Field Studies to Test Marine Cloud Brightening Efficacy

Marine cloud brightening (MCB) is a proposed climate intervention technique that uses injections of salt aerosol to increase the amount of sunlight reflected by low marine clouds.

Global (GCM) and regional climate model simulations are an important component of being able to quantitatively assess MCB impacts under different injection strategies, but GCMs cannot explicitly represent the cloud dynamical scales that control the interactions between aerosol and cloud, and therefore how much cloud brightening could be achieved with MCB. Observations and high-resolution (large eddy) modeling studies to resolve turbulent and microphysical processes are needed to fully quantify and understand cloud-scale processes. Large eddy simulations have shown that clouds can respond to the increase in droplet concentrations in ways that can either reduce or increase cloud liquid water amount and cloud lifetime, a response inconsistent with most GCMs. Yet even these critical process modeling tools are incomplete and still under-resolve some key processes such as cloud top entrainment and aerosol-cloud interactions more broadly. There is thus a great desire for field studies that can critically test our process models. Improved process models are needed to (a) quantify the full range of responses of clouds to the addition of the injected sea salt under different meteorological and background aerosol conditions; (b) to determine the optimal injected salt particle size distribution; (c) to design and improve both macrophysical and microphysical parameterizations of cloud processes.

We propose a field study design that builds on those used in previous field campaigns to study aerosol-cloud interactions, with the important difference of being able to control the aerosol perturbation to the cloud. This presentation will describe aspects of a new Cloud Aerosol Research Instrument (CARI) designed to release a sufficient number of small saline aerosol droplets to visibly perturb marine low clouds. Sea water would be used to produce an accumulation-mode sea salt aerosol plume, using a ship-based spray system, providing an aerosol source with well-defined properties (composition, size and aerosol number). Essentially, the goal with CARI is to create “salt tracks”, a variant of a ship tracks created with aerosol derived from natural seawater. In salt tracks, the addition of salt aerosol increases the droplet concentration, reducing droplet size, and thereby increasing overall cloud brightness. Salt aerosol injections would be carried out for a limited time and over a limited portion of the cloud field. Using a consistent and well-characterized aerosol source allows a more definitive separation of cloud changes driven by aerosol-cloud covariability associated with meteorology from those driven by local sources driving aerosol-cloud interactions. Systematically conducting such controlled perturbation experiments over a range of background aerosol and meteorological conditions would further allow for building statistical relationships between aerosol perturbations and cloud responses over a range of time-scales. By accurately characterizing the local meteorology, the injected and background aerosol, and the cloud microphysical, macrophysical and radiative responses, the field studies can be used to initialize and test how accurately process models are able to simulate these responses under a range of different conditions.