The Influence of Transient Mountain Waves on a Localized Barotropic Jet

The impact of transient mountain waves on a large-scale flow is investigated through idealized modeling over a horizontally periodic domain in which a dynamically self-consistent synoptic-scale jet passes over an isolated three-dimensional mesoscale mountain. The depth-independent cross-mountain flow U accelerates from zero to 20 m/s back to zero in 50 hours. Consequently, the flow field undergoes a progression through the low-level blocking/flow-splitting regime into nonlinear and linear wave regimes during the first 25 hours. A reverse progression takes place in the following 25 hours.

The mountain-induced spatial response is presented by computing difference fields, defined as the departure from the flow fields of a control simulation without a mountain. For nonlinear cases, as revealed by the difference in zonal momentum, the large-scale flow response is characterized by a broad region of flow deceleration extending far downstream from the mountain. Bands of flow acceleration are found both north and south of the mountain. In nonlinear cases, potential vorticity anomalies are continuously generated by breaking gravity waves and are advected downstream by the large-scale flow. Despite the small scale of these PV anomalies, they may be inverted by using quasi-geostrophic balance to obtain an accurate representation of the large-scale response to gravity wave drag.

In this study, a ``perfect'' conventional gravity wave drag (GWD) parameterization is implemented based on the momentum flux distribution computed from the full nonlinear simulation. Results show that this parameterization scheme tends to induce a much weaker spatial response and, more importantly, it fails to produce enough flow deceleration near the core of the jet. This result implies that the consideration of momentum re-distribution in association with the balanced response may be required for a better GWD parameterization.