WRF-Simulated Springtime Low-Level Jets over Iowa: Implications for Wind Energy

Abstract

High-resolution simulations with the Weather Research and Forecasting (WRF) model are analyzed to characterize the frequency, intensity, height, and duration of springtime low-level jets (LLJ) and their implications for wind energy resource assessment and planning in Iowa. The time evolution of short-duration LLJ is analyzed to understand wind behavior around LLJ events and to illustrate their importance for high-frequency (few hours) variability in wind speeds and rotor plane turbulent kinetic energy (TKE). During spring, the LLJ core height has a spatiotemporal mean value of 217 m, but the LLJ depth means it frequently intersects typical wind turbine rotor planes. Nearly one-quarter of LLJ exhibit a maximum within the height interval 50-150 m AGL. LLJ profiles are found to have higher mean wind speeds across typical wind turbine rotor planes than non-LLJ profiles and to exhibit lower values of TKE. LLJ occur under stable stratification (i.e. positive Richardson numbers) and are associated with low TKE and the occurrence of high vertical wind shear. The frequency and duration of LLJ exhibit geospatial variability across Iowa with highest values in the northeast of the state. Analyses of daytime and night-time LLJ indicate topographic variability is an important factor in the development of LLJ.

Power and Wind Shear Implications of Large Wind Turbine Scenarios in the US Central Plains

“…The regional flow is generally dominated by southeasterly and southwesterly flow in winter and spring and dominated by southwesterly flow in summer [54]. The LLJ is more frequently associated with north/south wind directions and is more common in the northeast of the domain in spring [31] and may account for the high frequency of extreme shear in March and at night.…”

“…Although LLJ are typically assumed to be centered at heights above 300 m above ground level (a.g.l.) [29], in some locations, these LLJ are located at heights of relevance to wind resources and WT operating conditions (e.g., in the Southern Great Plains [30]), and a recent study has demonstrated their prevalence at WT relevant heights over parts of the Midwest including Iowa during spring [31]. When present, such phenomena may result in very high shear across the rotor plane [30] or complex shear profiles across the rotor-swept area [32].…”