Abstract. Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RH ice ) 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, RH ice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RH ice 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 RH ice 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 numbers.Correspondence to: M. Krämer (m.kraemer@fz-juelich.de) 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.

[1] We report on in situ and remote sensing measurements of ice particles in the tropical stratosphere found during the Geophysica campaigns TROCCINOX and SCOUT-O3. We show that the deep convective systems penetrated the stratosphere and deposited ice particles at altitudes reaching 420 K potential temperature. These convective events had a hydrating effect on the lower tropical stratosphere due to evaporation of the ice particles. In contrast, there were no signs of convectively induced dehydration in the stratosphere. Citation: Corti, T., et al. (2008), Unprecedented evidence for deep convection hydrating the tropical stratosphere, Geophys. Res. Lett., 35, L10810,

In situ measurements of total water have been obtained during several airborne field experiments in the Arctic (POLSTAR 1997 and 1998; EUPLEX/ENVISAT 2003), at midlatitudes (ENVISAT 2002, Cirrus 2003 and 2004, TROCCINOX 2005), and in the tropics (APE‐THESEO 1999, TROCCINOX/ENVISAT 2005, SCOUT‐O3 2005) in 52 flights in cirrus using the Jülich Lyman‐α fluorescence hygrometer FISH. For a subset of 28 flights, the measurements are complemented by gas phase measurements of H2O. From the data set obtained in these experiments, the ice water content (IWC) in cirrus clouds is derived using two different approaches and functions of the minimum, mean, median, and maximum IWC are provided. The data are analyzed as a function of temperature in the range 183–250 K for Arctic, midlatitudinal, and tropical regions thus extending previous climatologies to much lower temperatures and lower detectable IWC. For each temperature, IWC covers a broad band, decreasing with temperature over the whole temperature range. In the tropics, several events of enhanced ice water content are observed which are related to recent impact of convection.

Abstract.A PSC was detected on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (∼10 −4 cm −3 ) of small nitric acid (HNO 3 ) containing particles (d<6 µm) were observed at altitudes between 18 and 20 km. Provided the temperatures remain below the NAT equilibrium temperature T NAT , these NAT particles have the potential to grow further and to remove HNO 3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures just slightly below T NAT (T >T NAT −3.1 K). This unique measurement of PSC formation at extremely low NAT saturation ratios (S NAT 10) constrains current NAT nucleation theories. We suggest, that the NAT particles have formed heterogeneously, but for certain not on ice. Conversely, meteoritic particles may be favorable candidates for triggering NAT nucleation at the observed low number densities.

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