Validation of an orographic source in a Lagrangian gravity wave parameterization

Many operational gravity wave parameterizations rely on the single column and steady state approximations, thus neglecting horizontal propagation and transience. Recent studies indicate that these assumptions can lead to faulty predictions, motivating the development of more complex models. MS-GWaM, a Lagrangian gravity wave parameterization that has been in development for about a decade, is one such model that is based on a multi-scale WKB theory allowing for both transience and horizontal propagation. So far, it has been validated mainly for non-orographic gravity waves, however, a simple orographic source has already been implemented in a test version of the model, which is coupled to a pseudo-incompressible flow solver (PincFlow). The present study investigates that source in an idealized setting. For this purpose, the model is adjusted to PincFlow's recently implemented terrain-following coordinate system. In addition, the orographic source is supplemented with a blocked flow drag and a wave amplitude reduction that accounts for blocked layer formation. These are derived from background flow tendencies and gravity wave momentum fluxes in highly idealized, wave-resolving simulations. The model is then tested against the latter, using both the transient configuration and a newly implemented steady state mode. The comparison shows that allowing for transience results in a more accurate forcing of the resolved mean flow, especially when the orographic source is changing in time.