Aircraft Observations of Transport and Diffusion in Cumulus Clouds

Abstract

A gaseous tracer, sulfur hexafluoride, was used to follow the path of two different AgI cloud seeding aerosols in cumulus clouds. The materials were released at cloud base or midlevels. Plumes sampled at midlevels were found to be relatively narrow and embedded within updrafts or downdrafts; relatively high concentrations of the tracer were observed in some downdrafts. Plumes with diameters comparable to the cloud diameters were found in the upper 20% of the clouds. These observations suggest only limited dispersion of the plumes in the clouds, with greater mixing occurring at cloud top. Similar behavior of the in-cloud plume is observed in results from a two-dimensional, numerical cloud model used to simulate the introduction of seeding materials into convective clouds. Observations of the ice crystal production rates are consistent with the results of recent laboratory findings concerning the properties of the seeding agents. The usefulness of this tracer technique in studying transport, diffusion and ice activation in cumulus clouds is discussed.