Nocturnal wind direction shear and its potential impact on pollutant transport

The potential effects of vertical wind direction shear on pollutant transpo rt at a complicated, semiarid site are examined using tower measurements. T his 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 wind s to develop frequently. During the night, winds that are oriented along th e valley frequently overlie shallow (<50-m deep) drainage winds. Results in dicate the nighttime direction, hear between the levels at 92 and 12 m abov e ground exceeds 60 degrees 20% of the time and exceeds 20 degrees 50% of t he time. Daytime shear is more modest: it is less than 20 degrees 90% of th e 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. 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, th e model predicts that the material impacts a different community located 10 km to the southeast. This simulation demonstrates that ignoring the vertic al shear effects can result in serious mistakes in responding to an emergen cy.