Sunday, 22 January 2012
Aerosols and the Surface Energy Balance: A Modelling Perspective
Hall E (New Orleans Convention Center )
Aerosols can have myriad impacts on the Earth's hydrological cycle through both direct and indirect effects. Both scattering and absorbing aerosols can deplete solar radiation reaching the surface, creating an imbalance in surface fluxes central in driving precipitation. Ground-based observations have detected dimming and brightening trends over Europe and East Asia during the last several decades that show strong correlation with trends in the concentration of anthropogenic aerosols over these regions. Observational and modelling studies have also found substantial hydrological changes over East Asia and the Indian subcontinent that are likely to be driven by aerosol impacts—via surface flux changes as well as other effects. General circulation models (GCMs) up to the most recent generation have exhibited limited success at recreating observationally established dimming and brightening trends over the last several decades that may have a significant impact on precipitation patterns. However, GCMs still remain an extremely useful tool for analysing aerosols' impact on the climate. The suite of models being utilized in the IPCC's Fifth Assessment Report (AR5) represent a great advance over the previous generation of GCMs, particularly in the field of aerosol physics/chemistry and microphysics. The Geophysical Fluid Dynamics Laboratory's CM3 General Circulation Model contains a significant upgrade in aerosol treatment, including internal mixing of aerosols and interactive dry and wet deposition. These advances, as well as those present in the latest generation models of other centers, are likely to make the AR5 models more capable of capturing and analysing aerosol effects previously obscured by model limitations. Using this new perspective, we analyse the CM3 model's performance at recreating the past global dimming and brightening trends, and investigate which aspects of the model's formulation contribute to its correlation with and deviation from the observed trend. This analysis is also extended to an investigation of aerosols' effects on precipitation, particularly the emergence of a predator-prey relationship between the hydrological impact of aerosol emissions and the scavenging of those same aerosols by the very precipitation that they affect.