Climate predictions for the rapidly changing Arctic are highly uncertain, largely due to poor understanding of the processes driving cloud properties. For example, cloud fraction and cloud phase have major impacts on energy budgets, but models do not represent these factors very well, often because of uncertainties in aerosol–cloud interactions and their impacts on precipitation and freezing processes. Using satellite and aircraft observations, along with an aerosol model, we have quantified the microphysical effects of combustion aerosols on Arctic cloud properties. We stratified by meteorological conditions, and focused on observations during polar night, when confounding direct and semi-direct aerosol effects are minimal. This method is unique in that it does not require detailed knowledge of freezing mechanisms, or the fraction of cloud-active aerosol particles to be known, to identify aerosol microphysical effects on clouds. Combustion aerosols over sea ice are associated with very large (~ 10 W m-2) differences in longwave cloud radiative effects at the sea ice surface. Co-varying meteorological changes on factors such as cloud fraction likely explain most of this signal. However, combustion aerosols do have significant impacts on cloud phase and precipitation frequency, and can explain up to 40 % of the cloud fraction differences. The types of effects, which vary by altitude and cloud type, offer insight into how combustion aerosols affect cloud freezing processes on an Arctic-wide scale.
Seminar Series Coordinators