  Low-level jets (LLJs) are the wind maxima in the lower regions of the
atmosphere with a high wind energy potential. Here we use large-eddy
simulations to study the effect of LLJ height on the flow dynamics in a wind
farm with $10\times4$ turbines. We change the LLJ height and atmospheric
thermal stratification by varying the surface cooling rate. We find that the
first row power production is higher in the presence of a LLJ compared to a
neutral reference case without LLJ. Besides, we show that the first row power
production increases with decreasing LLJ height. Due to the higher turbulence
intensity, the wind turbine wakes recover faster in a neutral boundary layer
than in a stably stratified one. However, for strong thermal stratification
with a low-height LLJ, the wake recovery can be faster than for the neutral
reference case as energy can be entrained from the LLJ. Flow visualizations
reveal that under stable stratification the growth of wind farm's internal
boundary layer is restricted and the wind flows around the wind farm. Wind
farms extract energy from LLJs through wake meandering and turbulent
entrainment depending on the LLJ height. Both effects are advantageous for wake
recovery, which is beneficial for the performance of downwind turbines. This
finding is confirmed by an energy budget analysis, which reveals a significant
increase in the kinetic energy flux in the presence of a LLJ. The jet strength
reduces as it passes through consecutive turbine rows. For strong
stratification, the combined effect of buoyancy destruction and turbulence
dissipation is larger than the turbulent entrainment. Therefore, the power
production of turbines in the back of the wind farm is relatively low for
strong atmospheric stratifications.
