Sea Technology

NOV 2013

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Conclusion The COARE V2.0 stability-based extrapolation scheme has been implemented to estimate winds at 80 meters above sea level, making use of the ASCAT 10-meter winds, the AVHRROI SST and the NARR SAT, relative humidity and air pressure. A neutral stability extrapolation is also formed as a reference. The smallest temporal resolution possible is daily, set by the availability of the satellite products. The daily values have then been averaged over varying time periods to estimate mean differences and uncertainties between the two extrapolation schemes. Monthly average winds at 80 meters for 2009 exhibit signifcant departures from neutral stability that vary systematically with geographic location and time of year. Unstable conditions (SST>SAT) dominate over the Gulf Stream, in the Sargasso Sea and over the outer shelf south of 35° N throughout most of the year. In these regions, the stability-based monthly mean winds at 80 meters are typically 0.5 to 1 meters per second less than expected, assuming neutral conditions, a result of enhanced vertical mixing of momentum. In winter (January to March), stable conditions, produced by cool water nearshore, extend along the length of North Carolina and 25 to 50 kilometers offshore. In spring (April to June), stable conditions are found only north of Cape Hatteras but have expanded across the shelf, with the last remnant in June associated with the shelfbreak. In the regions of stable conditions, the stability-based winds at 80 meters are signifcantly greater than expected when neutral stability is assumed. The validity of the speed estimate in the stable regions is diffcult to assess because the highly sheared stable layer grows quite slowly. The growth in the thickness of the stable layer is proportional to the square root of downstream horizontal distance from nonstable conditions and 38 st / November 2013 may take several tens of kilometers for the layer to impact the winds at 80 meters. Therefore, the study results identify regions of uncertainty in the winds where there may potentially be a greater wind resource than predicted using neutral extrapolation of low-level winds. Further study will be required to frm up speed estimates. Additional studies will include measurement of the wind profle over the lower 200 meters of the atmosphere in regions with stable stratifcation and more sophisticated model approaches. Several offshore platforms that can host a wind profler exist off North Carolina, and negotiations to deploy a profler are underway. Nested fne-scale atmospheric modeling, with 2-kilometer horizontal resolution and nine vertical levels under 200 meters, by WeatherFlow (Scotts Valley, California) is ongoing and may provide an improved estimate of average winds over regions of stable atmospheric conditions if suffciently validated. A fnal synthesis that considers these sources of information is anticipated to be completed in the next one to two years. References For a list of references, contact Harvey Seim at hseim@ n 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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