CWEX-10/11: Overview of Meteorological Measurements in a Wind Farm

During summers of 2010 and 2011 we have conducted experiments in a 200-turbine operating wind farm in Iowa to measure aerodynamic and microclimate properties in and near a wind farm in an area of intensely managed crops. Surface measurements at multiple locations were made of temperature, humidity, wind speed, wind direction, turbulence, and fluxes of heat, momentum, moisture and CO2. These observations, together with remotely sensed wind speed, wind direction and turbulence at hub height accumulated over two growing seasons, enable us to document differences in microclimate conditions in a wind-farm environment. Wind direction, wind speed, power produced, and temperature provided by the SCADA data from 13 turbines enable studies of wakes and their interactions with each other and with the surface. We find, in general agreement with past studies, that turbines modify flow at the surface by generally reducing the wind speed, increasing the night-time temperature and decreasing the daytime temperature. Going beyond previous reports, we find that spatial structures of changes to the mean wind, turbulence kinetic energy, thermal stability, and fluxes of heat, moisture and CO2 at the surface are driven by (a) the presence of overlying wakes, (b) wakes which impinge on the surface, and (c) an irregular standing wave pressure field created by lines of turbines presenting an obstacle to the flow. Control cases are provided by evaluating surface conditions under a wide variety of wind directions and evaluating surface conditions at times when turbines were turned off or abrupt wind shifts were documented. Measurements taken over two distinctly different growing seasons and multiple fields reveal that turbines have clearly measurable impact on surface fluxes, including CO2. Careful delineation of wind directions from the SCADA data reveal lateral characteristics of wakes and allow comparisons of power reduction for multiple interacting turbines with comparable results reported for off-shore wind farms. Surface stability observed concurrently with wake interactions provides some insights on diurnal variability of wind farm performance.