Aerosol effects on cloud-precipitation and land-surface processes

Aerosols not only directly scatter and absorb solar radiation (the aerosol direct effects), but also modulate cloud properties by acting as cloud condensation nuclei (CCN) to form cloud droplets (the aerosol indirect effects). The variability of marine warm cloud properties is derived from satellite multiple sensors, and is normalized by the variability of satellite-derived aerosol index (AI) and reanalysis-derived lower-tropospheric stability (LTS). Global statistics shows that higher AI (polluted air) are associated with reductions in the cloud droplet effective radius (Re) and warm rain processes, reductions in the cloud liquid water path (CLWP), and increases in the cloud covers. Higher LTS (strong inversion) is also associated with reductions in Re and increases in the cloud covers. Local variability of these cloud properties is explained by a combination of AI and LTS better than by either AI or LTS alone. Finally, the spatial mean and the spatial gradient of the aerosol direct and indirect radiative forcing are estimated and compared with the greenhouse-gases (GHG) radiative forcing over the tropical ocean.; The aerosol direct effect not only reduces global irradiance but also increases diffuse radiation. Diffuse radiation is more homogeneously absorbed by the plant canopy and more efficiently utilized for the plant photosynthesis process than is direct radiation. Thus, increases in aerosol loading are expected to increase plant productivity (the aerosol diffuse-radiation effect). Satellite-derived surface albedo and radiative temperature as well as ground-based observations of surface CO2 flux and heat fluxes are utilized to calibrate the parameters in a sun-shade canopy model by reducing model-observation discrepancies. The well-calibrated off-line sun-shade canopy model is run for the warm seasons in 2000 and 2001 with and without aerosol loading in all-sky conditions. The sensitivity experiments show that the aerosol loading increases plant productivity by only +0.5% in 2001 and -0.09% in 2000 over the eastern U.S. due to the combination of the positive and negative aerosols diffuse-radiation effect over the forests and the croplands, respectively. The spatio-temporal variability of aerosol diffuse-radiation effect is well explained by the variability of LAI, cloud optical depth, near-surface atmospheric temperature, and diurnal cycles.