Laboratory Simulation of Mountain Effects on Large-Scale Atmospheric Motion Systems. The Rocky Mountains

Abstract

An analysis concerning the modelling of large-scale atmospheric motions past orographic features in a linearly stratified rotating laboratory experiment is conducted; it is concluded that for an ƒ-plane model the similarity criteria include. matching the Rossby, Burger and Ekman numbers as well as a mountain height function normalized on the fluid depth. The fluid depth to topographic width parameter is not of zeroth order importance, and this allows for the use of distorted laboratory topographies; i.e., the laboratory model can employ exaggerated vertical to horizontal length scale ratios. Experiments are conducted for the westerly f-plane flow over a model of the Rocky Mountains for a range of parameters appropriate for the atmosphere. Horizontal streamline patterns at various depths and at ranges of the system parameters are presented and analyzed quantitatively. The experiments demonstrate ~ qualitative agreement concerning the ridge over the mountain, the downstream trough to the ease of the mountains, and the general orientation of the ridge and trough. The experiments also show that relative to an observer moving with the mean wind, a closed cyclonic eddy is found to the southeast of the central portion of the mountains. This cyclonic disturbance is located and farther to the northeast as the Rossby number is increased. Furthemore, a stationary anticyclonic vortex is located along the crest just to the north of the mountain center, the location of this anticyclone is relatively insensitive to the Rossby number. Experiments are also presented for vertical cross-section motions in the southern. central and northern regions of the model. These demonstrate the nature of the wake pattern, especially the vertical motion field Finally, experiments concerning the eastward advection of a trough with a cut-off low over the central portions of the model am presented. These show a weak northeasterly drift of the cut-off low upstream of the mountains and a sharp veer to the south on passing the mountain crest; the core of the cut-off low accelerates in passing over the mountains and then decelerates in the lee. These laboratory results are qualitatively similar to atmospheric observations of the advection of cut-off low across the Rocky Mountains.