To better understand the mechanism of and response to mountain induced wave breaking in the troposphere, a two-layer hydrostatic flow system with a free surface, over an isolated bump is investigated. This system is a coarse representation of the continuously stratified troposphere.
As a theoretical foundation, two layer normal shocks are reexamined in the context of classical hydraulic theory. It is demonstrated that for a two layer Boussinesq flow under a third passive layer, the physical conjugate state after a shock is unique, and can be specified by its upstream variables. A shock regime diagram is constructed based on the numerical solutions and theoretical considerations of wave steepening.
High-resolution, 3-D, two-layer model runs are carried out to nearly steady states for flows with or without upstream shear over isolated circular hills with a wide range of heights. Wavebreaking in a two layer system is classified into five different categories, namely, internal jump, external lower layer jump, external upper layer jump, double jump with an internal jump after an external upper layer jump, and double jump with an internal drop after an external lower layer jump. It is shown that all these different types of shocks can be consistently described using the shock regime diagram.
The wavebreaking and potential vorticity generation show considerable sensitivity to the upstream shears. A balanced vortex in the lower layer drifting over an elongated ridge is examined, as an example of a time dependent evolution in a real atmosphere. Potential vorticity generation in the upper layer is shown to be turned on and off by the position of the vortex.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics