A GCM Investigation of Aerosol-Cloud-Radiation Interactions Using A-Train Satellite Data

While direct radiative forcing of aerosols has been extensively studied in recent years, the impact of semi-direct and indirect forcings on global and regional climate remains largely unknown. To date, quantification of the indirect effects of aerosols on climate remains a challenging problem. Inadequate understanding of the relationship between microphysical and dynamical processes contributes to large uncertainties in model simulations of the aerosol effects on clouds and climate, especially for ice clouds, due primarily to the lack of accurate global-scale observations. New data from NASA's A-Train constellation coupled with recent developments in climate modeling provides an unprecedented opportunity to advance the understanding of aerosol-cloud-radiation interactions and their climatic impact. The semi-direct effect is associated with the aerosol absorption of sunlight which leads to heating of the lower troposphere and reduces large-scale cloud cover in a magnitude that could be comparable to the aerosol direct and/or indirect effects. At present, the semi-direct effect is poorly understood and its role in cloud distribution and regional climate change remains unresolved. The objective of this study is to investigate the impact of dust aerosols on regional climate with a focus on the North Africa region, by examining the responses of the regional climate system to direct, semi-direct, and first indirect (with a focus on ice clouds) aerosol radiative forcings in the UCLA AGCM. Parameterization of the effective ice particle size in association with the aerosol first indirect effect based on ice cloud and aerosol data retrieved from A-Train satellite observations has been employed in climate model simulations. Offline simulations reveal that the direct solar, IR, and net forcings by dust aerosols at the top of the atmosphere (TOA) generally increase with increasing aerosol optical depth. When the dust semi-direct effect is included with the presence of ice clouds, positive IR radiative forcing is enhanced since ice clouds trap substantial IR radiation, while the positive solar forcing with dust aerosols alone has been changed to negative values due to the strong reflection of solar radiation by clouds, indicating that cloud forcing associated with aerosol semi-direct effect could exceed direct aerosol forcing. With the aerosol first indirect effect, the net cloud forcing is generally reduced in the case for an ice water path (IWP) larger than 20 g m-2. The magnitude of the reduction increases with IWP. GCM simulations show that changes in precipitation, cloud cover, and OLR patterns due to the overall effect of dust aerosols is similar to those due to the aerosol direct and semi-direct effects, inferring that aerosol radiative forcing produced by these two effects could be more important than the aerosol first indirect effect in the context of North Africa regional climate.