Our field site was an Iowa wind farm consisting of about 100 1.5 MW turbines mounted on 80-m towers scattered over about 50 square miles. All turbines are located in intensively managed agricultural fields that are annually rotated between corn and soybeans. Flux stations (four provided by NCAR as a educational deployment and two provided by Iowa State University) enabled measurements of surface exchanges of momentum, moisture, heat and CO2 as well as leaf wetness at “undisturbed” sites (2) upwind of an E-W line of turbines and four sites downwind (under prevailing S to SE winds this time of year). Two vertically pointing lidars, one positioned upwind of the line of turbines and one located downwind, were operated by Professor Lundquist's research group from the University of Colorado. Leaf wetness sensors also were installed at the Iowa State sites and one of the NCAR sites to measure vegetation wetness period and mark points of sign change in latent heat. Instruments were deployed from late June to mid August 2011. Vegetation over the measured area and relevant fetch was exclusively corn, which reached a mature height of about 2.8 m by July 18.
Lidar-derived wind speed and direction at hub height enabled us to stratify data by periods when turbine waves were overhead of the flux stations. Preliminary results suggest that wake effects of the line of turbines can be detected most frequently and with largest magnitude at night under low-level jet conditions and (of the three downwind measurement sites) nearest the turbines (3.5 hub-heights downwind). Turbines in the E-W line are spaced about 266 m apart, which complicates analysis of data taken farthest from the turbine line due to possibly overlapping turbine wakes. On the other hand, these data may provide unique opportunities for validation of high-resolution numerical simulations of wind farms.
Results of ongoing analyses, which seek to quantify key moisture, heat and CO2 exchange processes over the crop and influences of the turbines, will be described in the presentation.