The Relationship between Overlying Synoptic-Scale Flows and Winds within a Valley

Abstract

The relationship between winds above and within the Tennessee Valley is investigated climatologically and with an atmospheric numerical model. For the climatological analyses, winds above the valley were determined by interpolation from four surrounding rawinsonde stations, while winds within the valley were measured on four 100-m towers. Tennessee Valley winds are generally weak and bidirectional, oriented along the valley's axis. The valley wind direction depends strongly on the component of the synoptic-scale pressure gradient that is superimposed along the valley's axis at ridge-top level, with winds blowing along the valley's axis from high toward low pressure. This relationship between winds above and within the valley can result in countercurrents similar to those observed in the Rhine Valley. While winds in the Tennessee Valley are driven primarily by this pressure-driven channeling mechanism, downward momentum transport can cause afternoon winds within the valley to approach the wind directions aloft when winds at ridge-top level are strong, and thermally driven valley circulations can appear at night when winds at ridge-top level are weak. A hydrostatic numerical model was used to provide additional insight into the physical processes governing the near-surface winds in the Tennessee Valley. The results support the identification of pressure-driven channeling, downward momentum transport, and thermal forcing as the principal mechanisms determining valley wind directions. They also illustrate the importance of topographical features in producing deviations from simple pressure-driven channeling. The relative importance of thermally driven and pressure-driven winds is examined, and guidelines are presented for estimating when one or the other process will dominate.