Can giant aerosol particles fail to help produce rain?

Numerical modeling studies have indicated that giant aerosol particles can expedite precipitation formation in small cumulus clouds, because their greater size requires less condensational growth prior to the onset of coalescence. It would therefore be expected that high concentrations of giant aerosol particles ingested into small cumulus would rapidly produce precipitation-sized drops, and perhaps cause a decrease in the cloud lifetime. However, data collected during the Rain in Cumulus over the Ocean (RICO) project, held in Antigua and Barbuda in December 2004 and January 2005, indicate that these expectations are only sometimes realized.

This presentation will report on observations from the NCAR C-130 aircraft and the NCAR Spol-Ka radar collected during RICO. Comparisons between estimated giant aerosol particles in clear air and radar echo evolution of small trade wind cumulus will be presented. Multiple cases will be discussed, including some in which weak radar echoes and long cloud lifetimes were observed on days with high giant particle concentrations. Possible factors that limited the development of precipitation in these cases, despite the presence of substantial numbers of giant aerosol particles, will be discussed.