Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

Abstract

Abstract. Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can also be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Opportunistic experiments have significantly improved process level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus, demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

Reducing Aerosol Forcing Uncertainty by Combining Models With Satellite and Within‐The‐Atmosphere Observations: A Three‐Way Street

“…Except in special circumstances and with stringent assumptions (e.g., Pahlow et al, 2006;Rosenfeld et al, 2016;Dawson et al, 2020), quantitative constraints on nearly all the key variables needed to characterize detailed ACI are beyond the capabilities of existing and currently planned space-based remote-sensing instruments. Nevertheless, satellite observations of well-defined aerosol sources such as ship tracks, industrial point sources of pollution, wildfire smoke and volcanic plumes, represent "natural laboratories" that have proven particularly useful in studying ACI, because the unaffected surroundings provide a control for the confounding factors associated with meteorology, at least to first order (Christensen et al, 2022). Also, the large datasets obtained from space can support fine-scale stratification of meteorological conditions, allowing the aerosol effects on clouds to be isolated statistically in some cases (e.g., Zamora & Kahn, 2020).…”

Impact of Holuhraun volcano aerosols on clouds in cloud-system resolving simulations

“…The latter is the focus of the present manuscript. A large volcanic eruption as a natural laboratory may help to better understand and quantify how cloud properties are modified in response to anthropogenic aerosols emissions (Inguaggiato et al, 2018;Christensen et al, 2021).…”