Mountain Wave Study at a Wind Farm Site in the Eastern Rocky Mountains

An observational and modeling study was performed to analyze wind flows over and around mountainous terrain in the vicinity of a proposed wind power project. Previous experimental and theoretical studies of flows over a hill have shown that under near-neutral atmospheric stratification the highest speed-ups are produced at the crest of the hill, whereas wake and flow separation can occur on both the windward and leeward faces of the mountain. On-site measurements at the proposed wind farm site indicated an anomalous wind speed distribution. Measured winds on the lee side of the mountain were much greater during the winter season than those observed 300 m higher on the mountain top. This raises several issues: 1) the reliability of the measurements, 2) the atmospheric mechanisms responsible for the observed change in wind speed distribution, and 3) the characteristic climatology of the region (i.e., the periodicity of the responsible phenomenon).

The measurement stations on the lee side and top of the mountain consist tall towers equipped with cup and 3-D sonic anemometers. A detailed analysis of the instrumentation ruled out any wind speed measurement errors or malfunction of the wind measurement equipment.

The behavior of an approaching flow in response to a mountain barrier can be determined by the speed and stability of the airstream and the characteristics of the topography. Several mechanisms can be responsible for generating stronger winds on the lee side of a mountain including katabatic flows and mountain waves followed by a hydraulic jump. Initially, katabatic flows were suspected of causing the higher winds observed at the lower elevation site. However, observed wind flow patterns and the topographic aspects of the site ruled out the generation of katabatic winds. The project area is perfectly situated and the terrain preferentially oriented for the generation and persistence of lee side mountain wave-induced downslope winds, since (1) the terrain is oriented perpendicular (SE to NW) to the prevailing southwesterly flow; (2) the winter climatology features frequent development of nearby low pressure systems (typically in the lee of the Rocky Mountains) in conjunction with higher pressure to the northeast and southwest; (3) the winter season position of the jet stream is in the general vicinity of area of interest; (4) the surface roughness at the site is low, and (5) the terrain features a long slope at a considerable gradient (roughly 3% or more) which favors additional acceleration of downslope winds. These flow regimes are most common during the cold season (November through April).

Advanced Regional Prediction System (ARPS) simulations were set up for four representative cases during January and February 2011 over a limited area centered on the proposed wind farm site. ARPS was initialized using the North-American Mesoscale (NAM) model analyses produced by the National Centers for Environmental Prediction (NCEP). ARPS was run in cascade mode from 12-km down to 4 km, 1 km and finally 400 m grid-point spacing. To realistically capture turbulent flows within the atmospheric boundary layer, ARPS simulations were conducted with at least 6 vertical levels within the first 200 m above ground level (AGL) and 13 levels within the first 1500 m AGL.

The ARPS simulations were consistent in depicting the evolution and decay of mountain waves. Animation sequences from the ARPS simulation illustrating the x-y and x-z views of the wind speed and potential temperature show mountain waves producing accelerated winds on the lee side of the mountain. Further downstream, wind speeds drop off abruptly, as the speed distribution, a product of the (wave) length and height (amplitude) of the terrain encounters a “trough” in the wave train. Over time, the wave propagates downstream and weakens, and speeds at the lower elevation site drop off. The typical duration of these events is 12 - 24 hours or more, indicating that they are driven mostly by interactions between the larger-scale meteorological patterns affecting the thermodynamic profile (that is, stability), the geostrophic wind direction, and the local terrain. Overall, this modeling study indicates that downslope winds associated with mountain waves are responsible for the higher wind speeds observed on the lee side of the mountain. This has important ramifications for wind power resource assessment, turbine choice, and turbulence characteristics.