Intercomparison of mid-latitude tropospheric and lower stratospheric water vapor measurements and comparison to ECMWF humidity data

Abstract. Accurate measurement of water vapor in the climate-sensitive region near thetropopause is very challenging. Unexplained systematic discrepancies betweenmeasurements at low water vapor mixing ratios made by different instrumentson airborne platforms have limited our ability to adequately address a numberof relevant scientific questions on the humidity distribution, cloudformation and climate impact in that region. Therefore, during the pastdecade, the scientific community has undertaken substantial efforts tounderstand these discrepancies and improve the quality of water vapormeasurements. This study presents a comprehensive intercomparison of airbornestate-of-the-art in situ hygrometers deployed on board the DLR (GermanAerospace Center) research aircraft HALO (High Altitude and LOng Range Research Aircraft) during the Midlatitude CIRRUS(ML-CIRRUS) campaign conducted in 2014 over central Europe. The instrumentintercomparison shows that the hygrometer measurements agree within theircombined accuracy ( ± 10 % to 15 %, depending on the humidity regime);total mean values agree within 2.5 %. However, systematic differences onthe order of 10 % and up to a maximum of 15 % are found for mixingratios below 10 parts per million (ppm) H 2 O . A comparison ofrelative humidity within cirrus clouds does not indicate a systematicinstrument bias in either water vapor or temperature measurements in theupper troposphere. Furthermore, in situ measurements are compared to modeldata from the European Centre for Medium-Range Weather Forecasts (ECMWF)which are interpolated along the ML-CIRRUS flight tracks. We find a meanagreement within ± 10 % throughout the troposphere and asignificant wet bias in the model on the order of 100 % to 150 % inthe stratosphere close to the tropopause. Consistent with previous studies,this analysis indicates that the model deficit is mainly caused by too weakof a humidity gradient at the tropopause.