The trend of wind speed over the United States during 1998 - 2011

Abstract

Through the analysis of the high-resolution radiosonde measurements over the U.S., we identified a decreasing trend near the surface and an increasing trend at the upper levels, which is consistent with previous studies. The decreasing trend of near-surface wind speed is statistically significant (p<0.01). However, the statistical significance of the decreasing trend drops with increases in altitude. We identified a largest negative trend in wind speed close to the surface. The magnitude of this largest negative trend is up to 15 times of the values reported in the previous studies. This might be due to the higher resolution in the vertical than data used in the previous studies, but it could also be a short-term phenomenon due to the shorter record considered as compared with the previous work. We also found a relationship between the elevation of the sites and the trend of near-surface wind and wind aloft. We identified a statistically negative relation between the magnitude of the decreasing trend and the elevation of the stations. The height of the planetary boundary layer (PBL) is estimated using three different methods. For non-elevated sites, a decreasing trend is seldom found for wind above the PBL, suggesting an important role for the land surface in determining the trend of wind speed. For elevated sites such as those in the mountainous west of the US, a decreasing trend is found for both the near-surface wind and the wind aloft, indicating that the changes in free-tropospheric circulation may also exert influence on changes in surface wind speeds for these sites.