Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

Michael Diamond

Summary: Clouds confound our understanding of how human activities are changing our climate system. The cooling effect of interactions between small airborne pollution particles (aerosol) and clouds has “masked” some of the warming from increasing greenhouse gas concentrations. Unknowns about how much such masking has already occurred and about how clouds will respond to warming make it difficult to estimate future climate changes. A major factor that complicates the study of aerosol-cloud interactions is the challenge of disentangling aerosol effects on cloud properties from other weather-related changes. "Natural experiments," in which there is a known aerosol change that is independent of the weather, offer a promising framework for improving our understanding of causation in aerosol-cloud relationships. Michael Diamond will work with Graham Feingold and Jen Kay to study one such natural experiment: a highly-trafficked shipping corridor in the southeast Atlantic in which cloud brightening from pollution effects has been observed. I will use a hierarchy of models to determine whether biases in simulated cloud changes are related to aerosol effects on how much clouds precipitate or how much dry air is mixed into the cloudy layer by turbulent motions. These processes are key to understanding how clouds respond to both pollution and global warming.