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.