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

Aug 27, 2026

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Accurate water-vapor measurements in the climate-sensitive region near the tropopause remain difficult because different airborne instruments show unexplained systematic discrepancies at low mixing ratios, limiting understanding of humidity distribution, cloud formation, and climate effects. This study intercompares advanced in situ hygrometers aboard the DLR HALO aircraft during the 2014 ML-CIRRUS campaign over central Europe, finding agreement within combined accuracy (±10–15%) and mean values within 2.5%, but persistent 10–15% differences below 10 ppm. Model comparisons show tropospheric agreement within ±10% and a wet stratospheric bias of 100–150% near the tropopause, attributed to a blurred humidity gradient.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages30

Reading Time1 h 16 min

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Abstract

Introduction

Atmos. Chem. Phys. Discuss., [https://doi.org/10.5194/acp-2018-744](https://doi.org/10.5194/acp-2018-744)[ ](https://doi.org/10.5194/acp-2018-744)Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 24 July 2018 􀀍c Author(s) 2018 . CC BY 4.0 License. Intercomparison of mid-latitude tropospheric and lower stratospheric water vapor measurements and comparison to ECMWF humidity data Stefan Kaufmann 1, Christiane Voigt 1,2, Romy Heller 1, Tina Jurkat-Witschas 1, Martina Krämer3, 5 Christian Rolf3, Martin Zöger4, Andreas Giez4, Bernhard Buchholz5, Volker Ebert5, Troy Thornberry6,7, Ulrich Schumann 1 1Deutsches Zentrum für Luft-und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, 82234, Germany 2Johannes Gutenberg-Universität, Institut für Physik der Atmosphäre, Mainz, 55128, Germany 3Forschungszentrum Jülich, Institute for Energy and Climate Research (IEK-7), Jülich, 52428, Germany 10 4Deutsches Zentrum für Luft-und Raumfahrt, Flight Experiments, Oberpfaffenhofen, 822234, Germany 5Physikalisch-Technische Bundesanstalt Braunschweig, Braunschweig, 38116, Germany 6NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA 7Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA 15 Correspondence to: Stefan Kaufmann ([Stefan.Kaufmann@dlr.de](Stefan.Kaufmann@dlr.de)) 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, 20 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 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 25 mean values even 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% 30 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 a blurred humidity gradient at tropopause altitudes. Atmos. Chem. Phys. Discuss., [https://doi.org/10.5194/acp-2018-744](https://doi.org/10.5194/acp-2018-744)[ ](https://doi.org/10.5194/acp-2018-744)Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 24 July 2018 􀀍c Author(s) 2018 . CC BY 4.0 License. 1. Introduction Water vapor is one of the most important trace gases in Earth’s atmosphere due to its large influence on the radiation budget and atmospheric dynamics. It absorbs and emits infrared radiation throughout the entire profile of the atmosphere (Kiehl and Trenberth, 1997) . The radiative effect of small changes in water vapo

FAQs

Why is measuring water vapor near the tropopause challenging?

Small absolute changes in humidity strongly affect UTLS radiation and dynamics, and different airborne instruments show unexplained systematic discrepancies at low mixing ratios.

How well do the in situ hygrometers agree in the ML-CIRRUS campaign?

Instrument measurements agree within combined accuracy (±10–15%, depending on humidity regime) and total mean values agree within 2.5%, while systematic differences of about 10% up to 15% occur below 10 ppm.

How do ECMWF model humidity results compare with the in situ measurements?

Along the ML-CIRRUS flight tracks, the troposphere shows mean agreement within ±10%, but the model has a wet bias in the stratosphere of about 100–150% close to the tropopause, linked to a blurred humidity gradient at tropopause altitudes.

Aug 27, 2026

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