The effects of orography on land-atmosphere turbulent exchange processes

An experimental investigation of the effects of orography on the boundary layer was conducted in February 1984 in the vicinity of Boulder, Colorado. Wind and thermodynamic data were acquired principally by the NASA B57-B aircraft but also with rawinsondes, the Boulder Atmospheric Observatory meteorological tower, the PROFS Mesonet surface stations, the radar wind profiler network, and the Boulder wind network.

The aircraft, which was instrumented to measure 3-axis mean winds and turbulence fluctuations as well as temperature and moisture information, was flown in a box-like pattern at multiple levels from approximately 305 m (1000 ft) to 3140 m (10,300 ft) above the surface. These data provided a three-dimensional view of the flow in the lee of the Rocky Mountains, including the front range, the Flat Irons.

The data acquired revealed the nature of the flow with varying directions of the prevailing wind relative to the mountain range. Profiles of momentum flux, heat flux, and turbulence kinetic energy were calculated and analyzed.

Consistent patterns emerged in the flow. Separation of the flow from the sharp edges of orographic features were detected. Indication of flow reattachment and even reverse flows on the plain downstream of the mountain range were evident. Monin-Obukhov lengths calculated from tower and aircraft for two days indicating that the day February 1, 1984, with flow normal to the mountain range was highly stable whereas the day February 2, 1984, was moderately unstable in the surface layer. TV Comparison of the data with parameterization models for general circulation numerical schemes is given. The results suggest strong orographic effects. Values of the universal similarity function of Yamada were calculated from the observations and compared with the theory. Discrepancies were found which were also attributed to unusual flow caused by the orographic features.