Low-Level Jets: Detection & Characterization
The term Low-Level Jet (LLJ) is applied to any confined wind speed maximum in the lower atmosphere. LLJ have important implications for loading on structures, moisture and heat advection, and atmospheric pollution dispersion/transport. For example, they play a key role in dictating wind resources, wind turbine rotor plane aerodynamic loading, turbine structural loading, and turbine performance. Detection algorithms for LLJ available in the literature vary widely and thus variations in LLJ frequencies and characteristics described by different studies in different regions may not reflect the true physical situation but rather be solely the result of methodological differences. Thus, it is imperative to assess the sensitivity of LLJ climatologies to the methodological decisions applied and data resolution and to advance robust repeatable metrics to describe LLJ. We present a method to characterize LLJ and apply it to output from simulations at 4km by 4km with the Weather Research and Forecasting (WRF) model. Sensitivity of LLJ characteristics to the (i) LLJ definition and (ii) model vertical resolution are also quantified.
Based on the output from WRF simulations over a domain centered on the U.S. state of Iowa, we find evidence of LLJs in the lowest 500 m of the atmosphere in at least one WRF grid cell within Iowa on 98 % of nights. These nocturnal LLJs are most frequently associated with stable stratification and low turbulent kinetic energy and hence are more frequent during the winter months. The spatiotemporal mean LLJ maximum (jet core) wind speed is 9.55 m s−1, and the mean height is 182 m. LLJ at these heights are likely to play important roles in dictating wind resources and wind turbine operating conditions. They may also be important to advection and transport of pollutants and allergens emitted at the surface.
High-resolution WRF simulations over the state of Iowa for December 2007–May 2008 are analyzed using a range of LLJ detection algorithms to generate a seasonal analysis of LLJs over the state and to assess the implications for wind energy resources and operating conditions. LLJ properties considered are maximum wind speed, the height of the wind speed maximum, frequency, duration, and flow direction. Using a detection algorithm in which the wind speed above and below the LLJ must decrease by at least 20 % of the jet core wind speed, approximately 95 % of LLJs have wind speed maxima between 3 and 25 m s−1, and the mean, modal and median heights of the LLJ core are approximately 183, 125 and 174 m, respectively. The low altitude of these jet core means that these LLJ have the potential to impact wind speeds and turbulence conditions in the rotor plane of the latest generation of wind turbines. LLJs are found to be associated with low turbulent kinetic energy across heights of a typical wind turbine rotor plane, to occur most frequently under stable conditions, and to cause comparatively high positive and occasionally negative wind shear across the rotor plane. Locations of highest regional LLJ frequency and duration are found to exhibit seasonal variability, likely due to changes in flow direction and the interaction between regional and locally forced flows.