Efficacy of Averaging Interval for Nonstationary Winds

Abstract

In comparison with atmospheric boundary-layer winds, which are generally regarded as stationary, windstorms such as hurricanes and thunderstorms/downbursts have strong nonstationary features characterized by rapid changes in wind speed and direction. The averaging interval associated with turbulent wind characteristics in boundary-layer winds is typically varied between 10 min and 1 h. A fixed averaging interval (FAI), which uses a constant mean to isolate the fluctuating wind component, has been effective in characterizing boundary-layer winds; however, the question remains as to whether the user-defined interval is appropriate for nonstationary winds. To address this concern, a variable averaging interval (VAI) scheme is proposed. For better understanding of the characteristics of nonstationary winds, traditional FAI methods are compared with alternative FAI approaches that use time-varying means and the proposed VAI approaches. In addition, the definitions for gust factor, turbulence intensity, and turbulence integral length scale are revised to ensure compatibility with the proposed schemes. Nonstationary data measured during the passage of Hurricane Lili and transient data measured during a rear-flank downdraft obtained from the 2002 thunderstorm outflow field experiment are used to examine the efficacy of FAI and VAI schemes and the associated turbulent wind characteristics.