Aerosol Indirect Effects
Schematic diagram of the many interactions of aerosols in the environment and with clouds.
Focus Area Lead - Steve Ghan
The role of aerosols in climate forcing is a critical factor in assessment and prediction of climate changes, as well as in advancing climate modeling frameworks. Aerosol forcing has two major components, direct and indirect. Direct effects of aerosols are the influence of the aerosols on the planet's radiation balance by the scattering of solar radiation, which result in the cooling of the Earth's surface. Aerosols can also have an indirect effect on climate, based on the ways in which aerosols interact with surrounding clouds. Direct effects of aerosols on the climate system are much better understood and quantified than the indirect effects.
Indirect effects of aerosols include their influence on the radiation balance and hydrology through their impact on cloud microphysical processes (first indirect effect) and amount (second indirect effect). There is also a semi-direct effect, in which the heating by aerosol particles due to absorption of solar radiation results in a decrease of cloud amount. The indirect effect is a significant source of uncertainty due to the complexity and number of the atmospheric interactions involved, as well as the wide range of scales on which these interactions occur.
PNNL is investigating the effects of aerosols on clouds of various types in order to develop a better understanding of the role that aerosol indirect effects play in the current climate and how they may influence future climate change. Our research efforts will provide the physical basis for the development of parameterizations that can be included in global models. Multiple possible effects have been noted in the literature and these effects often have contradictory impacts on clouds and climate. In many cases, existing modeling frameworks reflect only a partial set of known aerosol-cloud interaction processes. In some sense, these models produce a "partial derivative" result, by illustrating a result of a particular process, with all other processes being held constant.
The answer to the problem of incomplete specification of physical processes is the development of a unified framework in which all of the aerosol-cloud interaction processes can be treated in a physically consistent manner. This Aerosol Climate Initiative project addresses this issue by developing and using a 3-D cloud resolving model (CRM) that contains more efficient treatments of aerosol and cloud microphysical processes.