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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-176
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 May 2020

29 May 2020

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This preprint is currently under review for the journal AMT.

Understanding cryogenic frost point hygrometer measurements after contamination by mixed-phase clouds

Teresa Jorge1, Simone Brunamonti1, Yann Poltera1, Frank G. Wienhold1, Bei P. Luo1, Peter Oelsner2, Sreeharsha Hanumanthu3, Bhupendra B. Sing4,5, Susanne Körner2, Ruud Dirksen2, Manish Naja6, Suvarna Fadnavis4, and Thomas Peter1 Teresa Jorge et al.
  • 1Institute of Atmospheric and Climate Science, ETH Zürich
  • 2Deutscher Wetterdienst (DWD)/GCOS Reference Upper Air Network (GRUAN) Lead Center, Lindenberg, Germany
  • 3Forschungzentrum Jülich (FZJ), Institute of Energy and Climate Research, Stratosphere (IEK-7), Jülich, Germany
  • 4Indian Institute of Tropical Meteorology (IITM), Pune, India
  • 5Department of Geophysics, Banaras Hindu University, Varanasi, India
  • 6Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India

Abstract. Balloon-borne water vapour measurements in the (sub)tropical upper troposphere and lower stratosphere (UTLS) by means of frost point hygrometers provide important information on air chemistry and climate. However, the risk of contamination from sublimating hydrometeors collected by the intake tube may render these measurements difficult, particularly after crossing low clouds containing supercooled droplets. A large set of measurements during the 2016–2017 StratoClim balloon campaigns at the southern slopes of the Himalayas allows us to perform an in-depth analysis of this type of contamination. We investigate the efficiency of wall-contact and freezing of supercooled droplets in the intake tube and the subsequent sublimation in the UTLS using Computational Fluid Dynamics (CFD). We find that the airflow can enter the intake tube with impingement angles up to 60°, owing to the pendulum motion of the payload. Supercooled droplets with radii > 70 μm, as they frequently occur in mid-tropospheric clouds, typically undergo contact freezing when entering the intake tube, whereas only about 50 % of droplets with 10 μm radius freeze, and droplets < 5 μm radius mostly avoid contact. According to CFD, sublimation of water from an icy intake can account for the occasionally observed high water vapour mixing ratios (χ(H2O) > 100 ppmv) in the stratosphere. Furthermore, we use CFD to differentiate between stratospheric water vapour contamination by an icy intake tube and contamination caused by outgassing from the balloon and payload, revealing that the latter starts playing a role only at high altitudes (p < 20 hPa).

Teresa Jorge et al.

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Understanding cryogenic frost point hygrometers measurements after contamination by mixed-phase clouds - supplementary material Jorge, Teresa https://doi.org/10.3929/ethz-b-000411719

Teresa Jorge et al.

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Short summary
Balloon-borne frost point hygrometers are crucial for the monitoring of water vapour in the upper troposphere and lower stratosphere. We found that when traversing a mixed-phase cloud with big supercooled droplets, the intake tube of the instrument collects on its inner surface a high percentage of these droplets. The newly formed ice layer will sublimate at higher levels and contaminate the measurement. The balloon and instrument package are also investigated as source of the contamination.
Balloon-borne frost point hygrometers are crucial for the monitoring of water vapour in the...
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