News & Events - 2016
Where Clouds and Particles Meet Climate1 February 2016
Painting: Claude Monet "Spring by the Seine"
New Approach to Quantifying How Aerosol-Cloud Interactions Influence Climate
It's easy to understand why the influence of atmospheric particles (aerosol) on clouds is one of the most uncertain pieces in the climate puzzle: there are many unknowns about both the aerosol and the clouds. On the aerosol side, there's the wide variation in several aspects, such as what's in the aerosol (composition), how the particles affect light, how effective they are at initiating the formation of water droplets or ice particles, and their distribution and movement in the global atmosphere. And clouds are the atmosphere's ultimate shape-shifters: turbulent, ephemeral, and largely unpredictable.
A new paper led by Graham Feingold of the NOAA Earth System Research Laboratory's Chemical Sciences Division in Boulder, Colorado, takes fresh aim at the elusive target of understanding how the aerosol/cloud interaction affects climate, and finds that the key is to consider yet another layer of complexity: that the meteorological conditions that drive cloud formation are changing – or "co-varying" – along with the aerosol amount. Feingold and coauthors at NOAA ESRL, the University of Colorado Boulder, and University of Leeds in the UK describe the new conceptual approach in their paper, New Approaches to Quantifying Aerosol Influence on the Cloud Radiative Effect, published this week in the Proceedings of the National Academy of Sciences.
The authors focus on modeling how the aerosol affects the clouds' reflection and absorption of sunlight, using a set of inputs that describe both the aerosol and meteorology. They first illustrate the problem by making use of two different approaches that differ only in the co-variability between the meteorological conditions and aerosols. They get different outcomes for the role of the aerosols, even though both approaches used the same inputs. "This perplexing outcome motivated us to look further into how to model the complex system of clouds, aerosol, meteorology, and radiation," said Feingold.
To quantify the aerosol-cloud radiative effect the authors argue for studies that consider how the system would respond to inputs that are allowed to co-vary in a natural way, more accurately representing real-world conditions. They call for routine process model simulations in which the initial inputs for aerosol and meteorology are derived from observations – and hence are varying simultaneously in space and time. With a large volume of observations and successful model simulations of this kind, the authors argue that this approach could advance the study of aerosol-cloud interactions and their implications for the warming or cooling effects of clouds.
The new paper gives a good start on advancing the science on this topic, but it is far from the end of the story, notes Feingold. "To really quantify the warming and cooling effects of clouds, we will have to apply this approach in different cloud regimes in different places on the globe."
Citation: G. Feingold (NOAA ESRL CSD), A. McComiskey (NOAA ESRL GMD), T. Yamaguchi (University of Colorado CIRES at NOAA ESRL CSD), J. S. Johnson (University of Leeds), K. S. Carslaw (University of Leeds), and K. S. Schmidt (University of Colorado LASP), New Approaches to Quantifying Aerosol Influence on the Cloud Radiative Effect, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1514035112, 2016.
The topic of cloud radiative forcing associated with the atmospheric aerosol has been the focus of intense scrutiny for decades. The enormity of the problem is reflected in the need to understand aspects such as aerosol composition, optical properties, cloud condensation, and ice nucleation potential, along with the global distribution of these properties, controlled by emissions, transport, transformation, and sinks. Equally daunting is that clouds themselves are complex, turbulent, microphysical entities and, by their very nature, ephemeral and hard to predict. Atmospheric general circulation models represent aerosol−cloud interactions at ever-increasing levels of detail, but these models lack the resolution to represent clouds and aerosol−cloud interactions adequately. There is a dearth of observational constraints on aerosol−cloud interactions. We develop a conceptual approach to systematically constrain the aerosol−cloud radiative effect in shallow clouds through a combination of routine process modeling and satellite and surface-based shortwave radiation measurements. We heed the call to merge Darwinian and Newtonian strategies by balancing microphysical detail with scaling and emergent properties of the aerosol−cloud radiation system.