Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 13833-13848, 2017
https://doi.org/10.5194/acp-17-13833-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
21 Nov 2017
Sensitivity of surface temperature to radiative forcing by contrail cirrus in a radiative-mixing model
Ulrich Schumann1 and Bernhard Mayer2 1Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
2Ludwig-Maximilians-Universität München, Lehrstuhl für Experimentelle Meteorologie, Munich, Germany
Abstract. Earth's surface temperature sensitivity to radiative forcing (RF) by contrail cirrus and the related RF efficacy relative to CO2 are investigated in a one-dimensional idealized model of the atmosphere. The model includes energy transport by shortwave (SW) and longwave (LW) radiation and by mixing in an otherwise fixed reference atmosphere (no other feedbacks). Mixing includes convective adjustment and turbulent diffusion, where the latter is related to the vertical component of mixing by large-scale eddies. The conceptual study shows that the surface temperature sensitivity to given contrail RF depends strongly on the timescales of energy transport by mixing and radiation. The timescales are derived for steady layered heating (ghost forcing) and for a transient contrail cirrus case. The radiative timescales are shortest at the surface and shorter in the troposphere than in the mid-stratosphere. Without mixing, a large part of the energy induced into the upper troposphere by radiation due to contrails or similar disturbances gets lost to space before it can contribute to surface warming. Because of the different radiative forcing at the surface and at top of atmosphere (TOA) and different radiative heating rate profiles in the troposphere, the local surface temperature sensitivity to stratosphere-adjusted RF is larger for SW than for LW contrail forcing. Without mixing, the surface energy budget is more important for surface warming than the TOA budget. Hence, surface warming by contrails is smaller than suggested by the net RF at TOA. For zero mixing, cooling by contrails cannot be excluded. This may in part explain low efficacy values for contrails found in previous global circulation model studies. Possible implications of this study are discussed. Since the results of this study are model dependent, they should be tested with a comprehensive climate model in the future.

Citation: Schumann, U. and Mayer, B.: Sensitivity of surface temperature to radiative forcing by contrail cirrus in a radiative-mixing model, Atmos. Chem. Phys., 17, 13833-13848, https://doi.org/10.5194/acp-17-13833-2017, 2017.
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Short summary
It is generally assumed that a positive radiative forcing of the atmosphere implies a warming of the Earth surface. This assumption is valid for well-mixed greenhouse gases but is not guaranteed for disturbances which cause a vertically variable radiative heating rate profile with warming in the upper troposphere and cooling near the surface. This conceptual study shows that the warming induced by contrail cirrus prevails only for fast vertical heat exchange by mixing within the troposphere.
It is generally assumed that a positive radiative forcing of the atmosphere implies a warming of...
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