Can cloud droplet number increase with height?

As an air parcel ascends, its relative humidity increases until the parcel becomes supersaturated. The supersaturation continues to increase as the parcel ascends further, until the depletion of water vapor by the growing cloud droplets becomes faster than the increase of supersaturation due to cooling of the parcel, after which time the supersaturation has reached a maximum in the cloud. Although it is commonly thought that the maximum supersaturation, and thus largest droplet concentration, occurs just above cloud base, several research flights during the Rain In Cumulus over the Ocean (RICO) experiment showed otherwise. During these research flights, the maximum droplet concentrations were observed at higher altitudes in the small trade wind cumuli, far above cloud base.

This presentation will include the first three-dimensional cloud model simulation of the trade wind cumuli observed during the RICO field study on December 10, 2004. Condensation growth of the CCN observed below cloud base will be calculated along trajectories computed from the simulated cloud, and these drop size distributions will be compared to those observed during the RICO field study. Specifically, this study seeks to elucidate the influence of entrainment and mixing on the cloud droplet population, and to explain if entrainment and mixing can account for the increasing droplet concentration with altitude that was often observed in the trade wind cumuli during RICO.