Vertical distribution, seasonality and troposphericity of icesupersaturated air masses in the northern mid-latitudes from regular in-situ observations by passenger aircraft

The vertical distribution and seasonal variation of water vapour volume mixing ratio (H2O VMR), relative humidity with respect to ice (RHice) and particularly of regions with ice-supersaturated air masses (ISSR) in the extratropical upper troposphere and lowermost stratosphere are investigated at northern mid-latitudes over the regions Eastern North America, the North Atlantic and Europe for the period 1995 to 2010.Observation data originate from regular and continuous long-term measurements of H2O VMR, temperature and RHice by instrumented passenger aircraft in the framework of the European research program MOZAIC which is continued as European research infrastructure IAGOS (from 2011; see www.iagos.org). The observation data are analysed with respect to the thermal and dynamical tropopauses, as provided by ERA-Interim. Additionally, collocated O3 observations from MOZAIC are used as tracer for stratospheric air masses.Our key results provide in-depth insight into seasonal and regional variability and tropospheric nature of ice-supersaturated air masses at various distances from the tropopause layer. For the vertical distribution and seasonal variation of ISSR occurrence we show a comparison of our results to radio soundings and to satellite observations of cirrus cloud occurrence from AIRS and TOVs Path B instruments. Finally, for all three regions, we investigate the trends and the dependencies of ISSR occurrence on the North Atlantic Oscillation (NAO) index.</p