  A Lagrangian gravity-wave parameterization (MS-GWaM, Multi-Scale Gravity-Wave
Model) 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 pseudo-incompressible 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
model is able to reproduce the transient forcing of linearly generated mountain
waves that slowly propagate upwards, 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 model
but inhibited in steady-state simulations, due to the assumption of critical
level formation. This shows that transience can have a substantial impact in
the interaction between mountain waves and mean flow.
