Far-Field Mountain Waves Radiated from Nonlinear Orographic Flow Regimes

Weak atmospheric flows in mountainous regions tend to be highly nonlinear as low-level streamlines are deflected around mountains, rather than flowing over them. Additionally, nonlinearities common in these flows, such as upstream stagnation, low-level mountain wave breaking, and wake regions, can affect the morphology of the mountain wave fields that propagate to higher altitudes to potentially influence the middle atmospheric circulation. These effects must be parameterized in middle atmosphere models, but the wave responses in the presence of complex three-dimensional topography and realistic flows are not well understood. Here we investigate these issues with a novel method that couples fully nonlinear simulations of low Froude-number flows over three-dimensional mountains using the Weather, Research and Forecasting (WRF) model with linear mountain wave solutions from a Fourier-synthesized Ray (FR) model. The lower boundary of the FR model is initialized with a horizontal cross-section of the vertical velocity field from the WRF simulation at various heights above the mountain, and then forward traced to higher altitudes. We find that, even in presence of strongly nonlinear surface flow conditions, the linear FR model produces similar wave fields to WRF when initialized using the WRF solution at altitudes above the region where low-level wave breaking occurs (as characterized by moderate or intense levels of turbulent kinetic energy in the WRF solution). We then use the FR model to backtrace these upper-level WRF wave fields to the dividing streamline altitude near the surface to infer the “effective topography” responsible for forcing the far-field mountain waves in various low-Froude number flow regimes. Results are shown for several examples of constant flow over idealized three-dimensional mountains for a range of low Froude-number values. Preliminary implications for mountain wave parameterizations and plans for further work are discussed.