Global monitoring and retrievals of atmospheric aerosols and clouds

A knowledge of aerosol and cloud radiative properties and their variation in space and time is especially crucial to the understanding of the radiative forcing in all climate related studies. The role of clouds in modifying the Earth's radiation balance-albedo cooling effect in shortwave radiation and greenhouse warming effect in longwave radiation-is well recognized as a key source of uncertainty. Man-made processes, both burning of fossil fuels and biomass, generate greenhouse gases and aerosol particles. Directly, aerosol particles can scatter and absorb sunlight. Indirectly, these particles may serve as cloud condensation nuclei and alter cloud albedo. Ship tracks have provided intriguing examples of cloud albedo modification by anthropogenic aerosols. There has been a general concern that the potential climate forcing from the indirect effects of aerosol on clouds can be of comparable magnitude, but opposite in sign, to greenhouse gas radiative forcing. Global monitoring of aerosol and cloud radiative effects relies on advanced Earth Observing Systems (e.g., NASA's EOS and NASDA's ADEOS). Key advances include simultaneous observation of radiation budget and aerosol/cloud properties, additional information on particle size, phase, and vertical layer structure. Comprehensive radiation models are used to develop retrieval algorithms. This paper presents an overview of the science and technique in remote sensing and retrievals of atmospheric aerosols and clouds. High quality multi-spectral imagery, together with nadir propagating lidar measurements, acquired from high altitude aircraft in many field experiments are served as examples for discussion.