1FZ Jülich, ICG-I, Germany
2CAO, Region Moscow, Russia
3Univ. Mainz, Inst. Phys. Atmos., Germany
4ETH Zürich, Inst. Atmos. andClimate, Switzerland
*now at: Royal Met. Inst. of Belgium, Brussels, Belgium
**now at: Mühlenbachstraße 5, 52134 Herzogenrath, Germany
Abstract. Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixing ratios sometimes exceeding water saturation, yielding up to more than 200% relative humidities over ice (RHice) have been reported from aircraft and balloon measurements in recent years. From these observations a lively continuous discussion arose on whether there is a lack of understanding of ice cloud microphysics or if the water measurements are tainted with large uncertainties or flaws.
Here, RHice in clear air and in ice clouds is investigated: strictly 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 183–250 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 are caused by unexpected, frequent very low ice crystal numbers that could hardly be explained by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations with implications to 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.