Aerosol Radiative Forcing and Climate Response: A Regional Focus

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Regionally, anthropogenic aerosols can lead to large reductions in the surface solar irradiance, a corresponding increase in atmospheric solar heating, stronger low-level inversions, suppression of rainfall due to the direct microphysical effect, and less efficient removal of pollutants. These aerosol effects, in addition to altering the temperature distribution, can also lead to a weaker hydrological cycle, a major environmental issue for the 21st century. The overall goal of this project is to understand the above-mentioned aerosol effects. The specific objectives are twofold:1) To quantify the direct and the indirect aerosol radiative forcings on regional scales by integrating in-situ, surface and satellite observations with a comprehensive Monte-Carlo Aerosol-Cloud Radiation (MACR) model that accounts for hybrid mixing of inorganic, organic and black carbon species. MACR will also account accurately for aerosol forcing (direct and first indirect) in cloudy skies. These estimates will also be used to improve the treatment of aerosol radiative effects and aerosol-cloud interactions in the NCAR Community Climate Model (CCM). The focus will be on regional Aerosol Hot Spots, which contribute the most to the global mean anthropogenic aerosol loading including the Asian, African and N. American regions. The selected regions coincide with regions where field observations are available.2) To use the regional aerosol forcing as input to the NCAR CCM/Climate System Model (CSM) and estimate impacts of the forcing on climate and the hydrological cycle. Our focus will be primarily on the Asian aerosols which contribute as much as 3O~5O% of the global anthropogenic aerosol optical depth. The climate model will be run both in prescribed-surface sea temperature mode (CCM) as well as in fully coupled mode (CSM). So far, aerosol has been portrayed simply as counteracting greenhouse gas induced arming, and its counteracting extent has been the primary concern. The inclusion of absorbing carbonaceous aerosols is likely to challenge this traditional view substantially. Preliminary CCM experiments with the Indian Ocean Experiment (INDOEX) absorbing aerosols reveal that the South Asian haze regionally cools the surface, shifts the inter-tropical convergence zone northwards and remotely suppresses the convection in the equatorial western Pacific with implications to El Nino-Southern Oscillation cycles.The proposed research builds on the infrastructure developed under NSF Center for Clouds Chemistry and Climate (C4) and INDOEX, including: C4 Data Integration system (CIDS); MACR; Satellite aerosol retrieval based on chemical data; three dimensional aerosol assimilation model; Beowulf cluster computers; and the network of collaborators developed through INDOEX. An array of ground based, airborne and satellite observations will be used.The aerosol forcing and climate response results generated by this work should contribute directly to future national and international climate impact assessments. The aerosol forcing from this work will serve as an independent estimate from that provided by global models. It also should lead to significant improvements in GCM treatment of aerosol forcing.