@phdthesis{Kadasch2016, author = {Kadasch, Eckhard}, title = {Controlling entrainment in large-eddy simulation of stratocumulus clouds}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-40393}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {A front-tracking algorithm for large-eddy simulation (LES) is developed to untangle the numerical and physical contributions to entrainment in stratocumulus-topped boundary layers. The front-tracking algorithm is based on the level set method. Instead of resolving the cloud-top inversion, it is represented as a discontinuous interface separating the boundary layer from the free atmosphere. The location of the interface is represented as an isosurface of an evolving marker function the evolution of which is governed by an additional transport equation. The algorithm has been implemented in an existing LES code based on the anelastic approximation of the Navier-Stokes equations. The original LES algorithm is verified against direct-numerical simulation (DNS) data of an idealized two-dimensional cloud-top mixing layer. For this, the subgrid-scale model of the LES code was replaced by a constant molecular viscosity in order to focus on numerical errors only. A grid convergence study confirmed the anticipated global second-order rate of convergence and the convergence to the DNS solution. The slower convergence of the LES code as compared to the higher-order DNS yielded leading-order errors in the mixing layer growth at the coarsest resolutions, which were finer still than typical LES resolutions. The front-tracking algorithm is verified by LESs of two different convective atmospheric boundary layers: the smoke cloud, a solely radiatively driven boundary layer, and a stratocumulus-topped boundary layer based on data from the DYCOMS II field study. Specifying zero entrainment, it was shown that entrainment in LES can be controlled effectively by the front-tracking algorithm. The algorithm drastically reduces entrainment errors and reduces dependencies of the solution to numerical parameters such as the choice of flux-limiters and grid resolution.}, subject = {Entrainment; Large-eddy simulation; Level set method; Stratocumulus; Atmospheric boundary layer; Atmosph{\"a}rische Grenzschicht; Entrainment; Large Eddy Simulation; Level Set Methode; Stratocumulus; Kumulus; Atmosph{\"a}rische Grenzschicht; LES }, language = {en} }