Mechanisms of gravity wave focusing in the middle atmosphere

This paper uses a time-dependent ray-tracing model to study focusing of gravity wave energy propagating upward through a wavelike background whose phases are descending-the dominant middle-atmosphere case. It demonstrates that these rays experience noncritical caustic focusing when the background is not both periodic over an infinite domain and limited to a single vertical phase speed. The superposition of even a small number of descending-phase background waves with well-separated vertical phase speeds readily yields caustics for upward-propagating rays possessing a wide range of initial group velocities. Encounters with an isolated background packet also result in caustics as or after the ray passes through the background packet, depending on the strength of the spatial modulation. Finally, the presence of mean shear in combination with descending phase background wave motion introduces caustic Focusing at much lower altitudes than would occur without the shear. The caustics found in these three background configurations do not correspond to "c(gt) similar or equal to c(zb) events," in which a ray's vertical group velocity c(gz) matches the vertical phase speed c(zb) of a background wave motion. Their existence suggests a mechanism for enhanced noncritical wave breaking and mean flow acceleration in the mesosphere due to the time-dependent nonlinear interactions within the broad gravity wave spectrum. This mechanism is not currently included in middle-atmosphere gravity wave drag parameterizations.