Abstract
Some effects of the western mountain complex of North America on cyclones are investigated from the standpoint of how the vorticity tendency field might be influenced by orographically-induced vertical motion and divergence. A detailed case study is presented which includes computations of terms in the vorticity tendency equation during a lee cyclogenesis. The local increases in low-level vorticity which occurred during formation of the lee trough and cyclone were primarily due to horizontal convergence, while the horizontal and vertical advection of vorticity opposed the increases; the contribution of the tipping term was generally negligible. The study also provides evidence that orographic effects are insufficient to explain the genesis of major cyclonic storms.
