Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-1109
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
14 Dec 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
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 Institute for Atmospheric and Climate Science, ETH Zürich
Abstract. The complex microphysical details of cirrus seeding with ice nucleating particles (INP) 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 INP. We compare simulations where the ice crystal sedimentation velocity is increased at temperatures colder than −35 °C with simulations of cirrus seeding with INP 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 INP 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, on the contrary, lead to counteracting mixed-phase cloud responses. The increased sedimentation velocity simulations induce a 30 % larger surface temperature response, due to their lower altitude of maximum diabatic forcing compared with simulations of seeding with INP particles. They can counteract up to 60 % of the radiative effect of CO2 doubling with a maximum net top-of-the-atmosphere forcing of  2.2 W m−2.

Citation: Gasparini, B., Münch, S., Poncet, L., Feldmann, M., and Lohmann, U.: Is increasing ice crystal sedimentation velocity in geoengineering simulations a good proxy for cirrus cloud seeding?, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1109, in review, 2016.
Blaž Gasparini et al.
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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RC1: 'Review', Anonymous Referee #1, 10 Jan 2017 Printer-friendly Version Supplement 
 
RC2: 'Review of Gasparini et al.', Anonymous Referee #2, 12 Jan 2017 Printer-friendly Version 
 
RC3: 'Review of Gasparini et al.', Anonymous Referee #3, 17 Jan 2017 Printer-friendly Version 
 
AC1: 'Author's response', Blaz Gasparini, 23 Mar 2017 Printer-friendly Version Supplement 
Blaž Gasparini et al.
Blaž Gasparini et al.

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Short summary
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 of clouds and cloud properties differ significantly.
Cirrus clouds have, unlike other cloud types, a warming impact on climate. Decreasing their...
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