2D transient parameterization of gravity waves generated above an isolated mountain range

Most operational gravity-wave parameterizations use single-column and steady-state approximations, thus neglecting horizontal propagation and transience. Recent studies indicate that these simplifications can lead to inaccurate predictions. Orographic gravity waves, e.g., can propagate over substantial horizontal distances, leading to the deposition of momentum far from their sources. The neglect of this could be a cause of regional momentum-flux deficits in atmospheric models, e.g. downstream of the Andes. Moreover, the variability of low-level winds can make mountain-wave generation a highly transient process, challenging the legitimacy of the steady-state approximation. This motivates the development of more complex models.

  MS-GWaM is a Lagrangian gravity-wave parameterization that is based on a multi-scale WKB theory allowing for both transience and horizontal propagation. In a previous study (Jochum et al., 2025), it was used in simulations within the idealized atmospheric flow solver PincFlow to investigate its ability to correctly describe the interaction between orographic gravity waves and a large-scale flow. 2D flows over periodic monochromatic orographies were considered, using MS-GWaM either in its fully transient implementation or in a steady-state implementation that represents classic mountain-wave parameterizations. Comparisons of wave-resolving simulations (not using MS-GWaM) and coarse-resolution simulations (using MS-GWaM) showed that allowing for transience leads to a significantly more accurate forcing of the resolved mean flow. The present study supplements MS-GWaM (within PincFlow's successor PinCFlow.jl) with a new blocked-layer scheme and continues the investigation with the more realistic case of an isolated 2D mountain range, where the impact of upstream blocking and horizontal propagation increases substantially, resulting in a more complex wave-mean-flow interaction. The blocked-layer scheme uses a relatively simple approach to blocking that is consistent with MS-GWaM's spectral representation of the unresolved orography. Its two parameters are calibrated via Ensemble Kalman Inversion, using a wave-resolving simulation as reference. The results show that the inclusion of this scheme yields a slightly improved forcing of the mean flow.

References

Jochum, F., Chew, R., Lott, F., Voelker, G. S., Weinkaemmerer, J., and Achatz, U. (2025). The impact of transience in the interaction between orographic gravity waves and mean flow. Journal of the Atmospheric Sciences.