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.
Weak Mixing in the Eastern North Atlantic: An Application of the Tracer-Contour Inverse Method