THE MODIFICATION OF BAROCLINIC WAVES BY THE ROCKY-MOUNTAINS

The author diagnoses two observed cases of Rocky Mountain lee cyclogen esis and perform several idealized simulations to understand the effec t of the mountains on incident baroclinic waves. Several issues are ex amined: 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 sor ts of behavior may result in differing mean flows; and 4) what is a us eful conceptual framework in which to view lee cyclogenesis? The dynam ical underpinning for analysis of observations and idealized simulatio ns is the quasigeostrophic (QC) equations. The author finds that the m ost important effect of the mountains is to alter the mean distributio n of surface potential temperature and hence change the propagation ch aracteristics of the incident baroclinic wave. The presence of a mount ain enhances the gradient of theta, the QG approximation to surface p otential temperature, to the north of the peak and decreases it to the south. Thus the component of the baroclinic wave that is identified w ith 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 con sidered, systematically results in a smaller growth rate than one woul d expect without the mountain. The addition of a mean flow extends the influence of the mountain upstream and downstream from the obstacle a nd causes the waves to deviate to the north well upstream from the mou ntain, following the largest gradients of theta. The structure of the baroclinic waves over and downstream from the mountain varies substan tially depending on the location of the upper-level jet. For a jet to the north of the mountain, a strong, synoptic-scale ''cold surge'' dev elops in the lee, governed by QG dynamics. Upslope cooling reinforces horizontal temperature advection, and an anticyclone intensifies, movi ng southward along the contours of theta. Vith the jet to the south, the anticyclone weakens and the cyclone in the lee dominates.