A Climatology of Superrefractive Conditions for Assessing Wind Farm Impacts on Radars

Wind farms can interfere with operational radar with degradation impacts on weather detection and precipitation monitoring, air traffic control and tracking of aircraft, and in border protection. Wind turbine interference is not static, but rather is time varying due primarily to vertical gradients in atmospheric water vapor and temperature which affect atmospheric refractivity. At present, many of the wind farm installations within the United States are in areas with climatologically low moisture and therefore relatively lower atmospheric refractivity gradients, but an increased interest is being placed on coastal and border locations where additional wind power capacity exists and the traditional standard atmosphere approach to siting limitations is not well suited. Analysis of historical numerical weather model output demonstrates that wind turbine visibility to radar may be substantially underestimated along the coastal areas of the United States if a standard atmosphere profile is assumed. In order to examine the spatial and temporal characteristics of the refractive index, the North American Regional Reanalysis (NARR) is analyzed to produce monthly averages of refractivity at 3 hour output intervals utilizing near surface and boundary layer averaged data available in the archive database. A comparison with ducting frequencies produced from a 5-year global model climatology using refractive gradients computed directly from ECMWF hybrid level model data demonstrates that the boundary layer fields routinely available from NCEP can be used to simplify the computations for the climatology as well as provide a method for analyzing conditions from routinely available forecast grids. Prominent areas prone to superrefractive conditions are found off the southern California and Baja California coasts as well as the Gulf of Mexico coastal regions and the Atlantic seaboard. Also apparent is a seasonal advance of superrefractive conditions following the Mississippi River Valley northward, then expanding across the eastern United States from April through October before retreating back toward the Gulf of Mexico from November through March.