The Impact of Resolving Subkilometer Processes on Aerosol-Cloud Interactions of Low-Level Clouds in Global Model Simulations

Subkilometer processes are critical to the physics of aerosol-cloud interaction (ACI) but have been dependent on parameterizations in global model simulations. We thus report the strength of ACI in the Ultra-Parameterized Community Atmosphere Model (UPCAM), a multiscale climate model that uses coarse exterior resolution to embed explicit cloud-resolving models with enough resolution (250 m horizontal, 20 m vertical) to quasi-resolve subkilometer eddies. To investigate the impact on ACIs, UPCAM's simulations are compared to a coarser multiscale model with 4 km horizontal resolution. UPCAM produces cloud droplet number concentrations (N-d) and cloud liquid water path (LWP) values that are higher than the coarser model but equally plausible compared to observations. Our analysis focuses on the Northern Hemisphere (20-50 degrees N) oceans, where historical aerosol increases have been largest. We find similarities in the overall radiative forcing from ACIs in the two models, but this belies fundamental underlying differences. The radiative forcing from increases in LWP is weaker in UPCAM, whereas the forcing from increases in N-d is larger. Surprisingly, the weaker LWP increase is not due to a weaker increase in LWP in raining clouds, but a combination of weaker increase in LWP in nonraining clouds and a smaller fraction of raining clouds in UPCAM. The implication is that as global modeling moves toward finer than storm-resolving grids, nuanced model validation of ACI statistics conditioned on the existence of precipitation and good observational constraints on the baseline probability of precipitation will become key for tighter constraints and better conceptual understanding. Plain Language Summary How aerosol particles impact the climate through their interactions with clouds is a significant source of uncertainty in quantifying the drivers of climate change over the past hundred years. Global climate models have so far been heavily reliant on approximations of the physical processes that occur at subkilometer scales, even though processes at those scales are important for representing the physics behind aerosol-cloud interactions. To address this gap, we develop and run a multiscale global model that embeds a finer-scale model (250m in the horizontal and 20m in the vertical) within the columns of a coarser-resolution global model. A pair of simulations with preindustrial and present-day aerosol emissions is used to quantify the impact of human aerosol emissions. They show that the climate impact of resolving subkilometer resolutions is relatively small. However, this masks some key differences. The increase in cloud water with increasing aerosols is substantially weaker when subkilometer motions are resolved. Most of this weakening is due to a weaker response in nonraining clouds and there being fewer clouds that rain in the high-resolution model. The simulation results point to observations of specific processes that can help further constrain the impact of aerosols on clouds and climate.