Factors Affecting Cold-Air Outbreaks East of the Rocky Mountains

Abstract

A two-layer isentropic model is used to examine the parameters controlling the severity of cold-air outbreaks east of the Rocky Mountains, and in particular the channeling effect of the mountain range. The case that is scrutinized is the cold-air outbreak of early February 1989. The parameters investigated include (a) the baro-clinicity, expressed as the ratio of upper- to lower-layer zonal scale velocities, (b) the Rossby number; (c) the height of the topography;(d)the initial longitudinal position of the cold air mass relative to the mountain ridge; and (e) the presence of a wave in the upper-level flow. It is found that the Rossby number determines the intensity of a cold-air outbreak, whereas the baroclinicity as well as the height of the orography and the initial position of the cold air mass control the extent to which the outbreak is channeled by the mountain range. An upper-level trough over the central United States favors the evolution of an outbreak.