Nocturnal Wind Direction Shear and Its Potential Impact on Pollutant Transport

Abstract

The potential effects of vertical wind direction shear on pollutant transport at a complicated, semiarid site are examined using tower measurements. This high-elevation site is situated on a sloping plateau between mountains to the west and a river valley to the east. The local climate, which favors clear skies, low humidity, and light winds, permits terrain-generated winds to develop frequently. During the night, winds that are oriented along the valley frequently overlie shallow (<50-m deep) drainage winds. Results indicate the nighttime direction shear between the levels at 92 and 12 m above ground exceeds 60° 20% of the time and exceeds 20° 50% of the time. Daytime shear is more modest: it is less than 20° 90% of the time. The effects of vertical wind direction shear on plume transport are studied by performing two model simulations of release at 50 m above ground level during a period when strong directional shear persisted for several hours. In the simulation using the full wind profile, southwest winds above a shallow drainage layer initially transport material to a community located 2 km to the northeast of the release. However, when only the 12-m wind is used, the model predicts that the material impacts a different community located 10 km to the southeast. This simulation demonstrates that ignoring the vertical shear effects can result in serious mistakes in responding to an emergency.