| Quasi-geostrophic and primitive equations channel models are used to study the process of a short wave trough moving through a long wave trough in mid-latitude flow. We wish to gain an understanding of how the modification of the surface cyclone associated with the short wave comes about as it moves through the long wave trough.; A zonally independent, unstable basic state simulating the mid-latitude westerly jet stream has two small amplitude perturbations superimposed upon it. These are a long wave (wave 1) and two short waves (wave 2); their structures are those of the fastest growing normal modes of the basic state, for modes possessing a wavelength equal to the channel length (wave 1) and equal to one half of the channel length (wave 2). The short waves amplify and propagate eastward more rapidly than does the long wave trough.; For each of the models, two runs are made, one with both the long wave and the two short waves present initially and the other with only the two short waves. Emphasis is placed on what occurs within vertical columns directly above the surface cyclone centers.; We find that the surface cyclone associated with the short wave trough moving through the long wave trough deepens more and for a longer time than does the surface cyclone in the short wave only run. As the short wave trough moves through the long wave trough, the westward slope of the pressure trough with height above the cyclone center is maintained or reestablished, whereas for the short wave only surface cyclone the slope becomes and remains closer to vertical. This allows lower stratospheric, positive buoyancy (warm) advection to remain strong over the cyclone centers, for the two wave runs relative to the short wave only runs, since the relative maximum buoyancy region (warm pool) just above the tropopause is in the short wave trough. The strong lower stratospheric buoyancy (warm) advection, stronger than any tropospheric advection, drives the tropospheric and surface pressure falls at the cyclone center. |
