Research - Aerosols and Climate -Indirect Effect

Aerosol Indirect Effect Studies using Multi-sensor Satellite data sets

Modeling and observational studies indicate that both natural and anthropogenic aerosols affect cloud processes. Certain types of aerosols (e.g. sulfates) are water soluble and act as excellent CCN for cloud liquid water droplets resulting in an increase in available CCN for cloud formation. If atmospheric conditions such as temperature, water content, and vertical motion are held constant, increasing CCN will result in smaller water droplets compared to a less polluted region, increasing cloud albedo and reflecting more solar radiation back into space (First indirect effect. Research has shown that observed that liquid water drop effective radius (Rc) decreased by 32% when Aerosol Optical thickness (AOT) increased from 0.03 to 0.43 over oceanic regions between 5°N and –20°S. These changes were attributed to an increase in smoke aerosols from the Amazon (and central Africa) owing to an increase in biomass burning. The increase in albedo due to smaller drop sizes produced a cooling at the top of the atmosphere of –1.5 Wm-2. However, this relationship does not always occur and other studies have observed a positive correlation between AOT and Rc for highly polluted regions in the Arabian Sea. 

A decrease in cloud droplet size has the additional effect of delaying the onset of collision and coalescence in warm clouds, reducing precipitation efficiency and increasing the lifespan and the areal coverage of the cloud (second indirect effect).

Reducing precipitation efficiency also acts to increase water loading, leading to an increase in cloud liquid water path (LWP) and a corresponding increase in cloud thickness, complicating the identification of the first indirect effect in observations. Both the first and second indirect effects act to cool the atmosphere, thereby offsetting warming due to greenhouse gases. However, absorbing aerosols such as soot from biomass burning can suppress cloud formation by warming the atmosphere, increasing evaporation of water droplets and also increasing atmospheric stability (semi-direct effect).

We have studied the effect of aerosols on clouds AND precipitation using various satellite data sets.
Key references

1) Jones, T.A., S.A. Christopher, and J. Qaas, A six year satellite-based assessment of the regional variations of aerosol indirect effects, Atmos. Chem. Phys., 9, 4091-4114, 2009

2) Jones, T. A., and S. A. Christopher (2008), Seasonal variation in satellite-derived effects of aerosols on clouds in the Arabian Sea, J. Geophys. Res., 113, D09207, doi:10.1029/2007JD009118

3) Jones, T.A., and S. A. Christopher, Statistical properties of aerosol-cloud-precipitation interactions in South America, Atmospheric Chemistry and Physics, 10, 2287-2305, 2010.