Simulating Aerosol-Cloud Interactions Using a Climate Model with Explicit Embedded Boundary Layer Turbulence

A large source of the uncertainty in present-day anthropogenic radiative forcing stems from determining the extent to which changes in aerosol concentrations affect cloud radiative properties and morphology. Satellite-based estimates of the aerosol-cloud interaction tend to be weaker than those from many state-of-the-art climate models. Large-eddy simulations under idealized conditions also indicate the possibility that buffering mechanisms mute aerosol impacts on clouds. Indeed, studies using the Super Parameterized Community Atmosphere Model (SPCAM), where conventional cloud parameterizations are replaced with embedded cloud resolving models, show that the strength of the aerosol-cloud interaction is substantially reduced compared to conventional models.

In this study, we use Ultra-Parameterization (UP), a large-eddy permitting climate model based on SPCAM, to investigate the interaction between aerosols and clouds in the present-day climate. UP’s advantage is its ability to explicitly capture the scales of large-eddies in the boundary layer, strong inversions, and low-clouds in the atmospheric boundary layer. The model also better captures the vertical turbulence structure, which is crucial for accurately capturing cloud drop activation. We present how variations in aerosols and meteorology covary to influence cloud properties in UP and compare their sensitivities to those seen in the standard SPCAM model. Furthermore, we investigate the extent to which UP captures finescale features such as pockets of open cells, which are regions of open cellular convection surrounded by closed-cellular convection driven by strong aerosol-cloud-precipitation interactions, and ultraclean layers in the boundary layer with cloud condensation nuclei concentrations <10 cm-3 towards an attempt to assess whether differences in present-day and pre-industrial emissions can lead to changes in their relative occurrence.