Upper tropospheric ice sensitivity to sulfate geoengineering

Daniele Visioni, Giovanni Pitari, Glauco di Genova, Simone Tilmes, Irene Cionni
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Aside from the direct surface cooling that sulfate geoengineering (SG) would produce, investigations of the possible side effects of this method are still ongoing, such as the exploration of the effect that SG may have on upper tropospheric cirrus cloudiness. The goal of the present study is to better understand the SG thermodynamical effects on the freezing mechanisms leading to ice particle formation. This is undertaken by comparing SG model simulations against a
more » ... mulations against a Representative Concentration Pathway 4.5 (RCP4.5) reference case. In the first case, the aerosol-driven surface cooling is included and coupled to the stratospheric warming resulting from the aerosol absorption of terrestrial and solar near-infrared radiation. In a second SG perturbed case, the surface temperatures are kept unchanged with respect to the reference RCP4.5 case. When combined, surface cooling and lower stratospheric warming tend to stabilize the atmosphere, which decreases the turbulence and updraft velocities (<span class="inline-formula">−10</span><span class="thinspace"></span>% in our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative forcing (RF). This RF would go in the same direction as the direct effect of solar radiation scattering by aerosols, and would consequently influence the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given an 8<span class="thinspace"></span>Tg-<span class="inline-formula">SO<sub>2</sub></span><span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span> equatorial injection into the lower stratosphere, an all-sky net tropopause RF of <span class="inline-formula">−1.46</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span> is calculated, of which <span class="inline-formula">−0.3</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span> (20<span class="thinspace"></span>%) is from the indirect effect on cirrus thinning (6<span class="thinspace"></span>% reduction in ice optical depth). When surface cooling is ignored, the ice optical depth reduction is lowered to 3<span class="thinspace"></span>%, with an all-sky net tropopause RF of <span class="inline-formula">−1.4</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>, of which <span class="inline-formula">−0.14</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span> (10<span class="thinspace"></span>%) is from cirrus thinning. Relative to the clear-sky net tropopause RF due to SG aerosols (<span class="inline-formula">−2.1</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>), the cumulative effect of the background clouds and cirrus thinning accounts for <span class="inline-formula">+0.6</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>, due to the partial compensation of large positive shortwave (<span class="inline-formula">+1.6</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>) and negative longwave adjustments (<span class="inline-formula">−1.0</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>). When surface cooling is ignored, the net cloud adjustment becomes <span class="inline-formula">+0.8</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>, with the shortwave contribution (<span class="inline-formula">+1.5</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>) almost twice as much as that of the longwave (<span class="inline-formula">−0.7</span><span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>). This highlights the importance of including all of the dynamical feedbacks of SG aerosols.</p>
doi:10.5194/acp-18-14867-2018 fatcat:vyqpgnhjmve5pjoiktkamsc2eq