Sea Technology

NOV 2013

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Toward an Estimate of Wind Resource Offshore North Carolina Evaluation of a Stability-Based Extrapolation Scheme By Harvey Seim • Sara Haines • Natalie Thomas O ffshore wind resource assessments over the continental shelf off North Carolina have suggested that tens of gigawatts of power generation potential are possible from wind farms. An initial assessment undertaken by the University of North Carolina at Chapel Hill (UNC) took advantage of a variety of in-situ historical observations and employed simple extrapolation schemes (a power law and log-layer formulation) to estimate turbineheight winds and power potential. Subsequent work has investigated a variety of more sophisticated extrapolation schemes and established that 10-meter satellite Advanced Scatterometer (ASCAT) winds estimated by the European Organization for the Exploitation of Meteorological Satellites (2008 to present) are consistent with buoy-based winds. Here we describe work toward implementation and evaluation of a stability-based extrapolation scheme applied to the ASCAT winds to broaden the spatial coverage of the study. Motivation for this re-examination came from the infuence of oceanography offshore of North Carolina on atmospheric conditions. Large spatial changes in average sea surface temperature due to the poleward fow of the Gulf Stream and equatorward fow on the mid-Atlantic shelf produce strong variations in static stability of the overlying lower atmosphere. We seek to clarify the dependence of turbine-height winds (30 to 150 meters above sea level) off North Carolina on atmospheric stability and to document the sign and strength of the change in wind speed relative to neutral conditions, as well as how it varies along and across shore and over time. Formulations Two simplifying assumptions made in an earlier UNC study were temporally fxed variations in roughness length (i.e., only accounting for changes in roughness over land Annual average sea surface temperature (SST) for 2009 derived from daily Advanced Very High Resolution Radiometer-Optimally Interpolated data (AVHRR-OI). The black line offshore of the coast is the 100-meter depth contour. Temperatures above 25° C mark the Gulf Stream. Persistent convergent shelf fow toward 35° N produces cooler shelf waters to the north and warmer shelf waters to the south of 35° N. Black symbols with numbering indicate the position of National Data Buoy Center platforms used as ground truth in the studies. with no wind-speed dependence) and no accounting for atmospheric stability. Both assumptions can be relaxed through use of existing bulk formulae that are commonly used in oceanography when the necessary ancillary measurements are available. Surface roughness over water, typically represented as a quadratic drag law with multiple contributions, increases www.sea-technology.com November 2013 / st 35

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