Studying Scale Dependency of Aerosol Cloud Interactions using Multi-Scale Cloud Formulations.

The WRF-ACI model configuration is used to investigate the scale dependency of aerosol-cloud interactions (ACI) across the "grey zone" scales for grid and subgrid-scale clouds. The impacts of ACI on weather are examined across regions in the eastern and western U. S. at 36, 12, 4, and 1 km grid spacing for short-term periods during the summer of 2006. ACI impacts are determined by comparing simulations with current climatological aerosol levels to simulations with aerosol levels reduced by 90%. The aerosol-cloud lifetime effect is found to be the dominant process leading to suppressed precipitation in regions of the eastern U.S., while regions in the western U. S. experience offsetting impacts on precipitation from the cloud lifetime effect and other effects that enhance precipitation. Generally, the cloud lifetime effect weakens with decreasing grid spacing due to a decrease in relative importance of autoconversion compared to accretion. Subgrid-scale ACI are dominant at 36 km, while grid-scale ACI are dominant at 4 and 1 km. At 12 km grid spacing, grid-scale and subgrid-scale ACI processes are comparable in magnitude and spatial coverage, but random perturbations in grid-scale-ACI impacts make the overall grid-scale-ACI impact appear muted. This competing behavior of grid and subgrid-scale clouds complicate the understanding of ACI at 12 km within the current WRF modeling framework. The work implies including subgrid-scale-cloud microphysics and ice/mixed phase cloud ACI processes may be necessary in weather and climate models to study ACI effectively.