Aerosol Indirect Effects in a Coupled Global Aerosol and Atmospheric Circulation Model.

Abstract

The aerosol indirect effect remains one of the largest uncertainties in the projection of the future climate change. In this dissertation we improve both aerosol and cloud treatments in a coupled aerosol and atmospheric circulation model to advance our understanding of aerosol indirect effects. An empirical aerosol nucleation parameterization is implemented into the coupled model to better represent observed nucleation events in the boundary layer and is shown to improve the comparison of simulated aerosol size distributions with observations. Simulated cloud condensation nuclei (CCN) concentrations in the boundary layer range from 70 to 169 /cm3 from different nucleation mechanisms. Primary-emitted sulfate has the largest effect on simulated CCN concentration, while the effect of the boundary layer nucleation on CCN concentration strongly depends on the number of simulated primary particles. The first indirect forcing from various treatments of aerosol nucleation ranges from -1.22 to -2.03 W/m2. Including primary-emitted sulfate particles significantly increases the first aerosol indirect forcing, while whether particle formation from aerosol nucleation increases or decreases aerosol indirect effects largely depends on the relative change of primary particles and SO2 emissions from the preindustrial to the present day atmosphere. To better represent subgrid-scale supersaturation, a statistical cirrus cloud scheme is implemented into the coupled model and is shown to simulate the observed probability distribution of relative humidity well. Heterogeneous ice nuclei (IN) are shown to affect not only high level cirrus clouds through their effect on ice crystal number concentration but also low level liquid clouds through the moistening effect of settling and evaporating ice crystals. The change in net cloud forcing is less sensitive to the change in ice crystal concentrations because changes in high cirrus clouds and low level liquid clouds tend to cancel, while the net radiative flux change at the top of the atmosphere is still large because of changes in the greenhouse effect of water vapor. Changes in the assumed mesoscale temperature perturbation are shown to change ice crystal number and radiative fluxes with a magnitude that is similar to that from a factor of 10 change in the heterogeneous IN number.