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

Abstract

Abstract. Accurate measurement of water vapor in the climate sensitive region near the tropopause turned out to be 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 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 20 water vapor measurements. This study presents a comprehensive intercomparison of airborne state-of-the-art in situ hygrometers deployed onboard the DLR (German Aerospace Center) research aircraft HALO during the Mid-Latitude 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 even agree within 2.5%. However, systematic differences on the order of 10% and up to a maximum of 15% are 25 found for mixing ratios below 10 parts per million (ppm) H 2 O. 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 30 the tropopause. Consistent with previous studies, this analysis indicates that the model deficit is mainly caused by a blurred humidity gradient at tropopause altitudes.Atmos. Chem. Phys. Discuss., https://doi

Pico-Light H2O: Intercomparison of in situ water vapour measurements during the AsA 2022 campaign

“…Restraining our comparison to the same altitude and volume mixing ratio ranges, the Pearson's r coefficient is of 0.975 and the associated r 2 coefficient is of 0.998, similar to those reported in Singer et al, (2022) and Kaufmann et al, (2016). In Kaufmann et al, (2018), the reported r 2 values y from 0.948 to 0.996, depending on the instrument pair considered and environmental conditions, in the 1 to 1000 ppmv range. In the range 0-7.5 km, RH values measured by iMet-4 sondes compare really well with RH values calculated from FPH and Pico-Light measurements if the T (predominantly) and P measurements used for the calculation of RH come from the IMet sonde onboard.…”

“…Considering all 6 flights, Singer et al, (2022), reported average relative difference varying from -0.4 to 1.9%. Kaufmann et al, (2018) reported intercomparisons in the UTLS between in situ aircraft hygrometers during the ML-CIRRUS campaign: FISH, HAI (Buchholz et al, 2017), SHARC and AIMS (Kaufmann et al, 2016;Thornberry et al, 2013). The relative difference between each instruments was calculated against a reference value which was calculated using the average of measurements from a combination of several of these instruments, varying upon atmospheric conditions.…”

“…Differences as large as 20% (0.8 ppmv) are found for mixing ratios below 5 ppmv, depending on the instrument pair considered and the volume mixing ratio probed. (Kaufmann et al, 2018) reported stratospheric comparisons of FISH and AIMS to an average reference value calculated from measurements from AIMS, FISH, SHARC and HAI. Measurements have been performed onboard the HALO aircraft.…”