GACP Projects
Satellite Study of the Smoke Indirect Radiative Forcing
Lorraine A. Remer, Co-PI
Yoram J. Kaufman , Co-PI
Graham Feingold, Co-I
Eric Vermote, Co-I
Nazmi El Saleous, Co-I
Abstract: The magnitude of the aerosol indirect radiative forcing of climate, in which aerosols can affect the earth's climate by modifying cloud microphysics and altering cloud albedo and cloud fraction, is the largest unknown in estimating the total anthropogenic forcing that causes climate change. Due to it's complex geographic and seasonal variability, aerosol forcing was suggested as a possible fingerprint to the response of the climate system to human impact. Not much observational evidence of the indirect aerosol effect exists and existing observations present a confusing picture of partial answers and contradictions. Previous studies using AVHRR satellite data in the biomass burning regime of Brazil were successful in identifying the indirect effect on a regional scale and suggesting the intervention of an additional controlling factor - water vapor. Satellite data provide a large statistical sampling of the aerosol effect on clouds, an advantage in sorting between the many factors affecting the properties of individual clouds. We propose a study that would continue in the same spirit as these previous AVHRR studies, using three data sets with different spatial resolutions. The range of spatial resolutions spans the 50 m of the ER-2 MODIS Airborne Simulator to the 1 and 4 km of AVHRR and later EOS-MODIS which will be launched in the summer of 1998. We plan to use several proven retrieval methods to study the aerosol effect on clouds. The analysis will be applied independently to data from several years and locations. The study will concentrate on smoke aerosol because it is this aerosol type that presents the most established link between optical thickness measured by satellite and the cloud condensation nuclei (CCN) that influence cloud microphysics. The recently performed SCAR-B (Smoke Cloud And Radiation-Brazil) experiment provides a wide body of knowledge on the smoke properties that will be used in this study. The proposed study will attempt not just to document the indirect effect when it occurs but also to understand the factors that control and modify the aerosol effect on cloud droplet size, cloud albedo and possibly also cloud coverage. To this end, we plan to use model simulations of the interactions of aerosol and clouds in the presence of varying atmospheric conditions (e.g. water vapor concentration and temperature profile).
The expected results will include comprehensive analysis of the effect of smoke on clouds in South America and other regions. The impact of water vapor, other atmospheric parameters, cloud size and type on the indirect aerosol forcing of climate will be derived empirically from the satellite data and understood using simulations by models. The intended results will also include an algorithm that can be applied to any AVHRR data set and run independently in order to generate a comprehensive data base of cloud macro and microphysical properties, aerosol optical thickness and precipitable water vapor open to the wider community of climate researchers.
