Upper tropospheric ice sensitivity to sulfate geoengineering

Abstract

Abstract. Aside from the direct surface cooling sulfate geoengineering (SG) would produce, the investigation on possible sideeffects of this method is still ongoing, as for instance on upper tropospheric cirrus cloudiness. Goal of the present study is to better understand the SG thermo-dynamical effects on the homogeneous freezing ice formation process. This is done by comparing SG model simulations against a RCP4.5 reference case: in one case the aerosol-driven surface cooling is included and coupled to the stratospheric warming resulting from aerosol absorption of longwave radiation. In a second SG perturbed case, 5 surface temperatures are kept unchanged with respect to the reference RCP4.5 case. Surface cooling and lower stratospheric warming, together, tend to stabilize the atmosphere, thus decreasing turbulence and water vapor updraft velocities (-10% in our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative forcing (RF). This would go in the same direction as the direct effect of solar radiation scattering by the aerosols, thus influencing the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given a 8 Tg-SO 2 10 equatorial injection in the lower stratosphere, an all-sky net tropopause RF of -2.13 W/m 2 is calculated, of which -0.96 W/m 2 (45%) from the indirect effect on cirrus thinning (7.5% reduction in ice optical depth). When the surface cooling is ignored, the ice optical depth reduction is lowered to 5%, with an all-sky net tropopause RF of -1.45 W/m ). This highlights the importance of including all dynamical feedbacks of SG aerosols.

A statistical examination of the effects of stratospheric sulfate geoengineering on tropical storm genesis

“…This may be expected to be the dominant process controlling the SAI-induced changes in atmospheric static stability. Furthermore, recent work (Visioni et al, 2018) explores the surface cooling impact on uppertropospheric cirrus cloud formation, as well as the concomitant impact on static stability. Surface cooling and lower stratospheric warming, together, tend to stabilize the atmosphere, thus decreasing turbulence and updraft velocities.…”

“…The signal-to-noise ratio of the G4 experiment is not as large as that of G1 where solar dimming offsets quadrupled CO 2 concentrations. It is, however, more interesting for TC studies because the sulfate aerosol injected into the stratosphere causes radiative heating (Pitari et al, 2014) and other indirect effects on the upper troposphere (Visioni et al, 2018) that will potentially affect the deep tropospheric convention systems that characterize intense tropical storms.…”

Changes in clouds and thermodynamics under solar geoengineering and implications for required solar reduction

“…On the other hand, a reduction in low clouds over the ocean would make it more difficult to do marine cloud brightening at the same time as other forms of solar geoengineering. Changes in cirrus clouds are also relevant in the context of research on the effects of sedimentation of injected stratospheric aerosols on high clouds (Kuebbeler et al, 2012;Visioni et al, 2018) and proposals to intentionally thin cirrus clouds with nucleation-inducing aerosols in order to cool the earth by increased LW emission (Mitchell and Finnegan, 2009). The increase in high clouds in most models in G1 indicates that thermodynamic and radiative adjustments to the forcing scenario can have effects on high clouds that may counteract unintentional or intentional microphysical effects.…”