Observational and measurement requirements in the boundary layer for wind energy

Nocturnal stable conditions occupy about half of the diurnal cycle over land. In most land-based locations of interest for wind energy development, the nocturnal period is also the time of peak wind speeds above the surface in the turbine blade layer—at the heights of turbine rotor blades. Nocturnal conditions thus provide a significant resource opportunity, but pose a number of problems for wind energy, including: • Difficulties of obtaining high-quality measurements aloft in the blade layer • This layer aloft is generally decoupled from the layer near the surface, so that near-surface tower measurements are not representative of flow conditions aloft • Extrapolation formulae used to estimate tower-measured near-surface wind speeds to the blade layer produce significant errors at night • Representation of the stable boundary layer (SBL) in numerical weather prediction (NWP) models has been described frequently as a weakness in these models on all scales • Strong bursts of turbulence occur in the SBL that can damage hardware and drive up maintenance and repair costs These problems are compounded by the lack of long-term, high-quality measurements aloft at blade level. But some information is available from short-term field projects, and data from some of these experiments will be used here.

The need for meteorological information extends over a wide range of time and space scales, to support forecasting (especially from ~ 1 hr to ~ 1 da), siting (climatological time scales), and hardware design, for example. Types of flow that have been used for wind energy include complex terrain effects (ridgetop speedup, gap flows) and the low-level jet (LLJ) of the U.S. Great Plains. An instrument capable of providing the kind of high-resolution, high-accuracy data aloft is NOAA/ESRL's High-Resolution Doppler Lidar (HRDL), which has been involved in a number of field programs measuring these flows, notably the LLJ. Here we use HRDL data to explore the question, what are the temporal and spatial sampling requirements to adequately sample the flows used by wind energy at night and during morning and evening transitions. Examples include the kind of resolutions needed to determine the depth of the SBL from mean-profile data, the shape of the wind profiles, and the magnitudes of the shear encountered during nocturnal LLJ conditions.