Vertical Motions in Orographic Cloud Systems over the Payette River Basin. Part II: Fixed and Transient Updrafts and Their Relationship to Forcing

Abstract

Abstract Updrafts in wintertime cloud systems over mountainous regions can be described as fixed, mechanically driven by the terrain under a given ambient wind and stability profile (i.e., vertically propagating gravity waves tied to flow over topography), and transient, associated primarily with vertical wind shear and conditional instability within passing weather systems. This analysis quantifies the magnitude of fixed and transient updraft structures over the Payette River basin sampled during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). Vertical motions were retrieved from Wyoming Cloud Radar measurements of radial velocity using the algorithm presented in Part I. Transient circulations were removed, and fixed orographic circulations were quantified by averaging vertical circulations along repeated cross sections over the same terrain during the campaign. Fixed orographic vertical circulations had magnitudes of 0.3–0.5 m s−1. These fixed vertical circulations were composed of a background circulation in which transient circulations were embedded. Transient vertical circulations are shown to be associated with transient wave motions, cloud-top generating cells, convection, and turbulence. Representative transient vertical circulations are illustrated, and data from rawinsondes over the Payette River basin are used to infer the relationship of the vertical circulations to shear and instability. Maximum updrafts are shown to exceed 5 m s−1 within Kelvin–Helmholtz waves, 4 m s−1 associated with transient gravity waves, 3 m s−1 in generating cells, 6 m s−1 in elevated convection, 4 m s−1 in surface-based deep convection, 5 m s−1 in boundary layer turbulence, and 9 m s−1 in shear-induced turbulence.