Issues in modeling the aerosol direct
effects on climate
Chul Eddy
Chung
Scripps
Institution of Oceanography
Abstract:
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.