SEMINAR SERIES and COLLOQUIA

We show that the traditional view of transport by shallow cumulus clouds needs important refinement. On the basis of a straightforward geometrical analysis of Large Eddy Simulation results of shallow cumulus clouds, we conclude: 1) that the upward mass transport by clouds is strongly dominated by regions close to the edge of clouds rather than by the core region of clouds; and 2) that the downward mass transport is dominated by processes just outside the cloud. The latter finding contradicts the accepted view of a uniformly descending dry environment. We therefore advocate a refined view which distinguishes between the near-cloud environment and the distant environment. The near-cloud environment is characterized by coherent descending motions, whereas the distant environment is rather quiescent and plays no significant role in vertical transport. These ideas are tested against clouds observations from the Rain In Cumulus over the Ocean (RICO) field campaign. We discuss the relevance of the refined view for two areas: cloud lateral entrainment and dispersion of atmospheric compounds in a cumulus field. With regard to dispersion it is predicted that a tracer gas released in the distant environment will hardly diffuse and may stay there for a very long time. We show results obtained in a Virtual Reality Environment where we can study the dispersion of virtual dust released at arbitrary locations in the LES flow-field.

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