![]() It is now rather clear that the specific dynamical processes that underly the occurence of this phenomenon include, primarily, the “breaking” of upward-propagating internal waves that are forced orographically and a late stage Kelvin–Helmholtz instability of the low-level jet that appears in the lee of the obstacle in consequence of the wave–mean flow interaction induced by this primary wave- breaking event. These flows have a long history of serious investigation that has been reflected in the meteorological literature for at least 50 years. An especially interesting example of such flows is certainly provided by the severe downslope windstorms that are observed in the lee of major mountain ranges such as the foehn of Switzerland, the bora of Yugoslavia, and the chinook of North America. It is important to study hydrodynamic flows of geophysical origin not only because of their many practical connections to the understanding of atmospheric and oceanographic phenomena, but also because such flows often reveal the influence of fundamental hydrodynamic interactions that may be extremely difficult to adequately investigate in the laboratory. This instability erodes the downstream propagating K–H billows, eventually leading to the complete arrest of their continued propagation as they “dissolve” into fully developed turbulent flow. An instability of convective type first appears in the form of streamwise-oriented vortices of alternating sign. When the flow is allowed to access the third spatial dimension, the authors demonstrate that it develops intense three-dimensional motions in the regions where overturning of the isentropes in the otherwise stably stratified fluid takes place. A marked change in the global characteristics of the flow is shown to occur with increasing NU/ g, characteristics that include the speed of downstream propagation of the so-called chinook front, the drag exerted by the flow on the obstacle, and the intensity of the K–H instability induced pulsations of the surface velocity field. This nondimensional parameter represents the ratio of the acceleration that a fluid particle feels in the wave to the gravitational acceleration and measures the importance of non-Boussinesq effects. Results demonstrate that when the flow is restricted to evolve in two space dimensions, then the intensity of the Kelvin–Helmholtz-like (K–H) perturbations that form in the downstream shear layer that separates the accelerated low-level jet in the lee of the obstacle and the overlying region of decelerated flow increases dramatically with the governing parameter NU/ g ( U and N are, respectively, the velocity and buoyancy frequency characteristic of the upstream incident flow, while g is the gravitational acceleration). ![]() ![]() The authors present a series of new analyses of the problem of stratified flow over a localized two-dimensional obstacle, focusing upon the detailed dynamical characteristics of the flows that develop when the Froude number is such that the forced internal waves “break” above their topographic source. ![]()
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