Towards an implementation of topography in a next-generation gravity-wave parameterisation

The Multi-Scale Gravity Wave Model (MS-GWaM) uses raytracing-based modelling that supports transient gravity-wave parametrisation. The state-of-the-art implementation of MS-GWaM in the upper-atmosphere ICON model solves the raytracing equations in three dimensions and accounts for background (non-orographic) and convective gravity-wave sources. Our work extends the capabilities of MS-GWaM to include orography gravity-wave sources, and we present methods and preliminary results towards this goal. Specifically, we first determine the spectral representation of the topography in each ICON grid cell via Fourier fitting. We then apply linear theory to obtain a representation of the bottom boundary for the raytracer. Finally, the bottom boundary serves as an initial condition for the raytracer-based parametrisation of orographic gravity waves. Preliminary results indicate that a judicious setup of this bottom boundary allows for an optimal tradeoff between computational efficiency and a sufficiently accurate representation of the underlying topography.