June 25, 2024
Research Highlight

Aerosols Increase Size of Cloud Cells, Causing Greater Radiative Cooling Under Polluted Conditions

Applying a novel framework to a kilometer-scale model reveals new characteristics about elevated concentrations of aerosols’ effect on clouds

cloud cells

In the distance, Graciosa Island in the Azores. An Atmospheric Radiation Measurement site at the island provides continuous ground measurements of aerosol and cloud properties for scientists exploring elevated concentrations of aerosols and their effect on clouds.

The Science

Aerosols, often emitted alongside greenhouse gases, can brighten clouds and cause significant cooling. However, the uncertainty associated with aerosolcloud interactions (ACIs) is large and potentially significant enough to mask a sizable portion of greenhouse gas-related warming. A higher aerosol concentration generally suppresses rainfall and increases the abundance of droplets in clouds passing over Graciosa Island in the Azores, where an Atmospheric Radiation Measurement (ARM) site provides continuous ground measurements of aerosol and cloud properties. Kilometer-scale atmosphere model simulations capture the mesoscale structure of clouds and show agreement with satellite and aircraft observations of the cloud and aerosol properties. These results emphasize the importance of addressing mesoscale cloud-state transitions in the quantification of increased aerosol impacts on cloud reflection that cannot be attained from traditional climate models run at coarser scales.

The Impact

Researchers developed a Lagrangian framework that tracks the ACI along the trajectories of air parcels and embedded it into the Weather Research Forecast (WRF) model. This framework incorporates polar orbiting and geostationary satellite retrievals as well as aircraft measurements taken during the Intensive Operation Periods of clouds and aerosols throughout their lifetime, thereby providing a new constraint needed to quantify and understand highly nonlinear causal connections among cloud water, precipitation, and aerosols. New results reveal that as aerosol is increased in the simulations, drizzle is suppressed in clouds causing enhanced vertical velocity and detrainment near the top of the planetary boundary layer. Marine cloud cell area expands, narrowing the gap between shallow clouds and enhancing the reflection of solar radiation to space. These findings provide insights into model strength and weakness, which are useful for model development efforts to improve cloud controlling processes so that future climate change projections are better understood and quantified.


A series of WRF experiments were carried out at kilometer scales, increasing aerosol concentrations over 10 case studies coinciding with aircraft measurements taken during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA). These experiments determined the sensitivity of aerosol impacts on the microphysical and dynamical properties of marine stratocumulus clouds. In general, the results showed that precipitation is strongly suppressed and the clouds expand into otherwise cloud-free regions. This results in significant increases in liquid water path and cloud fraction that, when considered together in radiative-forcing calculations, provide a significant contribution to the total aerosol indirect effect. Despite the wide range of thermodynamic conditions, this response was found but was weaker on days with less precipitation. The role of precipitation in ACIs should be considered when parameterizing climate models using coarser-scale resolutions that are unable to capture the intricate details of the mesoscale structures in the clouds simulated using WRF. The researchers’ new framework is highly versatile and can be applied to land regions with ARM observations so a deeper investigation into aerosol activation, warm-rain processes, and/or turbulence representations can be explored further in future work.

PNNL Contact

Jerome Fast, Pacific Northwest National Laboratory, Jerome.Fast@pnnl.gov


This research has been supported by the Atmospheric System Research program as part of the U.S. Department of Energy, Office of Science, Biological and Environmental Research program under Pacific Northwest National Laboratory (PNNL) project 57131. 

Published: June 25, 2024

Christensen, M. W., Wu, P., Varble, A. C., Xiao, H., and Fast, J. D.: Aerosol-induced closure of marine cloud cells: enhanced effects in the presence of precipitation, Atmos. Chem. Phys., 24, 6455–6476, https://doi.org/10.5194/acp-24-6455-2024, 2024.