Research Highlights July 2011 : Donner et al.
Study describing the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL Global Coupled Model CM3
Earlier generations of climate models at GFDL have specified cloud
properties to be independent of atmospheric aerosol composition, despite
fairly well-understood (at least for liquid clouds) dependence of cloud
droplet number on aerosol composition and size distribution. Cloud drop
number in turn exerts a major control on cloud radiation, notably
albedo, and microphysics and macrophysics, notably cloud structure and
lifetimes. Aerosol-cloud interactions are likely to be important in
anthropogenic climate change. They have been difficult to incorporate in
climate models because they occur at scales much smaller than resolved
by climate models. CM3 parameterizes these smaller scales using
probability distribution functions for motions smaller than those it
explicitly resolves.
Aerosol-cloud interactions due to anthropogenic aerosol changes cool the Earth. The last two decades of the twentieth century warm by 0.32 C relative to 1881-1920 in CM3. CM2, which did not include cloud-aerosol interactions, warms by 0.66 C. Observational warming estimates range from 0.52 to 0.56 C. Figures 3 and 4 from this paper show that aerosol concentrations and absorption properties are simulated more realistically in CM3 than CM2. The direct effects of these aerosols are thereby more realistically simulated in CM3 than CM2. Figure 5 from this paper shows the clear-sky downward surface shortwave fluxes. These fluxes are biased low in CM2, indicating excessive aerosol direct forcing, consistent with the high bias in CM2 optical depths relative to observations. Note that even with cooling aerosol direct effects too large compared to observations, CM2 warmed too much over the twentieth century. On the other hand, CM3, whose aerosol direct effects are more realistic relative to observations but also includes cloud-aerosol interactions, does not warm enough over the twentieth century.
Figure
1. Climatological aerosol optical depths (550nm) from AERONET and
(a), (b)
CM2.1 and (c), (d) CM3. Dashed lines in (a) and (c) denote slopes of
0.5 and
2.
Figure2. Climatological aerosol co-albedos from AERONET (440nm) and (a),
(b) CM2.1
and (c), (d) CM3 (550nm). Dashed lines in (a) and (c) denote slopes of
0.5 and
2.
Figure 3. Surface clear-sky downward shortwave fluxes from Baseline Surface Radiation Network (BSRN) and (a) CM2.1 and (c) CM3. Differences in these fluxes: (b) CM2.1 minus BSRN and (d) CM3 minus BSRN. Model fluxes are obtained from the clear-sky radiative transfer code at all grid points and times (regardless of cloud characteristics), providing consistent sampling with respect to seasonal and diurnal cycles in CM2.1 and CM3.
