Transient Mountain Waves and Their Interaction with Large Scales

The impact of transient mountain waves on the large-scale flow is investigated through idealized modeling over a horizontally periodic domain in which a barotropic synoptic-scale system passes over an isolated three-dimensional mesoscale mountain. The cross-mountain flow U accelerates from zero to 20 m/s at 25 hours and decelerates back to zero at 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 influence of mountain-induced perturbations on the synoptic-scale flow is first diagnosed through global momentum budgets. It is found that the downward momentum flux in the upper troposphere could temporarily, before the maximum background flow arrives at the mountain, exceed the magnitude of pressure drag, which leads to mean flow acceleration. Nevertheless, the magnitude of the time-integrated pressure drag is still greater than that of the time-integrated momentum flux in the upper troposphere. Therefore, there is mean flow deceleration after a period of 50 hours as revealed by the domain-averaged absolute momentum budget.

The mountain-induced spatial response is also 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. This band of flow deceleration is accompanied by flow acceleration both north and south of it. 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 anomalies, they may be inverted by using quasi-geostrophic balance to obtain an accurate representation of the large-scale response to gravity wave drag.