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Dynamics of the Cloud-Environment Interface and Entrainment in Small Cumuli: Two-Dimensional Simulations in the Absence of Ambient Shear

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  • 1 National Center for Atmospheric Research, Boulder, Colorado 80307
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Abstract

We employ a two-dimensional numerical model with interacting nested domains to simulate the evolution of a small nonprecipitating cumulus cloud in the absence of shear. Grid nesting permits the use of a realistic boundary layer forcing to initiate cloud growth and, at the same time, the specification of very high spatial resolution in the vicinity of the cloud. The finest mesh employed in this study (5 m) gives about 160 points across the base of the cloud. Initially, the model produces a cloud which has a smooth upper surface. About eight to nine minutes after the onset of condensation, nodes appear on the upper cloud boundary. These nodes have a characteristic tangential length scale which is small compared to the width of the cloud base. In one of our simulations, a down-draft forms above the center of the cloud top and penetrates into the interior of the cloud. The entrainment of this unsaturated air reduces the liquid water content of the cloud below the adiabatic value and curtails growth of the cloud. In the present series of simulations, a penetrative downdraft is observed to form only in a cloud which develops a particular configuration of boundary nodes, a characteristic which is probably due to the assumed environmental conditions. Experiments were performed to assess the role which eddy mixing plays in the formation of the nodes and the entrainment process. It was found that while eddy mixing does not significantly affect the early nodal development, it does tend to inhibit the penetration of the downdraft. Our simulations indicate that entrainment in a growing cumulus is a well-ordered laminar phenomenon driven by inviscid dynamical processes rather than a turbulent phenomenon driven by mixing.

Abstract

We employ a two-dimensional numerical model with interacting nested domains to simulate the evolution of a small nonprecipitating cumulus cloud in the absence of shear. Grid nesting permits the use of a realistic boundary layer forcing to initiate cloud growth and, at the same time, the specification of very high spatial resolution in the vicinity of the cloud. The finest mesh employed in this study (5 m) gives about 160 points across the base of the cloud. Initially, the model produces a cloud which has a smooth upper surface. About eight to nine minutes after the onset of condensation, nodes appear on the upper cloud boundary. These nodes have a characteristic tangential length scale which is small compared to the width of the cloud base. In one of our simulations, a down-draft forms above the center of the cloud top and penetrates into the interior of the cloud. The entrainment of this unsaturated air reduces the liquid water content of the cloud below the adiabatic value and curtails growth of the cloud. In the present series of simulations, a penetrative downdraft is observed to form only in a cloud which develops a particular configuration of boundary nodes, a characteristic which is probably due to the assumed environmental conditions. Experiments were performed to assess the role which eddy mixing plays in the formation of the nodes and the entrainment process. It was found that while eddy mixing does not significantly affect the early nodal development, it does tend to inhibit the penetration of the downdraft. Our simulations indicate that entrainment in a growing cumulus is a well-ordered laminar phenomenon driven by inviscid dynamical processes rather than a turbulent phenomenon driven by mixing.

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