The Impact of Transience in the Interaction between Orographic Gravity Waves and Mean Flow

A Lagrangian gravity wave parameterization [Multiscale Gravity Wave Model (MS-GWaM)] that allows for fully transient wave–mean-flow interaction and horizontal propagation is applied to orographic gravity waves for the first time. Both linear and nonlinear mountain waves are modeled in idealized simulations within the pseudoincompressible flow solver PincFlow. Two-dimensional flows over monochromatic orographies are 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) show that allowing for transience leads to a significantly more accurate forcing of the resolved mean flow. The parameterization is able to reproduce the transient forcing of linearly generated mountain waves that slowly propagate upward, in contrast to the instantaneous distribution of wave energy in classic parameterizations. At high altitudes, wave breaking induces a wind reversal that is captured by the transient parameterization but inhibited in steady-state simulations, due to the assumption of critical level formation. This shows that transience can have a substantial impact on the interaction between mountain waves and mean flow.