Determination of entrainment and encroachment at the top of a convective boundary layer using Lagrangian particles
Responsible: Helge Knoop?
Project type: bachelor thesis
Duration: May 2013 - September 2013

Entrainment and encroachment processes have a major influence on the characteristics of a cloudless convective boundary layer. Entrainment is the mixing process at the top of this layer, and encroachment describes the thermodynamic growth of this layer. The underlying physics are, however, not well understood. A reliable and realistic entrainment parameterization is of great importance for numerical weather prediction models and atmospheric dispersion modeling. In this study, a new Lagrangian-particle-based entrainment rate determination method is developed. It is designed to provide the determination of three dimensional resolved entrainment rates from large-eddy simulations. The method tracks single Lagrangian particles, which are released inside the free atmosphere and represent a specific air volume. A set of criteria is used to identify the exact moment, when this particle is entrained into the turbulent boundary layer. The atmospheric properties, stored in the particles, are available for analysis. The method is able to resolve small scale entrainment characteristics and local fluctuations of the entrainment rate.
A large-eddy simulation of a cloudless convective boundary layer is used to test the new method and briefly validate the results. A good agreement with results determined by a known entrainment parameterization, is observed. Additionally, low frequent fluctuations of the entrainment rate, determined by the new method, are compared with fluctuations of the turbulent kinetic energy in the simulation. The good correlation between these two quantities suggests a high detection reliability of the new method.

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