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

Abstract

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 CO 2 doubling with a maximum net top-ofthe-atmosphere forcing of −2.2 W m −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.

A Process Study on Thinning of Arctic Winter Cirrus Clouds With High‐Resolution ICON‐ART Simulations

“…Besides two scenarios, where seeding is done either everywhere possible (SEED) or only where necessary to suppress homogeneous nucleation (TRGT), in TRGT2 (see Table 1), the seeding aerosol acts as IN only at high altitudes, that is, with T < 235 K. This setup is close to the assumptions made in former studies (e.g., Gasparini et al, 2017;.…”

“…However, during Arctic winter, characterized by low temperatures and lack of incoming solar radiation, convective activity is low (Figure 12a) and the troposphere is stably stratified (Figure 12b). In contrast to Gasparini et al (2017), in the upper troposphere, convective activity in both SEED and TRGT is reduced by up to one third (Figure 12a) and stability is slightly increased (Figure 12b). This hints at a self-enhancing feedback mechanism.…”

“…Furthermore, it should be nontoxic and affordable. Several studies assumed seeding scenarios with such a perfect substance (e.g., Gasparini et al, 2017;, hence disregarding methods of deployment but rather assuming that the aerosol particles were in place in necessary concentrations. Suitable agents could be Bismuth tri-iodide (BiI 3 ; Mitchell & Finnegan, 2009) and mineral dust .…”

“…For all scenarios except REF, temporally constant and horizontally homogeneous seeding aerosol concentrations are prescribed following the profile shown in Figure 6. In contrast to former studies (e.g., Gasparini et al, 2017;, the seeding aerosol can also form ice at temperatures above 235 K. Hence, in addition to SEED, where seeding can possibly occur at all temperatures below freezing level, results of two highly idealized, more targeted strategies are examined. In TRGT, the same concentrations and aerosol sizes as in SEED are assumed to be in place only where homogeneous nucleation would occur, thus avoiding formation of artificial cirrus clouds in supersaturated but cloud-free regions and theoretically achieving the maximum possible elimination of cirrus clouds.…”

“…Cirrus seeding in former studies was performed in a straightforward manner, by either prescribing an increased sedimentation as proxy (e.g., Muri et al, 2014) or assuming a "perfect" seeding scenario with regard to the ice nucleation properties of the INP, the concentrations, and the injection into altitudes most susceptible for seeding (e.g., Gasparini et al, 2017;Storelvmo et al, 2013).…”