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Journal of Advances in Modeling Earth Systems


© 2016. The Authors. We investigate the potential impact of the local environment on rising parcels in a convective boundary layer. To this end, we use data from an LES simulation of a shallow convective cloud field to feed a parcel model with a range of different local environments, representative of the heterogeneous environment inside a shallow cumulus cloud layer. With this method we can study the statistics of an ensemble of rising parcels, but also the behavior of individual parcels. Through the use of a heterogeneous environment, the interactions between different parcels are indirectly represented. The method, despite its simplicity, allows closer investigation of mechanisms like parcel screening and buoyancy sorting that have frequently been proposed in cumulus parameterization. The relative importance of the entrainment formulation can be assessed, considering various classic entrainment formulations. We found that while the entrainment formulation does affect parcel behavior, the impact of the local environment is significantly more important in determining the eventual fate of the parcel. Using a constant entrainment rate can already explain much of the variation in termination heights seen in nature and LES. The more complex entrainment models then seem to act on top of this mechanism, creating second-order adaptations in the main distribution as established by the heterogeneity of the environment. A parcel budget analysis was performed for two limit cases, providing more insight into the impact of the local environment on parcel behavior. This revealed that parcel screening inside cumulus clouds can be effective in enabling parcels to reach greater heights.




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