Demonstration of a Cut-Cell Representation of 3D Orography for Studies of Atmospheric Flows over Very Steep Hills

Sarah-Jane Lock Institute for Climate and Atmospheric Science, University of Leeds, Leeds, United Kingdom

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Heinz-Werner Bitzer DWD, Offenbach am Main, Germany

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Alison Coals National Centre for Atmospheric Science, University of Leeds, Leeds, United Kingdom

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Alan Gadian National Centre for Atmospheric Science, University of Leeds, Leeds, United Kingdom

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Stephen Mobbs National Centre for Atmospheric Science, University of Leeds, Leeds, United Kingdom

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Abstract

Advances in computing are enabling atmospheric models to operate at increasingly fine resolution, giving rise to more variations in the underlying orography being captured by the model grid. Consequently, high-resolution models must overcome the problems associated with traditional terrain-following approaches of spurious winds and instabilities generated in the vicinity of steep and complex terrain.

Cut-cell representations of orography present atmospheric models with an alternative to terrain-following vertical coordinates. This work explores the capabilities of a cut-cell representation of orography for idealized orographically forced flows. The orographic surface is represented within the model by continuous piecewise bilinear surfaces that intersect the regular Cartesian grid creating cut cells. An approximate finite-volume method for use with advection-form governing equations is implemented to solve flows through the resulting irregularly shaped grid boxes.

Comparison with a benchmark orographic test case for nonhydrostatic flow shows very good results. Further tests demonstrate the cut-cell method for flow around 3D isolated hills and stably resolving flows over very steep orography.

Corresponding author address: Sarah-Jane Lock, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail: s.j.lock@leeds.ac.uk

Abstract

Advances in computing are enabling atmospheric models to operate at increasingly fine resolution, giving rise to more variations in the underlying orography being captured by the model grid. Consequently, high-resolution models must overcome the problems associated with traditional terrain-following approaches of spurious winds and instabilities generated in the vicinity of steep and complex terrain.

Cut-cell representations of orography present atmospheric models with an alternative to terrain-following vertical coordinates. This work explores the capabilities of a cut-cell representation of orography for idealized orographically forced flows. The orographic surface is represented within the model by continuous piecewise bilinear surfaces that intersect the regular Cartesian grid creating cut cells. An approximate finite-volume method for use with advection-form governing equations is implemented to solve flows through the resulting irregularly shaped grid boxes.

Comparison with a benchmark orographic test case for nonhydrostatic flow shows very good results. Further tests demonstrate the cut-cell method for flow around 3D isolated hills and stably resolving flows over very steep orography.

Corresponding author address: Sarah-Jane Lock, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail: s.j.lock@leeds.ac.uk
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