Wind Direction Changes and Plume Behavior in Very Stable Conditions

Wind directions are commonly observed to exhibit large, rapid, and intermittent changes in very stable conditions. This presentation will explore the occurrence and magnitude of these short-term wind direction changes in very stable conditions and their associated meteorological factors. The evidence indicates that these large changes occur mainly at wind speeds less than 1.5 m s^-1 and are associated with momentum and sensible heat fluxes approaching zero in low wind shear conditions. This results in complete vertical decoupling. The presence of low-level-jets (LLJ) generates wind shear that tends to strongly suppress large wind direction changes. The nature of wind direction changes in very stable conditions has consequences for plume behavior and mixing. The Project Sagebrush Phase 2 field study found that wind directions tended to be quasi-steady within a relatively narrow range of arc for up to several tens of minutes at a time and then abruptly shifted by up to 100 degrees or more on time scales of less than 10 min. The standard deviations of the wind direction for the individual 10-min tracer sampling periods were small, generally less than 10-20 degrees. That is consistent with results from earlier studies such as Prairie Grass and what can be found in current EPA guidance for stable conditions. However, the large horizontal plume spreads actually observed during Project Sagebrush in very stable conditions are wholly inconsistent with the results from Prairie Grass and some modern plume modeling practices.