An intensified hydrological cycle in the simulation of geoengineering by cirrus cloud thinning using ice crystal fall speed changes

Abstract

Proposals to geoengineer Earth's climate by cirrus cloud thinning (CCT) potentially offer advantages over solar radiation management schemes: amplified cooling of the Arctic and smaller perturbations to global mean precipitation in particular. Using an idealized climate model implementation of CCT in which ice particle fall speeds were increased 2×, 4×, and 8× we examine the relationships between effective radiative forcing (ERF) at the top of atmosphere, near-surface temperature, and the response of the hydrological cycle. ERF was nonlinear with fall speed change and driven by the trade-off between opposing positive shortwave and negative longwave radiative forcings. ERF was À2.0 Wm À2 for both 4× and 8× fall speeds. Global mean temperature decreased linearly with ERF, while Arctic temperature reductions were amplified compared with the global mean change. The change in global mean precipitation involved a rapid adjustment (~1%/Wm 2 ), which was linear with the change in the net atmospheric energy balance, and a feedback response (~2%/°C). Global mean precipitation and evaporation increased strongly in the first year of CCT. Intensification of the hydrological cycle was promoted by intensification of the vertical overturning circulation of the atmosphere, changes in boundary layer climate favorable for evaporation, and increased energy available at the surface for evaporation (from increased net shortwave radiation and reduced subsurface storage of heat). Such intensification of the hydrological cycle is a significant side effect to the cooling of climate by CCT. Any accompanying negative cirrus cloud feedback response would implicitly increase the costs and complexity of CCT deployment.

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

“…2a) as already noted by Jackson et al (2016). This is because the cirrus CRE always decreases by about 30 % when doubling the IC sedimentation velocity, i.e.…”

“…As it is computationally demanding to simulate the detailed cirrus microphysical processes, climatic responses of seeding are often represented by increasing the IC sedimentation velocity in cirrus clouds (Muri et al, 2014;Crook et al, 2015;Jackson et al, 2016). Increasing the IC sedimentation velocity can, analogous to seeding, decrease the amount of cirrus cloud cover, ice water content (IWC), and ice crystal number concentration (ICNC).…”

Simultaneous stabilization of global temperature and precipitation through cocktail geoengineering

“…Numerous modeling studies have demonstrated the effect of stratospheric aerosol geoengineering on temperature and hydrological cycle [ National Research Council , , and references therein], which is further illustrated by the imperfect analog of volcanic eruption [ Rasch et al ., ]. Our simulated climate response to CCT geoengineering is in broad agreement with previous studies in that CCT reduces CO 2 ‐induced warming without substantially weakening the global hydrological cycle [ Muri et al ., ; Kristjánsson et al ., ; Crook et al ., ; Jackson et al ., ]. However, only a few studies have evaluated the climate effect of CCT, which merits further modeling studies preferably with improved representation of cloud microphysics [ Bardeen et al ., ].…”

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

“…Another type of studies used a prescribed increased sedimentation velocity of ice crystals as proxy for seeding (Crook et al, ; Jackson et al, ; Muri et al, ). This approach can reproduce surface climate responses to seeding but is unable to capture the changes in cloud microphysical processes that occur when considering seeding aerosol in a cirrus formation parameterization (Gasparini et al, ).…”