Impact of atmospheric stability on wind resource estimates off North Carolina

Abstract

Large spatial changes in average sea surface temperature offshore of North Carolina 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. The dependence of turbine-height winds (80–100 m above sea level) off North Carolina on atmospheric stability was examined using historical buoy observations as input to a variety of one-dimensional (vertical) extrapolation schemes. Stable stability conditions were common at buoy locations on the shelf north of Cape Hatteras and within roughly 10 km of the shoreline south of Cape Hatteras. Unstable conditions were more common on the mid- and outer shelf south of Cape Hatteras and in the Sargasso Sea. Monin-Obukhov similarity scaling was used to estimate winds during non-neutral conditions based on a modified version of the TOGA COARE 2.0 algorithms. Unstable conditions reduced extrapolated turbine height wind speeds relative to neutral stability conditions by as much as 5%. A power-law representation of wind speed using an exponent of 0.07, empirically determined in previous work, was consistent with the winds estimated for neutral conditions under a number of assumptions; variations between schemes were less than 2%. Stable conditions led to larger extrapolated winds relative to neutral conditions, sometimes by more than 10%. The vertical extent of stable conditions is unclear and could impact the validity of the estimated winds. Of particular concern are locations near the coastline where an internal boundary layer (IBL) would form in response to cool underlying sea surface temperatures. A correction for distortion of the wind field measured from buoys by surface waves led to a 2–3% increase in turbine height winds regardless of extrapolation scheme. Satellite winds at 10m elevation have been used to develop a more highly resolved spatial depiction of the wind field, both on the shelf, over the Gulf Stream and in the western Sargasso Sea. Winds from the QuikSCAT and ASCAT missions were used, each with 25 km resolution. Comparison of the two satellite wind products during an 18-month overlap time period found the QuikSCAT winds to be stronger by 0.2 to 0.9 m/s where unstable conditions dominate over the Gulf Stream and on the shelf immediately south of Cape Hatteras. Buoy-based winds were found to be most consistent with ASCAT winds. Both wind products suggest strongest winds at 10m elevation on the shelf lie between Cape Hatteras and Cape Fear. Strongest winds were observed in deep water over the Gulf Stream north of Cape Hatteras. Wind speeds increase moving offshore, making the shelfbreak the region of greatest wind resource where water depths are less than 50 m.