ATMOSPHERIC WAVES IN THE NORTHWESTERN UNITED STATES

Abstract

I. The flow of air in vertical planes across the Rocky Mountains is studied through the potential-temperature distribution. Sinusoidal configurations of the isentropes are observed which seem to correspond to theoretical patterns, i.e., wide separation of isentropic surfaces (low stability) and ascent overlying descent in the lee with a nodal surface at 17,000-20,000 ft. This configuration is consistent with known distributions of chinook-arch clouds and hail. Another pattern with a very low nodal surface and a second node at 30,000 ft is found. The occurrence of this second pattern may be a function of high stability. The theoretical interpretation of these phenomena is discussed. II. A theoretically likely relationship between the development of lee troughs and migratory pressure waves is pointed out. The mechanism of cyclogenesis is demonstrated by mean maps giving the life history of lee-side development. This deepening depends upon the twofold effect of a Pacific low entering the west coast. First, the low increases the flow over the mountains with consequent intensification of the standing lee trough. Second, the low then passes over the mountains and produces additional lee pressure falls. The nature of moving pressure waves is investigated. A large fraction of those clearly identifiable from barograms are found to be nonfrontal, indicating that frontal analysis may be overemphasized in the western United States. These nonfrontal waves are high-level, fast-moving, and nonadvective. They possess in-phase isobars and isotherms, and become frontal waves in the lee.