Abstract. The vertical distribution and seasonal variation of water vapour volume mixing ratio (H2O VMR), of relative humidity with respect to ice (RHice) and particularly of regions with ice-supersaturated air masses (ISSRs) in the extratropical upper troposphere and lowermost stratosphere are investigated at northern mid-latitudes over the eastern North American, North Atlantic and European regions for the period 1995 to 2010. Observation data originate from regular and continuous long-term measurements on board instrumented passenger aircraft in the framework of the European research programme MOZAIC (1994–2010), which continues as the European research infrastructure IAGOS (from 2011). Data used in our study result from collocated observations of O3 VMR, RHice and temperature, as well as H2O VMR deduced from RHice and temperature data. The in situ observations of H2O VMR and RHice with a vertical resolution of 30 hPa (< 750 m at the extratropical tropopause level) and a horizontal resolution of 1 km resolve detailed features of the distribution of water vapour and ice-supersaturated air relative to the thermal tropopause, including their seasonal and regional variability and chemical signatures at various distances from the tropopause layer. Annual cycles of the investigated properties document the highest H2O VMR and temperatures above the thermal tropopause in the summer months, whereas RHice above the thermal tropopause remains almost constant in the course of the year. Over all investigated regions, upper tropospheric air masses close to the tropopause level are nearly saturated with respect to ice and contain a significant fraction of ISSRs with a distinct seasonal cycle of minimum values in summer (30 % over the ocean, 20 %–25 % over land) and maximum values in late winter (35 %–40 % over both land and ocean). Above the thermal tropopause, ISSRs are occasionally observed with an occurrence probability of 1.5 ± 1.1 %, whereas above the dynamical tropopause at 2 PVU (PVU: potential vorticity unit), the occurrence probability increases 4-fold to 8.4 ± 4.4 %. In both coordinate systems related to tropopause height (TPH), the ISSR occurrence probabilities drop to values below 1 % for the next higher air mass layer with pressure levels p < pTPH−15 hPa. For both tropopause definitions, the tropospheric nature or fingerprint, based on O3 VMR, indicates the continuing tropospheric influence on ISSRs inside and above the respective tropopause layer. For the non-ISSRs, however, the stratospheric nature is clearly visible above the thermal tropopause, whereas above the dynamical tropopause the air masses show a still substantial tropospheric influence. For all three regions, seasonal deviations from the long-term annual cycle of ISSR occurrence show no significant trends over the observation period of 15 years, whereas a statistically significant correlation between the North Atlantic Oscillation (NAO) index and the deviation of ISSR occurrence from the long-term average is observed for the North Atlantic region but not for the eastern North American and European regions.

Abstract. The vertical distribution and seasonal variation of upper tropospheric humidity (UTH) and particularly of ice-supersaturated air masses in the extratropical upper troposphere and lowermost stratosphere (Ex-UTLS) is 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 water vapour volume mixing ratio (H2O VMR), temperature and relative humidity with respect to ice (RHice) by instrumented passenger aircraft in the framework of the European research program MOZAIC (1994–2010) which is continued as European research infrastructure IAGOS (from 2011). The in-situ observations of UTH with a vertical resolution of 30 hPa (

&lt;p&gt;The vertical distribution and seasonal variation of water vapour volume mixing ratio (H&lt;sub&gt;2&lt;/sub&gt;O VMR), relative humidity with respect to ice (RH&lt;sub&gt;ice&lt;/sub&gt;) 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.&lt;/p&gt;&lt;p&gt;Observation data originate from regular and continuous long-term measurements of H&lt;sub&gt;2&lt;/sub&gt;O VMR, temperature and RH&lt;sub&gt;ice&lt;/sub&gt; 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 O&lt;sub&gt;3&lt;/sub&gt; observations from MOZAIC are used as tracer for stratospheric air masses.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;