of 0.1×10−3–1.7×10−3 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 cirrus clouds. 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 2×10−4 g m−3. 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