Comparison of Water Vapor Measurements with Data Retrieved from ECMWF Analyses during the POLINAT Experiment

Abstract

During the POLINAT (Pollution from Aircraft Emissions in the North Atlantic Flight Corridor) experiment, water vapor content was measured with a frost-point hygrometer on board the DLR (Deutsche Forschungsanstalt für Luft-und-Raumfahrt) Falcon research aircraft. Model estimates of the water vapor mixing ratio along the flight track of the aircraft have been interpolated from 6-h ECMWF analyses. Comparisons are made between the in situ measurements and the model data for 16 POLINAT flights. In the high troposphere, the mixing ratio from the model is generally larger than the measured one, with differences amounting to a few percent up to 100%. But at mixing ratios below 20 ppmv and in the stratosphere, the mixing ratios from the model are generally smaller than the measured ones. The observed differences in the high troposphere can be explained by the low accuracy of radiosonde data at low temperatures and/or at small relative humidities. In the stratosphere, due to the technical limitations of radiosonde hygrometers, the water vapor concentration is set to a constant value at each ECMWF analysis step. Despite these discrepancies, many of the small-scale features are, at least in a qualitative sense, reproduced by the model.