Evidence for the aerosol indirect effect in shallow cumuli

Increases in the number concentration of cloud drops resulting from anthropogenic aerosol loading (commonly called the first aerosol indirect effect) have been well documented in field studies of stratocumulus clouds. However, observations of these effects have not been reported in shallow continental clouds. Observations during the US Department of Energy's Atmospheric Science Program Cumulus Humilis Aerosol Processing Study (CHAPS) examined the number concentration of cloud droplets within shallow cumuli as a function of the interstitial (non-activated) particle concentration and updraft velocity. During CHAPS observations were made inside shallow clouds using instruments to measure the size distribution of both cloud drops and interstitial particles, as was well as a gust probe that provided concurrent measurements of the cloud-scale updrafts and downdrafts. Elevated carbon monoxide levels were found to be associated with an increase in the total number of interstitial particles, indicating an anthropogenic source of these particles. Furthermore, under these conditions (elevated CO, elevated number of particles) the number of cloud droplets was generally large as well. These results are consistent with predictions of the first aerosol indirect effect and support this hypothesis as it applies to cumuliform clouds common over the continent throughout the summer.