Evaluating mesoscale WRF configurations for nested mesoscale-LES wind turbine wake simulations: boundary layer performance over cropland

Increased wind power utilization worldwide has sparked investigation into the effects that large wind farms have on local and regional microclimates. The Crop/Wind-energy Experiment (CWEX) seeks to quantify the relationship between wind turbine wakes, boundary layer processes, and crop health using measurements of surface fluxes and LIDAR wind profiles of the turbine-layer. In 2011, measurements were collected by Iowa State University, the University of Colorado at Boulder, the National Renewable Energy Laboratory, and the National Center for Atmospheric Research, focusing on a wind farm situated within and surrounded by central Iowa corn fields during the growing season; the characteristics of the land surface were defined by the progression of crop growth. Measurements included wind profiles north and south of a row of turbines collected by two Leosphere/NRG Windcube LIDARs and data from four surface flux stations. The instruments were aligned in a linear configuration to sample the free-stream boundary layer, the near-wake, and the far-wake during typical southerly flow conditions.

Concurrent measurements of upwind and downwind conditions are ideal for evaluating turbine wake simulations; because mesoscale conditions evolve, these wake simulations must be nested within mesoscale simulations. Our future goal is to compare CWEX wake observations with high-resolution large eddy simulation (LES) turbine wake simulations, which requires both accurate representation of mesoscale features as well as appropriate nesting of the LES domain within the mesoscale domain.

To ensure that we produce adequate mesoscale boundary conditions, we conduct preliminary investigations of planetary boundary layer (PBL) performance using the Weather Research and Forecasting (WRF) model. In this presentation, we compare CWEX-observed evolution of surface fluxes and boundary layer winds with simulated representations for the first half of August 2011 (1st-16th). Two days in particular, August 12th and 15th, featured extended periods of southerly winds ideal for comparison between turbine inflow and outflow. We focus on the rotor-layer winds between 40m and 120m, a region typically ignored in boundary-layer parameterization evaluation. The model representation of the interaction between surface forcing and elevated nocturnal wind accelerations, or low-level jets, is a primary focus of this study. Low-level jets are of particular interest to the wind industry as they enhance both the wind resource aloft and turbulent motions impacting the turbine rotors. WRF performance is evaluated using grid resolutions from 30 to 1km, PBL parameterizations including the MYJ, MYNN, YSU, and ACM2 schemes, and boundary conditions including the NARR, ERA-Interim, and GFS datasets. These comparisons will guide the construction of the model configuration used for the LES simulations.