Transient mountain waves in an evolving synoptic-scale flow and their interaction with large scales
Characteristics of transient mountain waves and their impact on the large-scale flow are examined through idealized numerical simulations during the passage of a time-evolving synoptic-scale flow over an isolated 3D mountain of height h. The dynamically consistent synoptic-scale flow U accelerates and decelerates with a period of 50 h; the maximum wind arrives over the mountain at 25 h. The synoptic-scale static stability N is constant, so the time dependence of the nonlinearity parameter, epsilon(t) = Nh/U(t), is symmetric about a minimum value at 25 h.The evolution of the vertical profile of mountain-wave induced momentum flux and the cross-mountain drag shows substantial asymmetry about the mid-point of the cycle even though epsilon is symmetric. Larger downward momentum fluxes are found in the mid and upper troposphere when the cross-mountain flow is accelerating and this basic asymmetry can be understood through the WKB ray theory. For mountains high enough to preserve a moderate degree of nonlinearity when the incident flow is strongest, a higher drag state tends to form during the accelerating phase.The impact of transient mountain waves on the synoptic-scale flow is diagnosed through momentum budgets and the spatial flow response. It is found that domain-averaged deceleration can be induced solely due to transience even when no wave dissipation takes place. For the h = 1.5 km case, it is found that a broad region of flow deceleration exists far downstream of the mountain at 50 h which significantly slows down the 20 m s-1 jet of the synoptic-scale flow. It is also found that a large portion of the spatial response can be explained by potential vorticity (PV) dynamics.A "perfect" conventional gravity wave drag (GWD) parameterization is implemented based on the momentum flux distribution computed from the full nonlinear simulation. It is found that this parameterization scheme tends to produce much weaker spatial response and, more importantly, it fails to produce enough flow deceleration near the 20 m s-1 jet. It is suggested that the consideration of momentum re-distribution in association with the balanced response may be required for a better GWD parameterization.
- Atmospheric sciences