Entrainment and detrainment in numerically simulated cumulus congestus clouds. Part III: Parcel analysis

This paper is the third in a three-part series in which a three-dimensional numerical cloud model is used to simulate cumulus congestus clouds. The au thors conduct a detailed parcel trajectory and conserved variable analysis of the modeled clouds, with the principal goal of understanding the mechani sms associated with entrainment and detrainment. At any point in their lifetime each of the modeled clouds contains multiple thermals that become detached from the boundary layer as they ascend. Undi lute regions of subcloud air occur within the simulated clouds at all level s up to the cloud top. In the upper portion of the clouds, such air is foun d within small (compared with the overall width of the cloud) thermals that are continually eroding yet vigorously ascending. Such thermals are respon sible for most of the entrainment and detrainment. Environmental air entrai ned by ascending thermals is shed in the wake of the thermal, which contain s dilute cloud-base air moving at low velocities. There is no evidence for thermals ascending through the remnants of their predecessors as a Favored means for new cloud growth. The: source of entrained air within both updraf ts and downdrafts is typically a few hundred meters above the observation l evel (although there is a tendency for updrafts at the highest levels to en train air from just below that level). Undilute cloud turrets tended to overshoot their level of neutral buoyancy by a considerable distance. Condensate loading triggers the collapse of ind ividual turrets, with additional reductions in buoyancy resulting from the evaporative cooling due to entrainment as well as the transport of entraine d environmental air upward. Strong, narrow downdrafts develop along the top and edges of overshooting turrets. These downdrafts are often marginally s aturated (which would be the most dense mixture of two air masses) and are composed of a mixture of cloud-base and cloud-top air. They descend to mid levels within the modeled clouds before being detrained laterally.