A new shape-preserving filter combined with the LMCSL scheme in Enviro-HIRLAM – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2009 > Master Thesis by Joaki...

A new shape-preserving filter combined with the LMCSL scheme in Enviro-HIRLAM

Master Thesis by Joakim Refslund Nielsen

The talk will be in Danish

The traditional semi-implicit semi-Lagrangian method is a popular numerical approach used in many ACTM and NWP models. The method offers accurate and efficient predictions but suffers from severe lack of local mass conservation. This deficiency has recently been overcome by the introduction of inherently locally mass conserving schemes, such as the newly proposed LMCSL scheme, providing the basis for future dynamical cores in atmospheric modeling. Although, these new schemes fulfill many desirable properties to a satisfactory extend, improvements are still neccessary.

Combining the LMCSL scheme with a new locally mass conserving and shape-preserving anti-diffusive filter has been proposed in one and two dimensions. The resulting scheme provides monotone and positive definite solutions and in addition enhances accuracy especially near sharp gradients.

The aim of this project is to combine the new locally mass conserving and shape-preserving anti-diffusive filter with the LMCSL scheme in a three dimensional NWP model provided by DMI. Modifications of existing semi-implicit discretizations of the governing equations for the CISL scheme, will provide the framework for the new dynamical core. The filter performs posteriori corrections of the prognostic fields through controlled redistributions of mass towards anti-diffused target values without demolishing the locally conserving property of the original scheme. The combined scheme has been tested within the European Tracer experiment (ETEX-1) showing good results.

Supervisor: Eigil Kaas, Niels Bohr Institutet, Copenhagen University