Wind Atlas Analysis and Application Program (WAsP) and WAsP Engineering (WEng) are tools that are used extensively in wind energy applications such as site assessments and wind farm planning. WAsP is used to generate wind atlas data, estimate wind climate, estimate wind power potential, and calculate wind farm production. WEng uses properties of extreme wind speeds, wind shears, wind profiles, and turbulence to help calculate structural loads on wind turbines and other civil engineering structures in both homogeneous and complex terrain.

One input into WAsP and WEng is the surface roughness parameter or roughness length z₀. Roughness length is the parameterization of the length scale as a function of the roughness element geometry (e.g., spacing and silhouette area). Roughness length is input into WAsP and WEng as a two-dimensional field along with terrain elevation. The literature indicates that the roughness length parameter can be a subjective value that is estimated based on various subjective criteria. For example, a roughness length of 0.10 m is described in one document as “farmland with closed appearance”, in another as “cultivated area, low crops, obstacles of height H separated by at least 20 H”, and in another as “Low crops: occasional large obstacles”. Roughness lengths have been calculated empirically using micrometeorological measurements of vertical wind profiles and semi-empirically using quantitative vegetation information from satellite data.

In this project, the sensitivity of the roughness length inputs on WAsP and WEng results have been evaluated by testing the following roughness length variations: • Uniform inflation and/or deflation of roughness length over the modeling domain • The inclusion of areas of increased or decreased roughness within limited areas surrounding the wind farm and individual turbine locations. • The use of fine-scale roughness maps as determined from satellite imagery compared to generalized maps drawn by a user using wind energy application software (e.g. WindPro).

The following results were analyzed: • Wind parameters: speed, turbulent intensity, and shear • Annual Energy Production (AEP) • Wind turbine lifetime fatigue loads • Wind turbine wake model calculations

In this paper, WAsP and WEng output were analyzed for several wind farms where roughness lengths were varied according to reasonable assumptions that an analyst would make using the objective roughness length descriptions in the literature. The analysis is ongoing and the results and conclusions on roughness length sensitivity will be presented in the paper. Recommendations will be presented on using roughness lengths specifically in WAsP and WEng along with general recommendations for the use of roughness lengths in wind energy applications.