The Modification of Baroclinic Waves by the Rocky Mountains

Abstract

The author diagnoses two observed cases of Rocky Mountain lee cyclogenesis and perform several idealized simulations to understand the effect of the mountains on incident baroclinic waves. Several issues are examined: 1) in what sense is the effect of the mountains cyclolytic and why; 2) what is the effect of adding a mean surface wind; 3) what sorts of behavior may result in differing mean flows; and 4) what is a useful conceptual framework in which to view lee cyclogenesis? The dynamical underpinning for analysis of observations and idealized simulations is the quasigeostrophic (QG) equations. The author finds that the most important effect of the mountains is to alter the mean distribution of surface potential temperature and hence change the propagation characteristics of the incident baroclinic wave. The presence of a mountain enhances the gradient of ?*, the QG approximation to surface potential temperature, to the north of the peak and decreases it to the south. Thus the component of the baroclinic wave that is identified with surface potential temperature perturbations propagates around the north side of the mountains and accelerates. This leads to a change in vertical structure of the incident wave that, for the wavelengths considered, systematically results in a smaller growth rate than one would expect without the mountain. The addition of a mean flow extends the influence of the mountain upstream and downstream from the obstacle and causes the waves to deviate to the north well upstream from the mountain, following the largest gradients of ?*. The structure of the baroclinic waves over and downstream from the mountain varies substantially depending on the location of the upper-level jet. For a jet to the north of the mountain, a strong, synoptic-scale ?cold surge? develops in the lee, governed by QG dynamics. Upslope cooling reinforces horizontal temperature advection, and an anticyclone intensifies, moving southward along the contours of ?*. With the jet to the south, the anticyclone weakens and the cyclone in the lee dominates.