Abstract
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.
Highlights
Contrails are similar to upper-tropospheric ice clouds, which tend to warm the troposphere by reducing outgoing longwave (LW) terrestrial radiation and cool it by enhancing shortwave (SW) solar radiation backscattering (Stephens and Webster, 1981; Liou, 1986; Sinha and Shine, 1994; Chen et al, 2000; Schumann and Heymsfield, 2017)
One simulation is run for a flux change in the lowest model layer above the surface, and 10 for flux changes in subsequent 100 hPa pressure intervals between the surface and top of atmosphere (TOA)
We find that the flux in equilibrium over a constant-temperature surface is in between the initial instantaneous flux values at the tropopause and at the surface
Summary
Contrails are similar to upper-tropospheric ice clouds (cirrus), which tend to warm the troposphere by reducing outgoing longwave (LW) terrestrial radiation and cool it by enhancing shortwave (SW) solar radiation backscattering (Stephens and Webster, 1981; Liou, 1986; Sinha and Shine, 1994; Chen et al, 2000; Schumann and Heymsfield, 2017). The contrail climate effects are expensive to compute because they are small compared to the interannual variability (“climate noise”) in climate models (Ponater et al, 1996; Hansen et al, 1997b), so most studies used increased disturbances by a factor of 10 to 100 All these model studies suggest a mean global warming from contrails. Horizontal advection and downward mixing before getting lost to space by radiation In this conceptual study, we investigate changes in temperature from additional thin cirrus (contrails) at midlatitudes in a radiative-mixing model where the vertical mixing may result from deep convection, from the large-scale circulation, and from turbulent diffusion.
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