Ice supersaturations and cirrus cloud crystal numbers M. Krämer1, C. Schiller1, A. Afchine1, R. Bauer1, I. Gensch1, A. Mangold1,*, S. Schlicht1,**, N. Spelten1, N. Sitnikov2, S. Borrmann3, M. de Reus3, and P. Spichtinger4 1Forschungszentrum Jülich, Institut fur Chemie und Dynamik der Geosphäre 1: Stratosphäre, 52425 Jülich, Germany 2Central Aerological Observatory, Dolgoprudny, Region Moscow, Russia 3Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Joh.-Joachim-Becher-Weg 21, 55099 Mainz, Germany 4ETH Zürich, Institut für Atmosphäre und Klima, Universitätstrasse 16, 8092 Zürich, Switzerland *now at: Royal Meteorological Institute of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium **now at: Mühlenbachstraße 5, 52134 Herzogenrath, Germany
Abstract. Upper tropospheric observations outside and inside of cirrus clouds indicate
water vapour mixing ratios sometimes exceeding water saturation. Relative
humidities over ice (RHice) of up to and more than 200% have been
reported from aircraft and balloon measurements in recent years.
From these observations a lively discussion continues on whether there is a
lack of understanding of ice cloud microphysics or whether the water
measurements are tainted with large uncertainties or flaws.
Here, RHice in clear air and in ice clouds is investigated. Strict quality-checked
aircraft in situ observations of RHice were performed during 28
flights in tropical, mid-latitude and Arctic field experiments in the
temperature range 183–240 K. In our field measurements, no supersaturations
above water saturation are found. Nevertheless, super- or subsaturations
inside of cirrus are frequently observed at low temperatures (<205 K) in our
field data set. To explain persistent RHice deviating from saturation, we
analysed the number densities of ice crystals recorded during 20 flights.
From the combined analysis – using conventional microphysics – of
supersaturations and ice crystal numbers, we show that the high, persistent
supersaturations observed inside of cirrus can possibly be explained by
unexpected, frequent very low ice crystal numbers that could scarcely be
caused by homogeneous ice nucleation. Heterogeneous ice formation or the
suppression of freezing might better explain the observed ice crystal
Thus, our lack of understanding of the high supersaturations, with
implications for the microphysical and radiative properties of cirrus, the
vertical redistribution of water and climate, is traced back to the
understanding of the freezing process at low temperatures.
Citation: Krämer, M., Schiller, C., Afchine, A., Bauer, R., Gensch, I., Mangold, A., Schlicht, S., Spelten, N., Sitnikov, N., Borrmann, S., de Reus, M., and Spichtinger, P.: Ice supersaturations and cirrus cloud crystal numbers, Atmos. Chem. Phys., 9, 3505-3522, doi:10.5194/acp-9-3505-2009, 2009.