Abstract : 2G.8
A generalized SLEVE Coordinate allowing for thin Near-Surface Levels

Daniel Leuenberger, Marcel Koller, Christoph Schär

Smooth vertical coordinate levels in atmospheric models can considerably reduce numerical errors in the simulation of flow over complex topography. The recently proposed Smooth LEvel VErtical (SLEVE) coordinate uses a scale-dependent exponential vertical decay of the topography in the computation of the 3D model level heights and allows for a rapid transition of terrain-following levels to smooth and eventually quasi-horizontal levels in the middle and upper troposphere. One disadvantage of the SLEVE formulation is that the thickness of the lowest model levels is considerably reduced above mountain tops, thereby potentially leading to stability problems.

In this paper we propose a new SLEVE formulation with a modified vertical decay of topography leading to a more uniform level thickness near the ground while preserving the fast transition to smooth levels in the upper atmosphere. Using numerical simulations of flow over complex topography, we demonstrate the feasibility of this approach. As a result, the new formulation allows for generally thinner near-surface levels and thus a better resolution of the boundary layer compared to the SLEVE coordinate, while retaining its advantages over classical sigma-coordinates in upper levels.