An observational and numerical study of a regional-scale downslope flow 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 low-level 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(-1) occurring 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. A simple analytical solution based on local equilibrium appears to agree fairly well with the observed layer mean downslope wind speed, but the classic Prandtl solution for maximum downslope wind speed fails to match the observations. The properties of the flow appear to be insensitive to changes in soil moisture, land cover, and surface roughness length. The contribution to the low-level wind maximum by inertial oscillation at night is found to be insignificant.