| This dissertation combines aircraft in situ and airborne mm-wave cloud radar measurements to make contributions to one fundamental aspect of the study of cumulus clouds: the effect of turbulent mixing. The objective is to improve the understanding of processes of entrainment and detrainment of cumulus clouds and their impacts on ambient air. The method used is compositing, i.e. the characterization of typical patterns, and their variations, by combining numerous samples, in this case samples of shallow to mediocre cumulus clouds. Clear evidence is produced to prove the existence of horizontal vortex-ring structures commonly observed within the top of cumulus clouds. Significant lateral and vertical entrainment of ambient air into clouds is described. The lateral entrainment occurs through relatively small eddies and the typical lateral mixing length is 10-15% of the cloud diameter, whereas vertical entrainment mainly occurs through a vortex-ring concentration within the upper half of cloud. In the developing stage of cumulus clouds, the cloud-scale vortex-ring circulation at cloud top has a more erosive effect on clouds than laterally-mixing eddies because the former penetrates deeper and thus reduces the buoyancy more effectively than the latter. The detrainment signature of a life cycle of non-precipitating moist convection is described as well. The layer containing orographic cumuli clearly becomes enriched with moist static energy immediately downwind of the cumuli. This enrichment reduces potential instability but increases ambient humidity, which may facilitate successive cumulus growth. |
