Detection and Documentation of Discontinuities in Annual Gust Factor Time Series at U.S. Observing Sites

Abstract Gust factors are widely used in engineering applications to adjust wind data to standard siting and roughness conditions. Multiyear average gust factors have been typically applied in the applications. Using 1-min Automated Surface Observing System (ASOS) data from over 500 stations in the contiguous United States, the gust factor record is assessed for significant discontinuities using an objective piecewise linear regression fitting technique. Discontinuities are identified at a majority of stations and generally affect multiple wind directions at each station. These results are made available as a dataset providing annual gust factors and identified breakpoints for each station. Aside from the known change from cup to ultrasonic anemometers in the mid-2000s, the most pervasive discontinuity is associated with a largely undocumented transition of the model of ultrasonic anemometer in use at ASOS stations that commenced in 2018. This systematic change has resulted in an increase in gust factors on the order of 0.1 (5%–7%). Other discontinuities are also prevalent and can be ascribed to documented station relocations and changes in land use. During the period with ultrasonic anemometers, approximately 75% of the discontinuities were characterized as trends with an average slope of +0.02 yr −1 . The remainder of the discontinuities were identified as jumps, averaging +0.08 in magnitude. The transition from cup to ultrasonic anemometers also caused the gust factor record to increase by an average of 0.1 (5%–7%). While cup anemometers were in use, discontinuities tended to cause gust factors to decrease, presumably due to mechanical wear of the anemometer components. Significance Statement Wind data are typically standardized to account for differences in anemometer characteristics and changes in land use. Gust factors (the ratio of wind gust speed to the wind speed averaged over several minutes) form the basis for these adjustments. A method to detect discontinuities in the gust factor record is described. Over 80% of U.S. stations experience discontinuities. In some cases, gust factor changes take the form of discrete jumps, while in others, the change is characterized by a linear trend. Based on these results, a dataset of annual gust factors that documents segments of the record with consistent gust behavior is provided to complement existing station metadata.