Decadal Wind Trends at the Savannah River Site

One possible consequence of global warming would be an observed decrease in mean wind speeds over the U.S. within a band between about 30-40 degrees north latitude (Zack, et al. 2005). Recent studies (Pittman, et al. (2004), Galletta, (2005), and Weber, et al. (2005)) at various locations in the eastern U.S. indicate that, indeed, mean surface wind speeds have decreased by 5 to 10% over the last couple of decades. Interestingly, while the overall wind speeds at five East Coast locations have decreased during the past 15 years, strong winds (above 11.2 mps, 25 mph) show no similar trend. This seeming contradiction may be due to the increased frequency of tropical storms affecting these locations over the time period examined.

The Savannah River National Laboratory (SRNL) operates numerous towers at different locations at the Department of Energy's (DOE) Savannah River Site (SRS), which covers 800 km2 in southwestern South Carolina. In this paper, we examine the trends in wind speed and vertical and horizontal turbulence intensity over the past 14 years associated with measurements at eight different 61-m towers and we speculate on some of the ramifications of these results.

SRNL's tower system has operated for a period of about 30 years collecting wind and air temperature data every 15 minutes. Since the tower system's primary purpose is to provide wind data for emergency response, during the past 14 years (1991 to 2004) there have been rigorous quality assurance and data archival procedures strictly imposed. The result of these practices is a data recovery and acceptance rate generally exceeding 95%. The tower instruments collect wind speeds using sensitive cup anemometers and turbulence intensity data from bidirectional wind vanes (bivanes). Averages from all eight towers for a given time period (month or year) are determined using only data that have been quality assured.

The results of analyzing trends in the annual averages of the 15-minute, measured vertical turbulence intensity show an increasing trend for the time series, while the horizontal turbulence intensity shows mostly periodic oscillations (including the quasi-biennial oscillation, QBO) with no statistically significant trend since the early 1990s. These results seem to support the notion that while the turbulent transfer of heat energy from surface convention has increased, the short-term lateral turbulence intensity may not have changed much. Further insight was gained by examining the diurnal and seasonal changes in the two turbulence intensities.