Scientific challenges to characterizing the wind resource in themarine atmospheric boundary layer

With the increasing level of offshore wind energyinvestment, it is correspondingly important to be able to accuratelycharacterize the wind resource in terms of energy potential as well asoperating conditions affecting wind plant performance, maintenance, andlifespan. Accurate resource assessment at a particular site supportsinvestment decisions. Following construction, accurate wind forecasts are neededto support efficient power markets and integration of wind power with theelectrical grid. To optimize the design of wind turbines, it is necessary toaccurately describe the environmental characteristics, such as precipitationand waves, that erode turbine surfaces and generate structural loads as acomplicated response to the combined impact of shear, atmosphericturbulence, and wave stresses. Despite recent considerable progress both inimprovements to numerical weather prediction models and in coupling thesemodels to turbulent flows within wind plants, major challenges remain,especially in the offshore environment. Accurately simulating theinteractions among winds, waves, wakes, and their structural interactionswith offshore wind turbines requires accounting for spatial (and associatedtemporal) scales from O(1 m) to O(100 km). Computing capabilities for theforeseeable future will not be able to resolve all of these scalessimultaneously, necessitating continuing improvement in subgrid-scaleparameterizations within highly nonlinear models. In addition, observationsto constrain and validate these models, especially in the rotor-swept areaof turbines over the ocean, remains largely absent. Thus, gaining sufficientunderstanding of the physics of atmospheric flow within and around windplants remains one of the grand challenges of wind energy, particularly inthe offshore environment. This paper provides a review of prominent scientific challenges tocharacterizing the offshore wind resource using as examples phenomena thatoccur in the rapidly developing wind energy areas off the United States.Such phenomena include horizontal temperature gradients that lead to strongvertical stratification; consequent features such as low-level jets andinternal boundary layers; highly nonstationary conditions, which occur withboth extratropical storms (e.g., nor'easters) and tropical storms; air-seainteraction, including deformation of conventional wind profiles by the waveboundary layer; and precipitation with its contributions to leading-edgeerosion of wind turbine blades. The paper also describes the current stateof modeling and observations in the marine atmospheric boundary layer andprovides specific recommendations for filling key current knowledge gaps.