Chih-Chieh Chen, Gregory J. Hakim, and Dale R. Durran
Department of Atmospheric Sciences, University of Washington,Seattle, WA
Journal of the Atmospheric Sciences, 63, submitted.
The impact of transient mountain waves on a large-scale flow is
examined through idealized numerical simulations of the passage of a
time-evolving synoptic-scale jet over an isolated 3D mountain. Both
the global momentum budget and the spatial flow response are examined
to illustrate the impact of transient mountain waves on the large-scale
flow. Additionally, aspects of the spatial response are quantified by
potential vorticity inversion.
Nearly linear cases exhibit a weak loss of domain-averaged absolute momentum
despite the absence of wave breaking. This transient effect occurs
because, over the time period of the large-scale flow, the momentum
flux through the top boundary does not balance the surface pressure
drag. Moreover, an adiabatic spatial redistribution of momentum is
observed in these cases, which results in an increase (decrease) of
zonally averaged zonal momentum south (north) of the mountain.
For highly nonlinear cases, the zonally averaged momentum field shows a
region of flow deceleration downstream of the mountain, flanked by
broader regions of weak flow acceleration. Cancellation between the
accelerating and decelerating regions results in weak fluctuations in
the volume-averaged zonal momentum, suggesting that the
mountain-induced circulations are primarily redistributing
momentum. Potential vorticity anomalies develop in a region of wave
breaking near the mountain, and induce local regions of flow
acceleration and deceleration that alter the large-scale flow.
A ``perfect'' conventional gravity-wave-drag parameterization is
implemented on a coarser domain not having a mountain, forced by the
momentum-flux distribution from the fully nonlinear simulation. This
parameterization scheme produces a much weaker spatial response in the
momentum field and it fails to produce enough flow deceleration near
the 20 m \s1 jet. These results suggest that the potential vorticity
sources attributable to the gravity-wave-drag parameterization have a
controlling affect on the long-time downstream influence of the
mountain.