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The trends of wind and wind power at a typical wind turbine hub height (80 m) are analyzed using the North American Regional Reanalysis (NARR) dataset for 1979-2009. Based upon the wind speeds at NARR’s vertical layers right above and below the 80 m level, the wind speeds at 80 m are estimated using two methods assuming the wind profile respectively as linear and power-law distribution with respect to the altitude in the lower boundary layer. Furthermore, we calculate the following variables at 80 m that are needed for the estimation and interpretation of wind power: the air density, zonal wind, meridional wind, and total wind speed. It is found that the difference between using the power-law and linear interpolation for the derivation of the 80 m wind generally results in less than 20% difference in the estimate of annually-averaged wind power in the majority of U.S. Statistically significant and positive annual trends are found to be predominant over the contiguous United States with spring and winter being the two largest contributing seasons. Positive trends of surface wind speed (up to ~0.15 m s-1 dec-1) are generally smaller with less spread than those (up to ~0.25 m s-1 dec-1) to 80 m, reflecting stronger increases of wind speed at altitudes above the 80 m level. In the regional averages, trends are positive and linearly continuous during 1979-2009 for the West region, but for the East and Central regions, a larger positive trend is found for wind speed and wind power during 1991-2009. Large and positive trends of wind and wind power over the southeast region and high mountain regions are primarily due to the increasing trend of the meridional wind, which supports previous studies reporting the enhancement of subtropical (Bermuda) highs and the (Mexican Gulf) low level jet in response to global warming. In contrast, large and positive trends of wind and wind power over the northern states (bordering Canada) are primarily due to the increasing trend of the zonal wind, again reflecting the previous reports of the poleward expansion of the tropospheric zonal jet. The positive trend of wind power found in this study supports recent studies using radiosonde and reanalysis data that showed a positive trend of wind at the lower troposphere, but is inconsistent with previous ground-based reports. Further studies are needed to resolve such inconsistencies and to explain the trend of wind in the context of climate change.
Adviser: Jun Wang