Aerosols shown to increase size of cloud cells, causing greater radiative cooling under polluted conditions

Aerosols, often emitted alongside greenhouse gases, can brighten clouds and cause significant cooling. However, the uncertainty associated with aerosol–cloud 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.

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. The findings are published in the journal Atmospheric Chemistry and Physics.

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.

The 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.