The Effect of Averaging Duration on Differences Observed between Gust Factors from Tropical and Extratropical Winds

Abstract

A higher gust factor (GF), the ratio of a short-duration gust speed normalized by a longer-duration mean wind speed, has been identified in tropical cyclone (TC) wind relative to synoptically-generated extratropical (ET) wind in a number of studies (Yu and Chowdhury, 2009; Miller, 2006; Paulsen and Schroeder, 2004). These studies have focused on a single gust length and averaging duration when making comparisons. Data collected from within fourteen landfalling tropical cyclones was compared with ET data collected during an experiment conducted west of Lubbock, TX. GFs were computed using a 2-second gust based on three mean lengths, 10minutes, 5-minutes, and 1-minute, and differences between the TC and ET GFs were evaluated. The percent difference between the TC and ET GFs was largest for the 10- and 5- minute segments and smallest for the 1-minute segment. Histograms for the various GF populations also revealed differences in distribution shape and position for the two environments that were more significant for the 10- and 5-minute data than for the 1-minute data. These findings make sense when comparing power spectral density (PSD) plots of data from the TC and ET environments. The two environments have similar distributions of high-frequency energy, but differences emerge between the two distributions at lower frequencies. As a result, the 2-second gust taken from the longer (5- and 10-minute) segments reflects the difference in lowfrequency variation between the two environments better than the 2-second gust taken from the 1-minute segment, which captures only the variation present in the portion of the spectrum the TC and ET data have in common. As a result, the distributions of GFs for TC and ET data are most alike when considering a shorter averaging duration.