Trends in US Surface Winds Over the Last Quarter of the 20C: Observations and Model Results

Trends in near-surface wind speed are of interest for wind power production, land-atmosphere interactions, ocean-atmosphere interactions, agricultural applications and many other purposes. Changes in wind speed can create changes in surface fluxes of heat, momentum, moisture, and trace gases. These, in turn, can affect low- level atmospheric stability and boundary-layer depth, thereby feeding back to the ability of the boundary layer to extract momentum from the free atmosphere. Impact of climate change on surface wind speeds has received relatively little attention despite the important impact of wind speed changes on fundamental balances of heat, momentum, and water at the earth's surface. We have examined trends in near-surface wind speeds from observations and results of a regional climate model. Pryor et al. (2007) reported reductions in wind speed generally in the eastern half of the US in the latter quarter of the twentieth century. To examine whether regional climate models can capture such trends we have examined output of such a model (MM5) for the US for the period 1979-2004 produced under the North American Regional Climate Change Assessment Program. This model was driven by reanalysis boundary conditions updated at 6-h intervals at the lateral boundaries. Preliminary results indicate that the model also reveals a general decline in wind speeds in the eastern US and a few isolated regions of wind speed decline in the western US. At almost no grid points over the continental US did the model produce wind speed increases over the latter quarter of the twentieth century. More detailed analyses of trends in seasonal and diurnal distributions are in progress.