What can Possibly go Wrong? Cloud Microphysical Implications for Marine Cloud Brightening

Abstract:

Marine cloud brightening is suggested to be a viable way to counteract global warming by artificially increasing the albedo of clouds due to the deliberate seeding of aerosols (the so-called Twomey effect). Stratocumulus decks, which cover wide swaths of the Earth’s surface, are considered the primary target for this geoengineering technique. However, it is also well-known that this cloud type, its morphology, and its optical properties are highly sensitive to cloud microphysics, which may be significantly altered by the seeded aerosols.

In this study, we apply a highly detailed particle-based (Lagrangian) modeling approach covering the entire range of hydrometeors from aerosols to cloud droplets and raindrops without artificial categorization. Furthermore, associated microphysical processes are based on first principles, minimizing the problems typically associated with their parameterization in more commonly used models. This modeling framework is applied in both large-eddy and parcel simulations to analyze a vast phase space of different seeded aerosol spectra.

Based on these results, we emphasize potential dangers resulting from the seeding of stratocumulus (so-called negative liquid water adjustments): While seeding only a small number of large aerosols, non-drizzling stratocumulus can be triggered to produced significant precipitation that reduces the cloud liquid water and may even break up the cloud deck, with attendant reduction in the cloud albedo compared to non-seeded stratocumulus. Similarly, seeding a large number of small aerosols increases entrainment, which also decreases the cloud liquid water and hence albedo by heating and drying. In between these extremes, there may exist an ideal seeded aerosol distribution that minimizes these negative adjustments and enables optimal marine cloud brightening.