Evaluation of Pulse Aerosol Forcing on Marine Stratocumulus Clouds in the Context of Marine Cloud Brightening

Abstract

We explore the effect of aerosol perturbations on stratocumulus clouds in the context of marine cloud brightening (MCB) using high-resolution large-eddy simulations. We use a Lagrangian cloud microphysical model with very detailed treatment of aerosol activation and droplet growth. The aerosol forcing is represented as a finite-width rectangular pulse in time (uniform in space). We analyze three stratocumulus cloud systems differing in their surface precipitation rate, namely – non-precipitating, intermediate, and precipitating. We report on the diurnal evolution of these cloud systems subjected to a range of perturbations characterized by varying the amplitude and duration of the aerosol forcing pulse. Our simulations show that in the non-precipitating system, the clouds are relatively insensitive to duration and amplitude, and are sensitive only to the total number concentration of the injected aerosol. In contrast, the precipitating cloud system is affected by the duration and the amplitude of the forcing, with the sensitivity conditional on the state of the cloud system before the injection of aerosol particles. We use these case studies to assess the efficacy of potential MCB spraying strategies. Our analysis shows that negative LWP adjustments offset a substantial fraction of the Twomey induced brightening in all three cloud systems. This is countered by substantial cloud brightening obtained through precipitation suppression induced cloud-fraction adjustments.