A New Methodology to Address Aerosol-Cloud Effects on Radiation
Research helps separate influences of aerosols versus meteorology on clouds.
Small atmospheric particles called aerosols directly influence Earth’s radiative energy balance by absorbing and scattering sunlight. Because aerosols are also critical to the formation of cloud droplets, aerosols can indirectly affect the energy balance by changing cloud properties. These indirect effects have been difficult to observe because of the strong correlation between aerosols and meteorology. In this study, a new approach to assessing aerosol effects on shallow, liquid clouds is used to analyze aerosol associations with cloud macroscopic variables and radiative properties of shallow liquid water clouds. A 14-year analysis over the Department of Energy’s Atmospheric Radiation Measurment (ARM) Southern Great Plains (SGP) site shows only a weak aerosol influence on the shallow cloud radiative effect and albedo compared to macroscopic cloud properties and thermodynamic parameters such as lower tropospheric stability and boundary-layer coupling.
In this study, a new approach to assessing aerosol effects on shallow, liquid clouds uses quantities that are more closely related to the cloud radiative effect, therefore representing a pragmatic pathway toward untangling cloud microphysical effects from dynamics.
The influence of aerosols on clouds and contribution to cloud radiative forcing represent one of the largest uncertainties in climate studies. Higher aerosol concentrations are linked to more cloud condensation nuclei (CCN) and, with all else equal, smaller cloud drops and higher cloud albedo. However, aerosol and meteorological drivers are interconnected and may result in mutually compensating effects and adjustments that are not fully understood. In this study, a new approach to assessing aerosol effects on shallow, liquid clouds is proposed. Instead of quantifying the usual metrics for microphysical response to an aerosol perturbation, the study focuses on analysis of aerosol associations with cloud macroscopic variables and radiative properties of shallow liquid water clouds. Long-term, ground-based observations from the ARM Climate Research Facility were analyzed to investigate the coincident effects of aerosols, macroscale cloud properties, and selected meteorological indices on clouds at ARM’s SGP observatory in Oklahoma. For this site, the results indicate that over the 14-year analysis period the aerosol influence on the shallow cloud radiative effect and albedo is weak and that macroscopic cloud properties and thermodynamic parameters such as lower tropospheric stability and boundary-layer coupling play a much larger role in determining the instantaneous cloud radiative effect compared to microphysical effects. On a daily basis, aerosols show no correlation with cloud radiative properties, whereas the liquid water path shows a clear positive relationship.
National Oceanic and Atmospheric Administration
The authors would like to thank the ARM program for processing and providing the datasets used in this work. This work was supported by FAPESP grants 2014/04181-2 and 2013/08582-9; Department of Energy, Office of Science, Office of Biological and Environmental Research, Atmospheric System Research program (grant DE-SC00145680); and National Oceanic and Atmospheric Administration.
Sena, E. T., A. McComiskey, and G. Feingold. 2016. “A Long-Term Study of Aerosol-Cloud Interactions and Their Radiative Effect at the Southern Great Plains Using Ground-Based Measurements,” Atmospheric Chemistry and Physics 16(17), 11301-318. DOI:10.5194/acp-16-11301-2016.