A regional-scale downslope wind observed during the METCRAX Experiment in Northern Arizona

Boundary layer observations taken during the METCRAX field study in October of 2006 near Winslow in Northern Arizona revealed the frequent presence of a near-surface wind maximum on nights with relatively quiescent synoptic conditions. Data from a sodar, a radar wind profiler, several surface stations, and frequent high-resolution rawinsonde soundings were used to characterize this boundary-layer wind phenomenon and its relation to synoptic conditions and the ambient environment. The data analyses are augmented by high-resolution mesoscale numerical modeling. It is found that the observed nocturnal boundary layer wind maximum is part of a regional-scale downslope flow converging from high terrain of the Colorado Plateau toward the Little Colorado River Valley. The depth of this downslope flow is between 100 and 250 m with a peak speed of 4-6 m s-1occurring usually within the lowest 50 m above ground. Opposing ambient winds lead to a longer evening transition period, shallower slope flows, and a smaller horizontal extent as compared to supporting synoptic winds. Changes in soil moisture produce little impact on the properties of the flow, but neglecting the Coriolis force, as is typically done in idealized studies of downslope flows, has noticeable effects on the speed, direction, and transition of this regional scale downslope flow.