Cloud Microphysical Implications for Marine Cloud Brightening: The Importance of the Seeded Particle Size Distribution

Abstract

Marine cloud brightening (MCB) has been proposed as a viable way to counteract global warming by artificially increasing the albedo and lifetime of clouds via deliberate seeding of aerosol particles. Stratocumulus decks, which cover wide swaths of the Earth’s surface, are considered the primary target for this geoengineering approach. The macroscale properties of this cloud type exhibit a high sensitivity to cloud microphysics, exposing the potential for undesired changes in cloud optical properties in response to MCB. In this study, we apply a highly detailed Lagrangian cloud model, coupled to an idealized parcel model as well as a full three-dimensional large-eddy simulation model, to show that the choice of seeded particle size distribution is crucial to the success of MCB, and that its efficacy can be significantly reduced by undesirable microphysical processes. The presence of even a small number of large particles in the seeded size spectrum may trigger significant precipitation, which will reduce cloud water and may even break up the cloud deck, reducing the scene albedo and hence counteracting MCB. On the other hand, a seeded spectrum comprising a large number of small particles reduces the fraction of activated cloud droplets, increases entrainment and evaporation of cloud water, also reducing the efficiency of MCB. In between, there may exist an aerosol size distribution that minimizes undesirable microphysical processes and enables optimal MCB. This optimal size distribution is expected to be case-dependent.

Solar Geoengineering Modeling and Applications for Mitigating Global Warming: Assessing Key Parameters and the Urban Heat Island Influence

“…Yet, solar geoengineering solutions provide an opportunity to lower global temperature quickly and are needed as warming is escalating faster than expected (Hugonnet et al, 2021). There have been numerous solar geoengineering solutions proposed for reversing global warming (Gaskill and Reese, 2003;Crutzen, 2006;Royal Society, 2009;Levinson and Akbari, 2010;Ferraro et al, 2011Ferraro et al, , 2015Dykema et al, 2014;Sánchez and McInnes, 2015;Weisenstein et al, 2015;Cho, 2016;Keith et al, 2016;Dunne, 2018;Field et al, 2018;Reynolds, 2019;Keutsch, 2020;Hoffmann and Feingold, 2021). Creative solar geoengineering may also be able to help drought-related problems and these are discussed in section Solar Geoengineering Ideas for Drought Relief.…”

Relationships between supermicrometer particle concentrations and cloud water sea salt and dust concentrations: analysis of MONARC and ACTIVATE data

“…The results of these types of studies are important in light of growing attention towards marine cloud brightening efforts that need to account for GCCN, which would offset the intended brightening desired from smaller CCN according to the Twomey effect. 88 Analysis of data analogous to this study for mixed-phase clouds will be helpful to advance this type of analysis for another category of clouds impacted in ways that may differ with warm clouds.…”

The Radiative and Cloud Responses to Sea Salt Aerosol Engineering in GFDL Models

“…The exact size of ideal particles for MCB remains uncertain and will likely depend on local conditions in different places (Hoffmann & Feingold, 2021). The size of seeded particles matters because activating aerosol into cloud droplets depends on size.…”