First Large Scale Demonstration of Aerosol-Cloud Interactions Increasing Cloud Fraction

April Highlights

Large aerosol plume (A)  results from SO2 degassing out of the Big Island, Hawaii. Enhancement of cloud optical depth (B), decrease of cloud droplet size (C) and increase of cloud fraction (D) as result of trade cumulus clouds interacting with volcanic aerosol (A). For the first time we have evidence of large-scale increase in cloud fraction (D) due to aerosol-cloud interaction.

June 1, 2011 — Click on image to enlarge.

Sulfur dioxide gases vented out of volcanoes can turn into tiny sulfate particles in the atmosphere, called aerosols. Aerosols are an essential ingredient for cloud formation and their amount can significantly change cloud properties. Kilauea is a volcano on the Big Island of Hawaii and it was constantly emitting sulfur dioxide gases during the summer of 2008. The resulting aerosol particles are observed from MODIS to form a large-scale plume (Figure 1A) downwind of the Hawaiian Islands. We show that these aerosol particles decrease cloud droplet size, increase cloud brightness, and ultimately enhance cloud fraction. For the first time we are able to observe large-scale increase in cloud fraction resulting from aerosol-cloud interactions. This is only possible with observations by a suite of instruments onboard the NASA A-Train satellites. The theory and observational evidence are detailed in Yuan et al. (2011) and can be summarized in the following chain reaction: aerosols increase the number of cloud droplets and decrease their sizes, suppress drizzle formation, make clouds brighter and larger and finally increase cloud coverage. Direct satellite observations suggest as much as 20Wm-2 more solar energy is reflected back to space as a result of aerosol increasing cloud brightness and coverage.