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
Received: 08 Dec 2016 – Accepted for review: 12 Dec 2016 – Discussion started: 14 Dec 2016
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.
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.