Satellite Monitoring of the First Indirect Aerosol Effect: Retrieval of the Droplet Concentration of Water Clouds

A model for the vertical variation of microphysical and optical properties of single-layer water clouds is used to design a procedure to obtain cloud droplet concentration from satellite cloud optical depth / effective radius retrievals. This method is applied to data from the MODIS instrument aboard the EOS-TERRA satellite platform over a region near Cape Grim, Tasmania over the Southern Ocean. At this location there are seasonally repeated well-described variations in cloud droplet concentration. The satellite observations show that there is a factor 2.5 difference in droplet concentration between the summer and winter seasons as measured over the four year period (Jul 2000 – July 2004), which agrees well with Cloud Condensation Nuclei concentrations observed at Cape Grim. Cloud droplet concentration is a key parameter to monitor the first indirect aerosol effect. This analysis demonstrate that the MODIS instrument on-board the EOS-TERRA and AQUA platforms can be applied in a strategy to monitor the indirect aerosol effect.