Impact of stability on the atmospheric boundary layer

We present an investigation of the impact of stability in a coastal environment´s wind resource estimate. The site, near Cape Hatteras, is interesting because of its proximity to an open sound condition to the north and west, as well as its oceanic boundary to the south and east. Early exploration of wind profiles collected during 2012-2014 from the surface to 200 m with a Scintec XFAS SODAR wind profiler revealed distinct characteristics unique to winds coming from the sound, as compared to those coming from the ocean. “Steady-state” cases were defined in order to examine time periods where the winds might be more readily compared with simple theory. While wind profile cases from the sound exhibited low level shear that reached a constant wind speed within instrument height, the oceanic profiles were characterized by shears higher in both magnitude and elevation that had not yet reached their maximum at 200 m. Although this feature of the oceanic profiles is typical of a stably stratified wind regime, further study of the environmental conditions affecting each case was necessary to determine the specific impact of the stability on the profiles. An aggregation of ancillary data was collected, including surface sea and air temperature, surface humidity, and vertically-distributed air temperature and humidity from surrounding airports, meteorological stations and ocean buoys. The COARE algorithm was utilized to calculate a Monin-Obukhov Similarity (MOS) scaling parameter, L, a stability parameter appended to a neutral logarithmic layer. An internal boundary layer algorithm is used to determine the height over which the MOS theory should reproduce the structure of the wind profiles. The nested boundary layers identified include those created by changes in roughness as well as those induced by thermal effects of the sea and land surface. Upwind topography and local aerodynamic roughness length values are used to assess the impact of roughness on the measured wind profiles, and the height to which this impact should remain significant. The high shear at low altitude observed in the sound cases is mostly explained by this process. After controlling for roughness induced shear, thermal effects will be integrated. The level of stability or instability gleaned from the sign and magnitude of L will be used to assess the differences in wind profile structure across the sound and ocean cases identified, and the seasonal changes associated with both. This work is part of a larger effort to improve upon the wind resource estimate off the Outer Banks of North Carolina. The results will be incorporated into the larger scale study of wind energy in order to adjust for the effects of atmospheric stability in this dynamic region.