Abstract. Accurate measurement of water vapor in the climate-sensitive region near the
tropopause is very challenging. Unexplained systematic discrepancies between
measurements at low water vapor mixing ratios made by different instruments
on airborne platforms have limited our ability to adequately address a number
of relevant scientific questions on the humidity distribution, cloud
formation and climate impact in that region. Therefore, during the past
decade, the scientific community has undertaken substantial efforts to
understand these discrepancies and improve the quality of water vapor
measurements. This study presents a comprehensive intercomparison of airborne
state-of-the-art in situ hygrometers deployed on board the DLR (German
Aerospace 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 instrument
intercomparison shows that the hygrometer measurements agree within their
combined accuracy (±10 % to 15 %, depending on the humidity regime);
total mean values agree within 2.5 %. However, systematic differences on
the order of 10 % and up to a maximum of 15 % are found for mixing
ratios below 10 parts per million (ppm) H2O. A comparison of
relative humidity within cirrus clouds does not indicate a systematic
instrument bias in either water vapor or temperature measurements in the
upper troposphere. Furthermore, in situ measurements are compared to model
data from the European Centre for Medium-Range Weather Forecasts (ECMWF)
which are interpolated along the ML-CIRRUS flight tracks. We find a mean
agreement within ±10 % throughout the troposphere and a
significant wet bias in the model on the order of 100 % to 150 % in
the stratosphere close to the tropopause. Consistent with previous studies,
this analysis indicates that the model deficit is mainly caused by too weak
of a humidity gradient at the tropopause.
Impacts of Ozone Changes in the Tropopause Layer on Stratospheric Water Vapor
