SOME EFFECTS OF THE WESTERN CORDILLERA OF NORTH AMERICA OF CYCLONIC ACTIVITY

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.