MMM SEMINAR NCAR
Mountain-Wave Momentum Flux in an Evolving Synoptic-Scale
Flow
Dale Durran, Chih-Chieh Chen
and Greg Hakim
Dept. of Atmospheric
Sciences, University of Washington
Stably stratified airflow over mountains produces gravity waves that transport momentum vertically. The divergence of mountain-wave momentum fluxes is known to produce an important drag on the large-scale atmospheric flow. Current estimates of orographic "gravity wave drag" have relied almost exclusively on steady-state representations of the mountain-wave response.
This talk will focus on the behavior of the mountain waves that arise as numerical solutions to an initial value problem in which a stagnant air mass over the topography begins to accelerate in a dynamically consistent three-dimensional synoptic-scale flow. The synoptic-scale flow over the mountain oscillates between zero and 20 m/s over a
meteorologically representative period of two days. At almost all times the magnitude and vertical profile of the momentum flux in the numerical solution is very different from than that which would be predicted using steady-state theory. In particular, the vertical
momentum flux is greatly enhanced during the accelerating phase of the synoptic-scale flow. The behavior of momentum flux in the time-dependent problem may be understood through ray tracing and the conservation of wave action.
Thursday, July 29 2004, 3:30
PM
NCAR-Foothills
Laboratory
3450 Mitchell Lane
Bldg 2, Rm 1022,
Auditorium