Royal Netherlands Meteorological Institute
Section of Atmospheric Composition
During the POLINAT experiment, devoted to the study of the pollution from aircraft in the North Atlantic flight corridor, water vapour content was measured with a frost-point hygrometer on board of the DLR Falcon research aircraft. Model estimates of the water vapour mixing ratio along the flight track of the aircraft have been interpolated from six-hourly ECMWF analyses. Comparisons are made between the in situ measurements and the model data for all 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 % 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 or/and at small relative humidities. In the stratosphere, due to the technical limitations of radiosonde hygrometers, the water vapour 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 qualitative sense, reproduced by the model.
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