On the Interpretation of Boundary-Layer Wind Profiles and Low-Level Shear Upwind vs. Downwind of Wind Farms in Complex Terrain

Shear is an important dynamic quantity in the lower atmospheric boundary layer (ABL). In near neutral conditions, surface-layer momentum flux is proportional to the near-surface shear. In the unstable BL over simple topography or ocean, weak shear leads to cellular convection, whereas strong shear generates linear roll vortices. In the stable BL, large differences in the structure of the weakly stable vs. very stable BL result from differences in the magnitude of the low-level shear. Over complex terrain, landscape effects alter the structure of the flow, but lower-BL shear remains an important dynamic quantity. Additionally, shear can be a key diagnostic for whether NWP models are accurately representing ABL dynamics. Despite its importance, shear has not been well measured above the atmospheric layer easily measured by towers, whether because of instrumentation issues or accessibility issues. Recently scanning, pulsed Doppler lidar has provided the capability to accurately measure shear through the lowest several hundred meters of the atmosphere.

Another area where shear is important is as an indication of wind-farm effects on atmospheric flow. Current thinking is that as turbine arrays in wind farms extract momentum, they enhance turbulence and reduce shear as the flow passes over the wind farm. Here we use wind profile data from two Doppler lidars, one located upstream of several operating wind farms, and the other downstream, to evaluated changes in the mean wind profile and shear. Wind data were taken as part of the Second Wind Forecast Improvement Project (WFIP-2), an 18-month field-deployment and NWP-modeling study in the Columbia River Basin of Oregon-Washington, undertaken to improve quantitative predictions of wind properties, such as speed, direction, and turbulence, for wind-energy applications. Detailed, precise measurements of the wind profile were available at 15-min intervals from Doppler lidars at two locations separated by 40 km, as part of a comprehensive deployment of remote-sensing and in-situ instrumentation. Turbine rotor-level wind flow in this region is predominantly terrain constrained and from the west, especially during non-winter months.

Monthly and seasonally averaged wind-speed profiles showed a reduction in wind speed and shear for westerly-component flow at the downwind site vs. the upwind profile as expected. However, the wind farms were located southwest (SW) of the downwind lidar, but not in an unimpeded corridor to the northwest (NW), so we further analyzed the profile data by considering SW and NW flows separately. The result was that the mean downwind profiles calculated from the SW sample exhibited stronger flow and shear than the upwind profiles, opposite to expectation. The expected profile behavior was shown only in the unhindered NW-sample profiles. The lesson is that trying to infer effects such as wind-farm impacts from two (one upstream, one downstream) measurement sites in complex terrain is complicated, and the profile differences must be interpreted with much caution. In this case, differences in profiles due to topography were stronger than differences due to wind farms, so that the wind farm effects were masked by the topographic effects. We also validate the ability of the operational HRRR model to predict shear. Low-shear values during the day were reasonably well predicted, but the larger nighttime values were significantly underpredicted.