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Discussion papers
https://doi.org/10.5194/acp-2019-735
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-735
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Ice-supersaturated air masses in the northern mid-latitudes from regular in-situ observations by passenger aircraft: vertical distribution, seasonality and tropospheric fingerprint

Andreas Petzold1, Patrick Neis1,3,a, Mihal Rütimann1, Susanne Rohs1, Florian Berkes1,b, Herman G. J. Smit1, Martina Krämer2,3, Nicole Spelten2, Peter Spichtinger3, Philippe Nedelec4, and Andreas Wahner1 Andreas Petzold et al.
  • 1Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research 8 – Troposphere, Jülich, Germany
  • 2Forschungszentrum Jülich GmbH, Institute of Energy and ClimateResearch 7 – Stratosphere, Jülich, Germany
  • 3Johannes Gutenberg University, Institute for Atmospheric Physics, Mainz, Germany
  • 4CNRS Laboratoire d'Aérologie, and Université Paul Sabatier Toulouse III, Toulouse, France
  • anow at: CGI Deutschland B.V. & CO. KG, Frankfurt, Germany
  • bnow at: P3 solutions GmbH, Aachen, Germany

Abstract. The vertical distribution and seasonal variation of upper tropospheric humidity (UTH) and particularly of ice-supersaturated air masses in the extratropical upper troposphere and lowermost stratosphere (Ex-UTLS) is investigated at northern mid-latitudes over the regions Eastern North America, the North Atlantic and Europe for the period 1995 to 2010. Observation data originate from regular and continuous long-term measurements of water vapour volume mixing ratio (H2O VMR), temperature and relative humidity with respect to ice (RHice) by instrumented passenger aircraft in the framework of the European research program MOZAIC (1994–2010) which is continued as European research infrastructure IAGOS (from 2011). The in-situ observations of UTH with a vertical resolution of 30 hPa (< 800 m at tropopause level) and a horizontal resolution of 1 km resolve detailed features of the distribution of water vapour and ice-supersaturated air relative to the thermal tropopause, including their seasonal and regional variability and chemical signatures at various distances from the tropopause layer. Annual cycles of the vertical distribution of UTH over the investigated regions demonstrate annually increasing H2O VMR above the thermal tropopause in the summer months, but without an associated increase in RHice. Over all investigated regions, upper tropospheric air masses close to the tropopause level are nearly saturated with respect to ice and contain a significant fraction of ice-supersaturated regions (ISSR) with a distinct seasonal cycle of minimum values in summer (30 % over the ocean, 20–25 % over land), and maximum values in late winter (35–40 % over both land and ocean). Above the thermal tropopause, ISSR are occasionally observed with an occurrence probability of 1.5 ± 1.1 %, whereas above the dynamical tropopause at 2 PVU, the occurrence probability increases 4-fold to 8.4 ± 4.4 %. In both tropopause-height (TPH) related coordinate systems, the ISSR occurrence probabilities drop to values below 1 % for the next higher air mass layer with pressure levels p < pTPH − 15 hPa. For both tropopause definitions, the tropospheric nature or fingerprint, respectively, based on O3 VMR, indicate continuing dominant tropospheric influence on ISSR inside and above the respective tropopause layer. For the non-ISSR, however, the stratospheric nature is clearly visible above the thermal tropopause whereas above the dynamical tropopause the air masses show still a relevant tropospheric influence. For all three regions, seasonal deviations from the long-term annual cycle of ISSR occurrence show no significant trends over the observation period of 15 years, whereas weak but significant dependencies of ISSR occurrence on the North Atlantic Oscillation (NAO) index are observed.

Andreas Petzold et al.
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IAGOS Data Portal D. Boulanger https://doi.org/10.25326/20

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Short summary
The first analysis of 15 years of global-scale water vapour and relative humidity observations by passenger aircraft in the MOZAIC programme resolves detailed features of water vapour and ice-supersaturated air in the mid-latitude tropopause. Key results provide in-depth insight into seasonal and regional variability and chemical signatures of ice-supersaturated air masses, including trend analyses, and a close link to cirrus clouds and their highly important effects on climate.
The first analysis of 15 years of global-scale water vapour and relative humidity observations...
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