A Numerical and Analytical Investigation of Lee Cyclogenesis.

The effect of topography on the evolution of a disturbance in a baroclinically unstable mean flow is studied using analytical and numerical simulations. In particular, dynamic mechanisms involved in cyclogenesis in the lee of long, meridional barriers similar to the Rocky Mountains are explored. The rapid growth observed in lee cyclogenesis is highly dependent on the superposition of a growing baroclinic wave with a steady, orographically forced wave of the same scale. As the baroclinic wave moves over the mountain, development is masked by the orographically-forced, high-pressure ridge. As it moves down the lee side of the mountain, the baroclinic wave appears to grow rapidly due to superposition with the forced, lee-side trough. Indications of enhanced, lee-side baroclinic instability are present in the numerical simulations, but the effect on wave development is minor compared to the effect of superposition. The rapid continuous-mode growth, which was demonstrated by Farrell (1982), is not observed in any of the simulations. Keywords: Numerical weather prediction; Topographic effects; Cyclogenesis, Baroclinic instability(Theses).