Periods of constant wind speed: How long do they last in the turbulent atmospheric boundary layer?

Abstract. A non-investigated feature of the atmospheric turbulent wind, named periods of constant wind speed, is introduced and investigated. We hypothesize that such periods of constant wind speed are related to characteristic wind field structures (e.g., ramps or jets), which when interacting with a wind turbine may induce particular dynamical responses. Therefore, this study focuses on the characterization of the constant wind speed periods in terms of their lengths, probability of occurrence, and extreme events. Atmospheric offshore wind data are analyzed. Our findings reveal that the statistics of long constant wind speed periods are an intrinsic feature of the atmospheric boundary layer and show the challenging power law behaviour of extreme events, which depends on the local conditions and the precise definition of wind speed thresholds. A comparison to wind time series generated with standard synthetic wind models and to time series from ideal stationary turbulence suggests that these structures are not characteristics of small-scale turbulence but seem to be consequences of larger-scale structures of the atmospheric boundary layer, and thus are a typical multi-scale effect. Given the conclusive results, we show that the Continuous Time Random Walk model, as a non-standard wind model, can be adapted to generate the statistics of those periods of constant wind speed measured from the atmospheric turbulent wind.