Characteristics of Wind Speeds over the US from 1982-2004 as Simulated by Regional Climate Models
Eugene S. Takle, Iowa State Univ., Ames, IA; and S. C. Pryor, R. J. Barthelmie, T. Andersen, and T. NARCCAP-Team
Near-surface wind speeds are strong regulators of the exchange of heat, momentum, and moisture with the Earth's surface. Trends in near-surface (10-m) wind speeds from 1973-2005 (Pryor et al., 2008), as revealed by 50th and 90th percentile wind speeds, show a distinct decrease over the US in general, with particularly large decreases in the eastern US. Assessing trends in surface winds from observations is particularly challenging because of changes in site exposure over periods long enough to detect trends. And changes in instrumentation have created discontinuities in the datasets. Analysis of surface winds from the reanalysis data sets provides an alternative estimate of surface winds, but the various reanalyses have inconsistencies: the NCEP Reanalysis 1 shows little change over US land areas over this period, while the NCEP Reanalysis 2 shows increases over the Southwest and decrease in the mid south, and ERA-40 results show decreases in the Southwest and increases over Wyoming and the Northwest (Pryor, 2008). Regional climate model (RCM) results available from the NARCCAP archive provide another perspective on wind trends during this period. Runs produced by five RCMs driven by the NCEP Reanalysis 2 for the period 1979-2004 offer additional estimates of changes over this period. The MM5 model results from the NARCCAP archive show generally decreasing winds over this period except for a weak increase across the southern tier of states, a narrow band from central Minnesota to northern Kansas, and the Central Valley of California. Plots of the differences in surface wind between 1979-1990 and 1991-2004 by season from MM5 results show largest decreases (order of 0.05 m/s/yr) to be in winter (even though winter is not everywhere the season of strongest winds), particularly in the northern US with very slight increases in the south. This is in contrast to fall, which has a nearly reversed pattern with a small positive change the north and a weak decrease in the south. Largest increases are only about half of the largest decreases and generally cover smaller areas. Analyses of results from other models are in progress.
Session 16, Regional modeling - NARCCAP Part II
Thursday, 15 January 2009, 3:30 PM-5:00 PM, Room 129A
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