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Toward an Estimate of Wind Resource Offshore North Carolina

By Harvey Seim • Sara Haines • Natalie Thomas

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 flow 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.


Offshore 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 turbine-height 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 influence of oceanography offshore of North Carolina on atmospheric conditions. Large spatial changes in average sea surface temperature due to the poleward flow of the Gulf Stream and equatorward flow 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 fixed variations in roughness length (i.e., only accounting for changes in roughness over land 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 with wind speed due to the growth of the wave field. These schemes, referred to as neutral stability schemes, are relatively simple to implement because they require only wind observations at a specified height above the sea surface. A limitation in these formulations is that the wave field can be represented by the wind speed alone, which introduces uncertainty in the application of these simple schemes from a lack of dependence on wave age, especially in shallow waters and fetch-limited settings.

Atmospheric stability also impacts the vertical profile of wind speed. Unstable conditions are associated with convection, enhanced near-surface fluxes and reduced shear aloft, whereas stable conditions are associated with reduced near-surface fluxes and increased vertical shear aloft. Monin-Obukhov Similarity (MOS) scaling, with some modifications to improve its application over the oceans (including variations in surface roughness), has been found to adequately represent the variations in surface fluxes and wind shear in a number of settings. Implementations of MOS scaling have found wide application and continue to be refined.

However, use of these formulations requires a number of variables in addition to wind speed to be measured simultaneously [surface air temperature (SAT), relative humidity, sea surface temperature (SST), downwelled short- and long-wave radiation and barometric pressure]. A commonly available implementation of the Coupled Ocean-Atmospheric Response Experiment bulk algorithm (COARE V2.0), includes allowances for some missing variables (specifically downwelled short- and long-wave radiation). The challenge to this stability-based extrapolation scheme is to find appropriate sources of ancillary data on the same time and space scales as the ASCAT winds.


ASCAT, NARR, AVHRR-OI
The ASCAT product used provided daily estimates of wind speed on a 25-kilometer spatial grid starting in January 2008. After exploring possible sources for surface sea and air temperature, relative humidity and barometric pressure, we found that the North American Regional Reanalysis (NARR) product from the National Centers for Environmental Prediction was largely compatible in spatial resolution with the ASCAT winds. We then evaluated the NARR fields. To continue this article please click here.


Harvey Seim is a professor of marine sciences at the University of North Carolina at Chapel Hill and chairman of the department. For the last 15 years, Seim has been active in the development of coastal ocean observing systems.

Sara Haines is a research associate in marine sciences at the University of North Carolina at Chapel Hill.

Natalie Thomas is an undergraduate student at the University of North Carolina at Chapel Hill, majoring in mathematics and environmental science.




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