UNIVERSITY OF LEEDS Results III The basic Microscale Model has been extended to include moist dynamics - including the contribution from latent heat exchange Exploring a cut-cell representation of terrain in a microscale model Abstract Moves to increasingly high-resolution models give rise to more variations in the underlying orography being captured by the model grid. Consequently, high-resolution models must overcome instabilities associated with terrain-following (see Fig. 1a) approaches. This work further explores the capabilities of a cut-cell representation of orography for idealised orographically-forced and moist microscale flows. The model is based on terrain-intersecting coordinates (see Fig. 1b) and solves flow in the resulting cut-cells using a finite-volume approximation. The model has been designed for the purposes of very high-resolution simulations. Comparisons with benchmark orographic and moist test cases demonstrate very good results. Further tests show the potential for the cut-cell approach for stably resolving flows over very steep orography. Institute for Climate & Atmospheric Science SCHOOL OF EARTH AND ENVIRONMENT, UNIVERSITY OF LEEDS S. Lock, A. Coals, A. Gadian, S. Mobbs Microscale Model is a research model for very high-resolution studies - of grid-resolutions x ~ z < 100m. The project has focussed particularly on flows over steep and complex terrain. The model is 3- dimensional, nonhydrostatic and fully compressible, with prognostic variables u,v,w,’,’. Integration is computed with a time-splitting approach (based on Klemp & Wilhelmson, 1978) using 2nd-order leapfrog for the long timestep and a 1st-order forward-backward scheme for the short step. A 2nd-order centred scheme is used for spatial differencing on a staggered grid (Arakawa-C in the horizontal, Charney-Phillips in the vertical). The orographic surface is represented by a piecewise bilinear function, and a finite-volume approach is used to solve flows through the resulting cut-cells. Background Cut-cells were first explored in environmental modelling by Marshall et al. (1997) for representing ocean-bottom topography. Results showed a piecewise linear representation of the surface compared favourably with step representations, particularly for surfaces that show large variation at the grid-resolution. The cut-cell work was extended to a compressible atmospheric model by Bonaventura (2000) and results were seen to compare well with a traditional terrain-following model. Steppeler et al. (2002, 2006) further extended the work, implementing a 3D piecewise bilinear representation of orography in a full atmospheric model (COSMO-DE). Tests showed good comparison with the equivalent terrain-following model, and studies of real cases showed evidence of improved precipitation forecasts from the cut-cell approach. Model equations describe a 3D, nonhydrostatic, fully compressible system. Written in time-split form, with fast modes (acoustic and gravity) on the LHS and slow modes on the RHS, the equations take the form: Cut-cell approach is based on the method discussed in Steppeler et al. (2002, 2006). The orographic surface is represented by continuous piecewise bilinear surfaces, that intersect the model grid – Fig. 2. The orographic surface results in some cells that are cut – partially beneath the surface; partially above. Flow through the cut-cells is solved by Fig. 1: Vertical levels in (a) a terrain-following, and (b) a terrain-intersecting grid a) b) Fig. 2: Example orographic surface intersecting a grid- column, from Steppeler et al. (2006) using a finite-volume method to compute the divergence term in the pressure equation, using Gauss’s theorem over a volume V, bounded by surface area A: Applied to a cut grid-cell, the method requires the fluxes of the wind-field across the cell surfaces. For surfaces that coincide with the grid, the wind components are conveniently located (staggered grid), and there is zero flux across the orographic