Issues in modeling the aerosol direct effects on climate

Chul Eddy Chung

Scripps Institution of Oceanography 

Aerosol radiative forcing is defined as the effect of aerosol, both natural and anthropogenic, on the radiative fluxes at the top of the atmosphere (TOA) and at the surface and on the absorption of radiation within the atmosphere. The direct effect of aerosols on radiation (i.e., direct radiative forcing) is different from the radiative effect of greenhouse gases in many respects: 1) the aerosol forcing positive in the atmosphere and negative at the surface while the greenhouse gas forcing is positive everywhere; 2) the aerosol forcing is very heterogeneous in space while the greenhouse gas forcing is quite homogenious; and 3) the aerosol forcing fluctuates on a wide range of time scales while the greenhouse gas forcing slowly changes. My seminar addresses how some of the complications of the aerosol forcing translate into the modeling of the climatic effects of aerosols.
Chung and Zhang (2004) focused on the vertical profile of the aerosol forcing, and found that a lifted layer of aerosols results in a significant cooling of the underlying surface while a PBL profile makes very little cooling. They also found that direct aerosol heating of the near-surface air increases the Convective Available Potential Energy (CAPE) whereas heating above the boundary layer decreases CAPE. Chung and Ramanathan (2005) and Ramanathan, Chung et al. (2005) showed that the South Asian haze slows down the Indian summer monsoon. The proposed mechanism is associated with the facts that this haze is concentrated in S. Asia and nearby oceans, and in early summer the surface forcing outweighs the atmosphere forcing. Chung (2005) demonstrated that ignoring sub-monthly fluctuations of aerosol-induced atmospheric diabatic heating leads to underestimation of the precipitation increase by the aerosols in case of the Indian haze and not so in case of the Chinese haze.