Satellite based assessment of aerosol radiative forcing over land with improved smoke angular models

The impacts of aerosols on climate are relatively less understood and therefore it is imperative to study the aerosol radiative effects. In this dissertation, the cloud-free direct radiative effect (DRE) of aerosols (in shortwave region) over land has been investigated using observations from multiple satellite sensors. The aerosol radiative effect is defined as the difference in the TOA shortwave (<5 mum) fluxes in the absence and presence of aerosols. The shortwave radiative flux information from the Clouds and the Earth's Radiant Energy System (CERES) and the cloud and aerosol information from Moderate Resolution Imaging SpectroRadiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR) onboard Terra satellite were used in this study. The three datasets were merged in space and time using a data fusion algorithm which is developed in this study.;The 24-hr average global cloud-free mean shortwave DRE from this data was estimated to be -5.1+/-1.1 Wm-2 although substantial regional variability in DRE over land was found to exist due to differences in aerosol properties and land cover types. This is the first observational estimate of the global DRE over land using satellite data alone. A five year (2000-2005) analysis of the TOA forcing from biomass burning aerosols over Amazonia is found to vary between -5.2 Wm-2 and -9.4 Wm -2 with a mean value of -7.6 +/-1.4 Wm-2. One of the uncertainties in the forcing estimates above comes from the angular models used by CERES to convert radiance to flux. In this study, new cloud free empirical angular distribution models (EADM) for smoke aerosols were developed using TOA shortwave radiance measurements from CERES and aerosol measurements from MODIS. The broadband CERES radiances are then converted to TOA broadband shortwave fluxes using the new set of EADM. Results indicate that the shortwave aerosol radiative forcing of biomass burning aerosols is under estimated by ∼3 Wm-2 and the aerosol forcing efficiency is lower if the CERES angular models do not account for the aerosol properties.;The results from this study suggest that observations from multiple satellite sensors over land can be used to reduce the uncertainty in aerosol radiative impacts. This dissertation also recommends changes in the current CERES (and future radiation budget sensors) angular distribution models to include aerosol properties that account for the anisotropy in the scattered radiation due to aerosols.