Investigating transport, mixing and the formation of ice in cumuli with gaseous tracer techniques

Abstract

Applications of tracer techniques using insoluble sulfur hexafluoride (SF 6) to studies of transport, mixing and the activation of silver iodide (AgI) aerosols in cumuli are presented. One cumulus was treated with SF 6 and the aerosol near the cloud top (−13.5°C), in a region of little vertical transport. Up to 24% of the potential nuclei produced measurable ice particles 7 min after treatment, in accord with the results of recent laboratory measurements of activation of this aerosol by contact nucleation. A second cumulus was treated at the cloud base with SF 6 and the aerosol. The materials were transported to and mixed through the upper regions of the cloud. Ice particles evidently formed near the cloud top (estimated cloud top temperature −13°C). Only low concentrations of natural ice were found in untreated regions of the cloud. In the treated regions the ice particle concentrations in the cold, upper part of the cloud and in downdrafts at lower levels were consistent with the concentrations of AgI nuclei estimated from the tracer measurements. At lower levels of the cloud the materials were not so well mixed, the most concentrated regions being found on the upshear side of the cloud and dilute regions down shear. Mid and upper level ice concentrations were greatest in downdrafts on the downshear side, suggesting that the downdraft was important in transporting the ice to lower levels of the cloud.