Exploration of Aerosol
Impacts on the Land-Atmosphere Interactions and Climate
Aerosols
heat the atmosphere through absorption and cool the land surface even more
through scattering and absorption. A figure shown here is an example that 
the dry-season biomass burning aerosol in the
southern hemispheric tropics heats the atmosphere and cools the surface
substantially. As suggested by our radiative
calculations, about a half of surface cooling is compensated by the atmospheric
heating globally. Such perturbation and redistribution of energy will influence
atmospheric stratification, turbulent mixing, atmosphere-surface interaction,
and atmospheric circulation. Our objectives are to examine (1) how aerosols
affect the surface energy balance and its partitioning into sensible heat flux
and evapotranspiration (ET); (2) how the changes in
ABL temperature and moisture feedback such partitioning; (3) how the evolution
of the ABL is influenced by the changes in surface heating and entrainment at
the top of the ABL; (4) how the
aerosol-land interactions depend on land cover and atmospheric conditions; and
(5) how aerosol’s regulation on the partitioning of ET into different time
scale fluxes, namely canopy transpiration, canopy evaporation, and ground
evaporation, consequently correlates with convective precipitation, circulation
patterns, and the overall hydrological cycle.
To
achieve these goals, we are employing both 1-D and 3-D models to examine such
impacts, focusing on its diurnal cycle. Although perhaps unrealistic in some
aspects, 1-D models provide high resolution and flexibility in conducting
sensitivity studies, allowing the atmospheric response to radiative
perturbations to be examined more easily than possible with complex 3-D models,
thus complementing and facilitating the interpretation of 3-D model results.
A
high-resolution ABL model has been used to investigate the impacts of aerosols
on the evolution of the ABL for dry subsiding regions [Yu et al., 2002]. It shows that tropospheric
aerosols have substantial effects on land surface processes including
temperatures and latent fluxes that for absorbing aerosols are largely
unconnected with TOA radiative forcing. In
particular, aerosols affect the ET through reducing the solar radiation reaching
the surface and changing the entrainment at the top of the ABL, depending on
the loading, absorption, and vertical profile of aerosols, as well as soil
moisture. We are extending our study to tropical ascending regions using the NCAR SCCM, a
single-column version of NCAR CCM3.
We examine how absorbing aerosol perturbs the diurnal cycle of cloud,
atmospheric stratification, surface fluxes, and convection and what controls of
land cover exert on such impacts.
We
have also used NCAR regional climate model at a 60km horizontal resolution,
namely RegCM, to examine how aerosols affect the land
surface energy, water balance, and atmospheric circulation in the 
aerosols in changing atmospheric circulation and
climate.
These modeling activities have also inspired us to analyze climatology data for fingerprints of aerosol impacts on climate. The 22-year TOMS data show that the magnitude and duration of biomass burning aerosol vary interannually over Amazon (0-20S, 40-70W), which can be correlated with the deviations of wet season onsets from their climatological mean (the positive numbers of pentads represent delayed onset and negative numbers for early onset). In collaboration with Prof. Fu’s group and through a combination of multiple data sets (e.g., ISCCP, TRMM, CERES, TOMS, MODIS, and others), we are examining (1) how biomass burning smoke would affect the land surface processes and modify the atmospheric boundary conditions needed for the onset of wet season over South American Monsoon region, and (2) how such effects interact with the influence of large-scale circulation anomalies.