After a volcanic eruption, together with SO2, there’s also a large amount of 
ash injected in the upper troposphere that might favor heterogeneous nucleation 
against homogeneous. This is mostly the reason why after volcanic eruption a 
slight increase in citrus coverage is found.
On the other hand in a geoengineering simulation, the lack of this effect 
combined with a dynamical response of a continuous injection would produce a 
difference response.

We briefly discuss in the conclusion of our study the differences between an 
eruption and a SG scenario.

////////////////////////////////////////////////////////////
Daniele Visioni
PhD Student 
Dipartimento di Scienze Fisiche e Chimiche, Universita' dell'Aquila     
Via Vetoio, 67100 - Coppito, L'AQUILA
e-mail: daniele.visi...@aquila.infn.it
////////////////////////////////////////////////////////////


> On 13 Feb 2018, at 04:54, Andrew Lockley <andrew.lock...@gmail.com> wrote:
> 
> Hasn't observational data from Pinatubo constrained this variable reasonably 
> well? It would be surprising if there was "hidden" cooling of such magnitude. 
> This would also presumably apply to Tambora, etc. which have left temperature 
> (measured and proxy) and ash records.
> 
> A
> 
>> On 5 Feb 2018 18:28, "Daniele Visioni" <daniele.visi...@aquila.infn.it> 
>> wrote:
>> Hi Andrew, thank you.
>> 
>> No, you did not misunderstand our paper. If by particle rain-out you are 
>> referring to sulfate particles settling from the stratosphere to the upper 
>> troposphere and thus affecting freezing
>> by increasing the number of available IN for homogeneous freezing, what we 
>> found, in agreement with Cirisan et al. (2013), is that this is a negligible 
>> effect (some mW/m^2), expecially compared to the thermo-dynamical response 
>> that we show in our paper.
>> 
>> Best,
>> Daniele
>> 
>> ////////////////////////////////////////////////////////////
>> Daniele Visioni
>> PhD Student 
>> Dipartimento di Scienze Fisiche e Chimiche, Universita' dell'Aquila  
>> Via Vetoio, 67100 - Coppito, L'AQUILA
>> e-mail: daniele.visi...@aquila.infn.it
>> Check out our latest published paper:
>> https://www.atmos-chem-phys.net/17/11209/2017/acp-17-11209-2017.html
>> ////////////////////////////////////////////////////////////
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>>> On 5 Feb 2018, at 10:42, Andrew Lockley <andrew.lock...@gmail.com> wrote:
>>> 
>>> Poster's note: this is very important paper, as it constrains a key 
>>> side-effect of SAI. I may misunderstand the paper, but I don't think it's 
>>> looking at particle rain-out - which may provide a further mechanism
>>> 
>>> Upper tropospheric ice sensitivity to sulfate geoengineering
>>> Daniele Visioni1,2, Giovanni Pitari1, and Glauco di Genova2
>>> 1Department of Physical and Chemical Sciences, Universitá dell'Aquila, 
>>> 67100 L'Aquila, Italy
>>> 2CETEMPS, Universitá dell'Aquila, 67100 L'Aquila, Italy
>>> Received: 30 Jan 2018 – Accepted for review: 02 Feb 2018 – Discussion 
>>> started: 05 Feb 2018
>>> Abstract. Aside from the direct surface cooling sulfate geoengineering (SG) 
>>> would produce, the investigation on possible side-effects 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, 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-SO2 equatorial injection in the lower stratosphere, an all-sky net 
>>> tropopause RF of −2.13 W/m2 is calculated, of which −0.96 W/m2 (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/m2, of which −0.21 W/m2 (14 %) from cirrus thinning. Relatively to the 
>>> clear-sky net tropopause RF due to SG aerosols (−2.06 W/m2), the cumulative 
>>> effect of background clouds and cirrus thinning accounts for −0.07 W/m2, 
>>> due to close compensation of large positive shortwave (+1.85 W/m2) and 
>>> negative longwave adjustments (−1.92 W/m2). When the surface cooling is 
>>> ignored, the net cloud adjustment becomes +0.71 W/m2, with the shortwave 
>>> contribution (+1.97 W/m2) significantly larger in magnitude than the 
>>> longwave one (−1.26 W/m2). This highlights the importance of including all 
>>> dynamical feedbacks of SG aerosols.
>>> 
>>> 
>>> Citation: Visioni, D., Pitari, G., and di Genova, G.: Upper tropospheric 
>>> ice sensitivity to sulfate geoengineering, Atmos. Chem. Phys. Discuss., 
>>> https://doi.org/10.5194/acp-2018-107, in review, 2018.
>>> 
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