Quantifying the Radiative Effects of Aerosols and Clouds on Climate from Airborne Field Studies

Peter Pilewskie
University of Colorado

Laboratory for Atmospheric and Space Physics
Department of Atmospheric and Oceanic Sciences
Boulder, CO

In the fourth assessment report of the Intergovernmental Panel on Climate Change, large uncertainties persist in estimates of climate forcing by aerosol particles. One contributor to this uncertainty is the poorly quantified vertical distribution of solar radiation absorbed by aerosol particles, from the regional to global scale. Another is the spectral and spatial variability of surface albedo, an effect that can dominate the top-of-atmosphere perturbations due to aerosol scattering and absorption, particularly over land. Over the past three years a number of intensive airborne field experiments (ICARTT, MILAGRO, GoMACCS) have contributed significantly to our understanding of the impact of pollution outflow from urban-industrial centers on radiative forcing, using spectrally resolved radiometric measurements and novel observationally-based methods to derive forcing efficiency and flux divergence. We present an overview of some recent advances in quantifying direct radiative forcing realized by these studies, new observations of cloud radiative properties from TC4 and PACDEX, and some potential impacts of aerosols on cloud radiative properties (the so-called indirect effect), a potentially greater climate effect, but even more uncertain than direct radiative forcing.