Large eddy simulations to explore processes controlling the responses of marine low clouds to aerosol injections

Marine cloud brightening (MCB) climate intervention involves deliberate injections of aerosol into the marine boundary layer (MBL) to increase cloud albedo and sunlight reflection. Although climate modeling has demonstrated the potential for significant global cooling from MCB, doubts remain about the ability of large scale models to accurately represent the subgrid processes and their interactions that are involved in producing an albedo response to aerosol injections. Here, the University of Washington version of the System for Atmospheric Modeling (SAM-UW) is employed to explore the dependence of cloud responses to aerosol injections under a variety of different environmental conditions. SAM-UW has a prognostic representation of the aerosol budget in the marine boundary layer, and the simulations are performed in a domain that is sufficiently large to simulate mesoscale dynamical responses to an aerosol injection over a period of 1-2 days. A novel method is used to separately quantify the contributions to brightening from increases in droplet concentration (the Twomey effect) and from adjustments of liquid water path and cloud cover. Cloud responses to different levels of background aerosol and to different amounts of free-tropospheric (FT) moisture are explored using quasi idealized steady-state stratocumulus conditions with diurnally varying radiation. In all cases, aerosol injections result in more sunlight reflection. The strongest brightening occurs under pristine background conditions, where increases in cloud cover driven by precipitation suppression strongly augment the Twomey effect. As the background aerosol level increases, negative cloud adjustments tend to offset Twomey brightening to some extent, and these adjustments involve PBL drying in response to increases in cloud top entrainment. Under polluted background conditions with minimal precipitation, cloud droplet sedimentation drives cloud top entrainment increases. Free tropospheric moisture levels influence these responses in complex ways. A dry FT increases the strength of turbulence in the MBL by increasing cloud-top longwave cooling, but also results in lower LWP in the unperturbed state, which reduces the sedimentation-entrainment feedback to increased aerosol. Cloud responses are found to change through the diurnal cycle, and reasons for this behavior are explored.