Challenges for Quantifying Climate Effects of Aerosol-Cloud Interactions in Warm Low Clouds (Invited Presentation)

ABSTRACT: Warm low cloud systems are key mediators of aerosol radiative forcing globally and yet there remains much uncertainty about these effects and the processes controlling them. This stems in part from an inability to fully understand and simulate the key contributing processes in large scale models, and from our limited ability to observationally constrain aerosol-mediated perturbations to cloud systems. Although many studies have attempted to constrain processes controlling aerosol-cloud interactions, this presentation will argue that quantifying regional and global indirect aerosol effects on warm low clouds is particularly challenging for two distinct but interconnected reasons:

(a)    to exert globally relevant radiative forcing, anthropogenic aerosols must perturb cloud systems that are increasingly understood to be characterized by strong internal aerosol-cloud coupling and whose resilience to external perturbations is barely understood;

(b)    aerosol indirect forcing is nonlinear, spatiotemporally variable, and strongly dependent upon the poorly characterized aerosol state prior to human-induced perturbations;

I present: (i) a brief review of recent observational and modeling research that provides justification for the aforementioned challenges; (ii) suggestions for how we might shape future research to address these challenges.