Estimating Twomey forcing sensitivity to aerosol plume spreading rates

Abstract. The amount of sunlight that reaches Earth's surface can be reduced by increasing cloud droplet number and decreasing droplet size (i.e., Twomey forcing), a central idea underpinning the Marine Cloud Brightening strategy. Cloud albedo depends nonlinearly on cloud droplet concentration, meaning the spatial extent of aerosol plumes could be an important constraint on the brightening potential. Horizontal spreading of aerosols can be simulated by Langevin particle models, which depend on turbulence characteristics and precipitation. The turbulence information that drives the particle model is derived from a library of realistic, 2-day LES cases, spanning northeast Pacific stratocumulus conditions. The 2-D reflectance fields from the LES are superimposed onto the 2-D perturbed aerosol concentrations to calculate the 2-D Twomey forcing response.

The Day 1 and Day 2 LES regimes have distinct meteorological, aerosol, and turbulence characteristics associated with equatorward movement of the LES domain. Our results indicate that the Day 2 regime has substantially faster plume spreading than Day 1, with ensemble median differences exceeding 2 km hr^-1. Despite these differences, Twomey forcing is insensitive to the natural variability in spreading rate. This study suggests that Twomey forcing is resilient to variations in meteorology, aerosols, and turbulence, with its efficacy governed primarily by aerosol lifetimes and assumptions surrounding cloud adjustments. Although the natural variation in spreading rate does not materially affect Twomey forcing, idealized plume spreading simulations suggest that current GCM assumptions of an infinitely fast spreading rate could lead to a 10-200% overestimation of cooling.