Comparison of water vapor measurements from POLINAT 2 with ECMWF analyses in high-humidity conditions

During the Pollution From Aircraft in the North Atlantic Flight Corridor (POLINAT 2) experiment, water vapor content was measured during 12 flights of the Deutsches Zentrum fuer Luft und Raumfahrt Falcon research aircraft by means of a frost-point hygrometer. The last three flights were coordinated with the flights of the NASA DCS aircraft, taking part in the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX). The measurements were generally performed in high-humidity and cloudy environments. Synthesized profiles of humidity and cloud cover along the flight tracks were retrieved from the European Centre for Medium-Range Weather Forecasts analyses. It is shown that the measurements of supersaturation are generally correlated to the occurrence of clouds as indicated by the model cloud cover product. However, there are large discrepancies between the mixing ratio from the analyses and the measurements: the model underestimates the water vapor content. Some reasons for this underestimation are put forward, and the consequences for the investigation of the occurrence of contrail formation and persistence, when using such analyses, are pointed out.