Extreme winds and trends in the U.S

The wind climate is poorly understood. The predominant scientific emphasis and political focus is on the re-distribution of temperature and precipitation regimes. However, the magnitude, direction, and temporal distribution of the wind is crucial for producing accurate site assessments for wind energy generation, identifying susceptible areas for air pollution, and estimating the terms of the surface energy budget. Furthermore, realistic depictions of wind speed trends and return periods of extreme wind events are critical to engineering applications (especially in the renewable energy industry), climate change, and resource characteristic studies. Thus, the purpose of this presentation is to 1) unveil a high resolution (200 m) extreme winds map of the U.S. for several levels of the lower atmospheric boundary layer, and 2) discuss an in-depth observational and modeling analysis more closely examining detectable trends in wind speed linked with potential mechanisms responsible for these changes.

Extreme winds. Although there are a few sources of information regarding extreme winds and return periods for the U.S., these studies suffer from a lack of representativeness —. virtually all long-term climate stations reporting wind speed are located at airports, sites specifically chosen for their relative lack of wind. Here, we present a model-derived high resolution (200 m) climatology of extreme wind speed and return periods, validated using available estimates from long-term tall towers, high altitude surface observation stations, and previous in-situ mountain studies, for the coterminous U.S. and southern Canada at levels of 60, 80, 100, 120, and 200 m above ground level (AGL).

Recent trends — wind speed. A variety of studies of surface-based observations show no significant trends in wind speed throughout the U.S. (see e.g. Figure 1). Individual stations do show detectable trends (± 1 m s^-1 or more per decade) but there is no apparent geographical pattern or large-scale atmospheric mechanism responsible; these trends may be the result of local land use changes, instrument discontinuities, or station re-locations. Moreover, trends in extreme wind events are also difficult to ascertain, owing to the dearth of long-term, continuous, homogeneous wind speed data sets at representative sites. However, wind speeds at 850 hPa (about 1500 m or 500 ft above sea level), as measured from rawinsondes, show a distinct increase during the past 20 years, especially over northeast North America (> 0.5 m s^-1 decade^-1; Figure 2). Whether this is a natural fluctuation, a result of changes in surface roughness, or a consequence of long-term trends related to global climate change is still an open question.

Figure 1

Figure 2