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. 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 lowermoststratosphere are investigated at northern mid-latitudes over the eastern North American, North Atlantic and European regions for the period 1995 to2010. Observation data originate from regular and continuous long-term measurements on board instrumented passenger aircraft in the framework ofthe European research programme MOZAIC (1994–2010), which continues as the European research infrastructure IAGOS (from 2011). Data used in our studyresult from collocated observations of O3 VMR, RHice and temperature, as well as H2O VMR deduced from RHice andtemperature data. The in situ observations of H2O VMR and RHice with a vertical resolution of 30 hPa (&lt; 750 mat the extratropical tropopause level) and a horizontal resolution of 1 km resolve detailed features of the distribution of water vapour andice-supersaturated air relative to the thermal tropopause, including their seasonal and regional variability and chemical signatures at variousdistances from the tropopause layer. Annual cycles of the investigated properties document the highest H2O VMR and temperatures above thethermal tropopause in the summer months, whereas RHice above the thermal tropopause remains almost constant in the course of theyear. Over all investigated regions, upper tropospheric air masses close to the tropopause level are nearly saturated with respect to ice andcontain a significant fraction of ISSRs with a distinct seasonal cycle of minimum values in summer (30 % over the ocean, 20 %–25 % over land) andmaximum values in late winter (35 %–40 % over both land and ocean). Above the thermal tropopause, ISSRs are occasionally observed with an occurrenceprobability of 1.5 ± 1.1 %, whereas above the dynamical tropopause at 2 PVU (PVU: potential vorticity unit), the occurrence probability increases 4-fold to8.4 ± 4.4 %. In both coordinate systems related to tropopause height (TPH), the ISSR occurrence probabilities drop to values below 1 % forthe next higher air mass layer with pressure levels p &lt; pTPH−15 hPa. For both tropopause definitions, the tropospheric natureor fingerprint, based on O3 VMR, indicates the continuing tropospheric influence on ISSRs inside and above the respective tropopauselayer. For the non-ISSRs, however, the stratospheric nature is clearly visible above the thermal tropopause, whereas above the dynamical tropopausethe air masses show a still substantial tropospheric influence. For all three regions, seasonal deviations from the long-term annual cycle of ISSRoccurrence show no significant trends over the observation period of 15 years, whereas a statistically significant correlation between the NorthAtlantic 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.

atmospheric chemistry and physics

Ice-supersaturated air masses in the northern mid-latitudes from regular in situ observations by passenger aircraft: vertical distribution, seasonality and tropospheric fingerprint