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

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 (