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The most common option for numerical models of the atmosphere is to use model layers following the surface of the earth, using a terrain-following vertical coordinate. The present paper investigates the forecast of clouds and precipitation using the z-coordinate nonhydrostatic version of the Lokalmodell (LM-z). This model uses model layers that are parallel to the surface of the sphere and consequently intersect the orography. Physical processes are computed on a special grid, allowing adequate grid spacing even over high mountains. In other respects the model is identical to the nonhydrostatic terrain-following version of the LM, which in a number of European countries is used for operational mesoscale forecasting. The terrain-following version of the LM (LM-tf) is used for comparison with the forecasts of the LM-z. Terrain-following coordinates are accurate when the orography is shallow and smooth, while z-coordinate models need not satisfy this condition. Because the condition of smooth orography is rarely satisfied in reality, z-coordinate models should lead to a better representation of the atmospheric flow near mountains and consequently to a better representation of fog, low stratus, and precipitation. A number of real-data cases, computed with a grid spacing of 7 and 14 km, are investigated. A total of 39 real-data cases have been used to evaluate forecast scores. A rather systematic improvement of precipitation forecasts resulted in a substantial increase of threat scores. Furthermore, RMS verification against radiosondes showed an improvement of the 24-h forecast, both for wind and temperature. To investigate the possibility of flow separation at mountain tops, the flow in the lee of southern Italy was investigated.

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, highresolution 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.