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Volume 17, issue 7 | Copyright

Special issue: The Geoengineering Model Intercomparison Project (GeoMIP):...

Atmos. Chem. Phys., 17, 4871-4885, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 13 Apr 2017

Research article | 13 Apr 2017

Is increasing ice crystal sedimentation velocity in geoengineering simulations a good proxy for cirrus cloud seeding?

Blaž Gasparini, Steffen Münch, Laure Poncet, Monika Feldmann, and Ulrike Lohmann Blaž Gasparini et al.
  • Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

Abstract. The complex microphysical details of cirrus seeding with ice nucleating particles (INPs) in numerical simulations are often mimicked by increasing ice crystal sedimentation velocities. So far it has not been tested whether these results are comparable to geoengineering simulations in which cirrus clouds are seeded with INPs. We compare simulations where the ice crystal sedimentation velocity is increased at temperatures colder than −35°C with simulations of cirrus seeding with INPs using the ECHAM-HAM general circulation model. The radiative flux response of the two methods shows a similar behaviour in terms of annual and seasonal averages. Both methods decrease surface temperature but increase precipitation in response to a decreased atmospheric stability. Moreover, simulations of seeding with INPs lead to a decrease in liquid clouds, which counteracts part of the cooling due to changes in cirrus clouds. The liquid cloud response is largely avoided in a simulation where seeding occurs during night only. Simulations with increased ice crystal sedimentation velocity, however, lead to counteracting mixed-phase cloud responses. The increased sedimentation velocity simulations can counteract up to 60% of the radiative effect of CO2 doubling with a maximum net top-of-the-atmosphere forcing of −2. 2Wm−2. They induce a 30% larger surface temperature response, due to their lower altitude of maximum diabatic forcing compared with simulations of seeding with INPs.

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Cirrus clouds have, unlike other cloud types, a warming impact on climate. Decreasing their frequency therefore leads to a cooling effect. Cirrus ice crystals grow larger when formed on solid aerosols, inducing a shorter cloud lifetime. We compare simplified simulations of stripping cirrus out of the sky with simulations of seeding by aerosol injections. While we find the surface climate responses to be similar, the changes in clouds and cloud properties differ significantly.
Cirrus clouds have, unlike other cloud types, a warming impact on climate. Decreasing their...