Jet Streak Dynamics and Geostrophic Adjustment Processes during the Initial Stages of Lee Cyclogenesis

A numerical experiment is described which explores the relationship between upper-level potential vorticity advection and cyclogenesis on the leeward side of mountain barriers. A multilevel primitive equation model framed in isentropic coordinates is used to simulate the growth of a wave disturbance on the cold front associated with a preexisting “parent” cyclone. The effect of a mountain barrier placed in the path of the advancing cold front and the effect of an enhanced upper-level jet streak on the growth rate of the disturbance are investigated. Enhancement of the jet streak is accomplished by altering the geostrophic potential vorticity in a region upstream of the mountain barrier and solving for the corresponding man and geostrophic velocity field. The experiment suggests a strong connection between the intensity of the jet weak impinging on the barrier and the pressure fall in the lee. We also find that the strongest leeside pressure fall in this experiment is not accompanied by a conversion of available potential energy to kinetic energy. This suggests that geostrophic adjustment processes, rather than baroclinic instability, may cause the rapid initial growth of some, though possibly not all, lee cyclones.