| Boundary layer observations taken during the METCRAX field study in northern Arizona revealed the frequent presence of a regional scale downslope flow. Field observations are augmented by high-resolution mesoscale numerical modeling. The depth of this downslope flow is between 100 and 250 m with a peak speed of 4-6 ms-1 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 ambient winds. A simple analytical solution agrees fairly well with the observed 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. Contribution to the low-level wind maximum by an inertial oscillation at night is also found to be insignificant. |
