A STUDY OF THE EFFECT OF GENTLY VARYING TERRAIN ON ATMOSPHERIC FLOW NEAR THE SURFACE.

摘要

An extensive dataset obtained at the Boulder Atmospheric Observatory (BAO), a site with gently varying terrain, is investigated. Included in the analysis are wind measurements obtained from NCAR's Portable Automated Mesonet (PAM); wind and turbulent flux measurements from the BAO's 300 meter tower; and wind and temperature profile and flux measurements obtained from University of Washington's surface layer instrumentation. The objective of the study is the isolation of specific terrain effects on the observed winds.;Pairs of PAM stations are compared to isolate effects of the stations' local environments on their measured winds. Several statistically significant signals are found and related to the terrain. These are grouped into 'smooth', 'obstacle' and 'steep-sloped' situations. Potential flow computations are performed for three of the 'steep-sloped' situations. Potential flow computations are performed for three of the 'steep-sloped' situations. The potential flow predictions and the observed flow behavior are found to substantially agree; but in two of the cases departures are observed and attributed to flow separation or wake effects.;Roughness lengths determined from the surface layer measurements at all three measuring sites are found to be consistently between 1 and 2 cm., a reasonable value for the sites. The momentum and sensible heat fluxes determined from mean wind and temperature profiles are found to agree with directly measured fluxes, indicating that the lower surface layer was in adjustment with the immediate surroundings.;The boundary layer as measured by the BAO Tower is found not to be in adjustment with the adjacent surface during periods of moderate to strong winds. The roughness length inferred from Tower profiles is significantly larger than that determined from the University of Washington profiler, indicating that the Tower 'feels' the cumulative effect of obstacles and terrain undulations.;The wind direction data from the PAM are corrected to eliminate spurious biases, presumably introduced when the network was installed. The correction reduces the standard deviation of the wind direction among the stations by 50%. Periods of diffluence and confluence of the PAM wind fields are observed for westerly flow.