9530662 Durran Mountains exert a profound influence on the earth's weather and climate. The Principal Investigator will investigate two types of terrain-induced atmospheric phenomena through a series of numerical simulations. One focus of this effort will be to investigate the three-dimensional response of the cross-mountain flow to mountain wave generation and breakdown. The second focus will be to determine the relative importance of gap winds and downslope winds in the generation of strong orographically-forced surface winds. Atmospheric scientists have long been aware that orographically generated gravity waves may exert an important drag on the atmosphere. Early discussions of this phenomena date back to the late 1950's. Continued interest in gravity wave drag (GWD) has been fueled by experiments with high-resolution general circulation models, which suggest that the inclusion of GWD parameterizations in such models can significantly improve the simulated climatology of the zonally averaged westerly winds and the surface pressure fields. GWD parameterizations are now standard features in most high-resolution general circulation and medium-range weather forecast models, yet the nature of the mesoscale atmospheric response to GWD forcing remains largely unexplored. Recent results obtained by the Principal Investigator under prior NSF support suggest that the mean-flow deceleration that develops in response to gravity wave drag is spread over a surprising large spatial domain upstream and downstream from the mountain. These results also suggested that a different diagnostic variable, the pseudomomentum, could provide a better description of the local response to gravity wave drag. The Principal Investigator's previous results were obtained using a high-resolution two-dimensional numerical model. These investigations will be extended to three dimensions using a newly developed 3D nested-grid model. As part of the proposed research, the Principal Investigator will also derive expressions for the pseudomomentum in 3D compressible stratified flow using Hamiltonian fluid dynamics. Strong surface winds can be generated by the interaction of the synoptic-scale flow and topography through two different mechanisms: gap winds and downslope winds. Gap winds are produced when air is forced through a narrow break in a mountain barrier. Examples of this occur in the Columbia River gorge and in the mistral, which blows through the Rhone valley of France. Strong downslope winds may be generated when the air flows across mountains without significant cross-mountain gaps; examples include the Rocky Mountain Chinook and the Croatian Bora. In still other cases, such as the strong easterly winds that blow from the Cascades toward the town of Enumclaw, Washington, the distinction between gap winds and downslope winds is not so clear. Previous research with a shallow-water model has suggested that the gap wind mechanism is more effective on large spatial scales (very broad mountain ranges pierced by a gap) whereas the downslope wind mechanism is most effective on small spatial scales (narrow ridges). These early results will be extended to three dimensions and to a variety of more realistic atmospheric configurations, using the new 3D nested grid model. As before, the goal will be to characterize the large-scale forcing with a minimal number of dynamical and terrain-shape parameters and to systematically investigate the sensitivity of gap winds and downslope winds to variations in these parameters. Possible synergetic combinations of the two wind regimes will also be investigated. ***
