17 Aug 2021

17 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

Potential limitations of using a modal aerosol approach for sulfate geoengineering applications in climate models

Daniele Visioni1, Simone Tilmes2, Charles Bardeen2, Michael Mills2, Douglas G. MacMartin1, Ben Kravitz3,4, and Jadwiga H. Richter5 Daniele Visioni et al.
  • 1Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA
  • 2Atmospheric Chemistry, Observations, and Modeling Laboratory, National Center for Atmospheric Research, Boulder CO, USA
  • 3Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN, USA
  • 4Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
  • 5Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder CO, USA

Abstract. Simulating the complex aerosol microphysical processes in a comprehensive Earth System Model can be very computationally intensive and therefore many models utilize a modal approach, where aerosol size distributions are represented by observations-derived lognormal functions. This approach has been shown to yield satisfactory results in a large array of applications, but there may be cases where the simplification in this approach may produce some shortcomings. In this work we show specific conditions under which the current approximations used in modal approaches might yield some incorrect answers. Using results from the Community Earth System Model v1 (CESM1) Geoengineering Large Ensemble (GLENS) project, we analyze the effects in the troposphere of a continuous increasing load of sulfate aerosols in the stratosphere, with the aim of counteracting the surface warming produced by non-mitigated increasing greenhouse gases concentration between 2020–2100. We show that the simulated results pertaining to the evolution of sea salt and dust aerosols in the upper troposphere are not realistic due to internal mixing assumptions in the modal aerosol treatment, which in this case reduces the size, and thus the settling velocities, of those particles and ultimately changes their mixing ratio below the tropopause. The unnatural increase of these aerosol species affects, in turn, the simulation of upper tropospheric ice formation, resulting in an increase in ice clouds that is not due to any meaningful physical mechanisms. While we show that this does not significantly affect the overall results of the simulations, we point to some areas where results should be interpreted with care in modeling simulations using similar approximations: in particular, the evolution of upper tropospheric clouds when large amount of sulfate is present in the stratosphere, as after a large explosive volcanic eruption or in similar stratospheric aerosol injection cases. Finally, we suggest that this could be avoided if sulfate aerosols in the coarse mode, the predominant species in these situation, are treated separately from other aerosol species in the model.

Daniele Visioni et al.

Status: open (until 28 Sep 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on acp-2021-678', Olivier Boucher, 18 Aug 2021 reply

Daniele Visioni et al.

Data sets

Data from: Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering? Visioni, Daniele; MacMartin, Douglas G.

Stratospheric Aerosol Geoengineering Large Ensemble Project - GLENS Simone Tilmes, Jadwiga H. Richter, Michael Mills, Ben Kravitz, Douglas G. MacMartin

Daniele Visioni et al.


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Short summary
Aerosols are simulated in a simplified way in climate models: in the model analyzed here, they are represented in every grid as described by three simple logarithmic distributions, mixing all different species together. The size can evolve when new particles are formed, particles merge together to create a bigger one or particles are deposited to the surface. This approximation normally works pretty well. Here we show however that when large amount of sulfate are simulated, there are problems.