COMPARISON OF WATER-VAPOR MEASUREMENTS WITH DATA RETRIEVED FROM ECMWFANALYSES DURING THE POLINAT EXPERIMENT

During the POLINAT (Pollution from Aircraft Emissions in the North Atl antic Flight Corridor) experiment, water vapor content was measured wi th a frost-point hygrometer on board the DLR (Deutsche Forschungsansta lt fur Luft-und-Raumfahrt) Falcon research aircraft. Model estimates o f the water vapor mixing ratio along the flight track of the aircraft have been interpolated from 6-h ECMWF analyses. Comparisons are made b etween the in situ measurements and the model data for 16 POLINAT flig hts. In the high troposphere, the mixing ratio from the model is gener ally 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 str atosphere, the mixing ratios from the model are generally smaller than the measured ones. The observed differences in the high troposphere c an be explained by the low accuracy of radiosonde data at low temperat ures and/or at small relative humidities. In the stratosphere, due to the technical limitations of radiosonde hygrometers, the water vapor c oncentration is set to a constant value at each ECMWF analysis step. D espite these discrepancies, many of the small-scale features are, al l east in a qualitative sense, reproduced by the model.