Intercomparisons of stratospheric water vapor sensors: FLASH-B and NOAA/CMDL frost-point hygrometer

Studies of global climate rely critically on accurate water vapor measurements. In this paper, a comparison of the NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL) frost-point hygrometer and the Fluorescent Advanced Stratospheric Hygrometer for Balloon (FLASH-B) Lyman-alpha hygrometer is reported. Both instruments were part of a small balloon payload that was launched multiple times at Sodankylä, Finland. The comparison shows agreement well within the instrumental uncertainties between both sensors in the Arctic stratospheric vortex. The mean deviation between both instruments in the range between 15 and 25 km is -2.4% ± 3.1% (one standard deviation). The comparison identified some instrumental issues, such as a low mirror-temperature calibration correction for the NOAA/CMDL frost-point hygrometer as well as a time lag. It was found that the FLASH-B hygrometer measures water vapor reliably above 7 km in the polar atmosphere. Comparisons in the upper troposphere are affected by the gain change of the NOAA/CMDL hygrometer, causing a lag and a wet bias in the tropospheric low gain setting under the dry conditions in the upper troposphere.