Discipline: Technology and Engineering
Subcategory: Environmental Engineering
Kyra M. Bryant - Tennessee State University
Co-Author(s): Muhammad Akbar, Tennessee State University, TN
This study examines a variety of wind stress calculation techniques employed in forecasting models. As hurricanes transfer vast amounts of heat from tropical areas to cooler climates, they can be exceedingly destructive and, in unfortunate cases, cause death through storm surge. While wind serves a vital role in the development and behavior of a hurricane, surface wind stress provides the forcing for storm surge. Achieving accurate storm surge forecasting requires a high-performing model with valid wind field information. Many models incorporate Charnock’s proportionality constant, which was established during an era when high wind speed data was unavailable due to nonexistent meteorological observations for inaccessible areas. It was assumed that wind stress behaved identically in low and high wind speeds. Thus, Charnock’s parameter was an extrapolation from low to high wind speeds. Mostly, it implies a positive correlation between wind velocity and the wind-stress (drag) coefficient.
Since Charnock’s conception in 1955, technological advances have enabled data collection in remote areas during severe weather. The GPS dropwindsonde improved track forecasts by 32% and intensity forecasts by 20% in its inaugural year (1997), equating to the previous 20 to 25 years of accrued progress. A 2003 analysis of accumulated GPS dropwindsonde data revealed that wind velocity and the drag coefficient do not increase together incessantly. Wind velocity saturates after 40 m/s, and the drag coefficient, in fact, decreases.
Popular wind forcing formulae have been explored, old and new, via published journal articles, merited by numerous citations. Each method’s advantages and disadvantages are discussed. Despite the 2003 findings, research shows many models persist in using outdated formulae, treating low and high wind speeds the same. They follow previous beliefs that the drag coefficient increases with wind velocity. Here, a new wind stress method in the ADCIRC+SWAN storm surge model is under investigation. A storm surge hindcast will be simulated for a previous hurricane incorporating this new wind stress method. The results will be analyzed and compared against the ADCIRC+SWAN simulation using the available default wind stress. Currently ADCIRC+SWAN storm surge simulations using the default wind stress are being completed. After errors and shortcomings are discovered by comparing results, a wind stress profile recommendation will be made.
References: Charnock, H. (1955). Wind stress on a water surface. Quarterly Journal of the Royal Meteorological Society, 81(350), 639-640.
Aberson, S. D., & Franklin, J. L. (1999). Impact on hurricane track and intensity forecasts of GPS dropwindsonde observations from the first-season flights of the NOAA Gulfstream-IV jet aircraft. Bulletin of the American Meteorological Society, 80(3), 421-427.
Powell, M. D., Vickery, P. J., & Reinhold, T. A. (2003). Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422(6929), 279-283.
Funder Acknowledgement(s): This study was supported by an HBCU-UP Research Initiative Award grant from NSF to Dr. Muhammad Akbar, Assistant Professor, Tennessee State University, 3500 John A. Merritt Blvd, Nashville, Tennessee 37209.
Faculty Advisor: Muhammad Akbar, email@example.com