Analyzing the Direct and Indirect Radiative Effects of Dust Aerosol Concentrations

Abstract

Aerosols are one of the least understood climate-forcing agents. One type of aerosol, mineral dust, is of particular interest due to the role that fluctuations in atmospheric dust concentrations may have played in maintaining paleoclimates. This aerosol can directly affect the Earth’s radiative budget by reflecting and scattering shortwave light, as well as indirectly affect it by acting as an effective ice nucleus. The Single Column Atmospheric Model (SCAM) is a one-dimensional model with specified coordinates and dynamical states. This study ran four simulations over the West Pacific using the SCAM model: a control run with pre-industrial dust levels and three modified runs in which vertical dust profiles were relaxed to double, triple, and quadruple the pre-industrial levels. Comparing the control, doubled, and tripled runs reveals variations in cloud properties, which affect the top-of-atmosphere radiation budget. Cloud ice content and high-level cloud amounts increase with higher dust concentrations, likely due to the presence of more ice nuclei. This increase results in clouds reflecting more shortwave radiation back to space. However, this albedo increase is offset by an increase in the longwave trapping by clouds resulting in negligible net changes in cloud forcing. The simulation in which dust was quadrupled experienced dramatic cooling in its lower temperature profile indicating that the relaxation timescale was not sufficient to restore temperature to control values. The results from this simulation are therefore treated with caution. Our findings indicate that without temperature feedbacks, which were excluded in our simulations due to temperature restoration, the indirect effect of changing dust profiles is moderate due to large compensation between the long and shortwave cloud radiative forcing changes.