Investigating the Role of Dynamical Processes in Driving Aerosol-Cloud Interactions in Boundary Layers Over Ocean and Land

Abstract

Low clouds are maintained by complex interactions between turbulent motions and the population of cloud and aerosol particles. These processes are poorly understood and are challenging to represent in atmospheric system models. Responses of low clouds to greenhouse warming (“cloud feedbacks”) are typically treated as independent of the details of how aerosol particles influence clouds (“aerosol indirect effects”). Correlations between cloud feedbacks and indirect effects in models have been noted but are generally attributed to coincidence rather than to physically meaningful connections. Studies examining low cloud feedback processes typically do not consider aerosol-cloud interactions, and vice versa. However, there is emerging evidence that a model’s sensitivity to greenhouse gases is dependent upon the representation of aerosol-cloud interactions because the latter help set those properties of clouds that affect cloud feedbacks to warming. Conversely, the amount of condensed water in clouds helps determine the aerosol “base-state” in the preindustrial atmosphere through its importance for producing rain that removes aerosol particles. This can impact the strength of the anthropogenic aerosol indirect forcing.

Turbulent dynamics control low clouds and are at the heart of theories for both low cloud feedback processes and how low clouds respond to aerosol changes. Data collected as part of the Atmospheric Radiation Measurement program are providing new avenues to explore connections between turbulent processes and aerosol-cloud interactions.

Objectives: The overarching objective of the proposed research is to use observational data to examine how aerosol and low cloud microphysical properties are coupled for differing turbulent dynamics. Specific objectives of the proposed work are to examine how turbulence affects the seasonally varying relationships between the populations of aerosol particles and cloud and precipitation particles in low clouds over land and ocean.

Project methods: We will use a combination of ARM ground based and Aerial Facility (AAF) data from the Eastern North Atlantic (ENA) and Southern Great Plains (SGP) sites, together with AAF data from campaigns in the Amazon and over the far North Atlantic to focus on understanding the interplays between PBL turbulent motions and aerosol-cloud interactions under a range of different land and ocean meteorological and seasonal conditions.

Project Impacts: The proposed work will provide improved understanding how turbulent dynamical processes impact aerosol-cloud interactions in warm boundary layer clouds over both oceans and over land by combining the analysis of aircraft datasets with those from state-of-the-science ground sites. The work will lead to an improved understanding of how differences in boundary layer turbulent dynamics and aerosols shape cloud microphysical processes and precipitation properties that likely impact the strength of the cloud feedbacks and aerosol indirect forcing.