Interannual and Month-to-Month Variations of Wind Speed

Abstract

Spatial cross correlations and interannual and month-to-month variations of monthly mean wind speed were studied at 40 sites throughout the United States. Sites were selected on the basis of availability of 10 or more years of data from a fixed anemometer location and height and a climatological mean speed of 5 m s?1 (11 mph) or higher. Spatial cross correlations of monthly deviations from climatic means were found to be about 0.5 for sites separated less than 200 km, with annual mean wind deviations from the climatic mean correlated with coefficient value 0.32 for similarly separated sites. Applications for using nearby ?climatological? site wind speed data to adjust short-term ?candidate? site data are examined with several methods. The best results show only minimal improvement in estimating the long-term annual mean over that obtained from one year of on-site data. These results indicate that, for candidate wind energy site evaluation, on-site data must be relied on more than originally considered and climatological data relied on less. Probability distributions of monthly and annual mean speeds were found to be nearly Gaussian with respect to climatological montly or annual mean. These distributions were found to have median (50 percentile) values of V/V=0.99, with coefficients of variation σ/V ranging from 0.08 to 0.18 (average 0.11) for monthly means and from 0.03?0.12 (0.06 average) for annual means. The coefficients of variation were found to be independent of climatic mean speed V, and the median V/V and σ/V values for a given site were found to be independent of season. Time autocorrelations were found to be about 0.33 for monthly mean speed with one-month lag and for annual mean speed with one-year lag. This corresponds to a persistence probability (probability of deviation from climatic mean retaining the same sign over one time lag) of about 63%.