Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[Andrew Lockley]

But Daniele already constrained the seeding effect and it's swamped by the
stratification. He also explained that you can't necessarily map impulse
sources with continuous or near continuous sources.

This is perhaps one of the most important discussions in the field today.
Not least it has potential to affect aerosol coalescence, toxic rain out,
and also sky effects

A

On 15 Feb 2018 15:08, "Alan Robock"  wrote:

> I would completely agree with what Blaz wrote.
>
> Alan
> _
> Alan Robock, Distinguished Professor
>   Editor, Reviews of Geophysics
> Department of Environmental Sciences Phone: +1-848-932-5751 
> <(848)%20932-5751>
> Rutgers University Fax: +1-732-932-8644 
> <(732)%20932-8644>14 College Farm Road  E 
> -mail:
>  rob...@envsci.rutgers.edu
> New Brunswick, NJ 08901-8551  USA http://envsci.rutgers.edu/~robock
> ☮  http://twitter.com/AlanRobock2017 Nobel Peace Prize to ICAN!
> Watch my 18 min TEDx talk at http://www.youtube.com/watch?v=qsrEk1oZ-54
>
> On 2/15/18 7:08 AM, blaz gasparini wrote:
>
> Hi all,
>
> *On Tuesday, February 13, 2018 at 3:21:28 PM UTC+1, dvisioni wrote:*
>>
>> *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.*
>>
>
> I don't think there is any scientific agreement on the response of cirrus
> to the volcanic eruptions. Neither from observations (e.g. Meyer et al.
> 2015 ), nor
> from the model side.
> I wouldn't expect the ash to make a significant impact on the global
> scale, as it falls out out the atmosphere too quickly to significantly
> change the cirrus radiative forcing (looking
> from the global modeller's perspective)
>
> (+don't forget that a shift from homogeneous to heterogeneous freezing
> could additionally thin the cirrus and decrease further their warming
> effect, i.e. cirrus seeding effect)
>
> Best,
> Blaz
>
>>
>>
>> On 13 Feb 2018, at 04:54, Andrew Lockley  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"  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...@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  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
>>

Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[Alan Robock]

I would completely agree with what Blaz wrote.

Alan
_
Alan Robock, Distinguished Professor
  Editor, Reviews of Geophysics
Department of Environmental Sciences Phone: +1-848-932-5751
Rutgers University Fax: +1-732-932-8644
14 College Farm Road  E-mail: rob...@envsci.rutgers.edu
New Brunswick, NJ 08901-8551  USA http://envsci.rutgers.edu/~robock
☮  http://twitter.com/AlanRobock2017 Nobel Peace Prize to ICAN!
Watch my 18 min TEDx talk at http://www.youtube.com/watch?v=qsrEk1oZ-54

On 2/15/18 7:08 AM, blaz gasparini wrote:

Hi all,

/On Tuesday, February 13, 2018 at 3:21:28 PM UTC+1, dvisioni wrote:/

/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./


I don't think there is any scientific agreement on the response of 
cirrus to the volcanic eruptions. Neither from observations (e.g. 
Meyer et al. 2015 
), nor 
from the model side.
I wouldn't expect the ash to make a significant impact on the global 
scale, as it falls out out the atmosphere too quickly to significantly 
change the cirrus radiative forcing (looking

from the global modeller's perspective)

(+don't forget that a shift from homogeneous to heterogeneous freezing 
could additionally thin the cirrus and decrease further their warming 
effect, i.e. cirrus seeding effect)

Best,
Blaz



On 13 Feb 2018, at 04:54, Andrew Lockley > 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" > 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...@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
> 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 % 

Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[blaz gasparini]

Hi all,

*On Tuesday, February 13, 2018 at 3:21:28 PM UTC+1, dvisioni wrote:*
>
> *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.*
>

I don't think there is any scientific agreement on the response of cirrus 
to the volcanic eruptions. Neither from observations (e.g. Meyer et al. 2015 
), nor from 
the model side. 
I wouldn't expect the ash to make a significant impact on the global scale, 
as it falls out out the atmosphere too quickly to significantly change the 
cirrus radiative forcing (looking
from the global modeller's perspective)

(+don't forget that a shift from homogeneous to heterogeneous freezing 
could additionally thin the cirrus and decrease further their warming 
effect, i.e. cirrus seeding effect)
 
Best,
Blaz

>
>
> On 13 Feb 2018, at 04:54, Andrew Lockley  > 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"  > 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...@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 > > 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

Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[dvisioni]

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  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"  
>> 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  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 (−

Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[Andrew Lockley]

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" 
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  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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Re: [geo] (must read) Upper tropospheric ice sensitivity to sulfate geoengineering

[Daniele Visioni]

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  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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