  Low-level jets (LLJs) are the wind maxima in the lowest 50 to 1000 m of
atmospheric boundary layers. Due to their significant influence on the power
production of wind farms it is crucial to understand the interaction between
LLJs and wind farms. In the presence of an LLJ, there are positive and negative
shear regions in the velocity profile. The positive shear regions of LLJs are
continuously turbulent, while the negative shear regions have limited
turbulence. We present large-eddy simulations of wind farms in which the LLJ is
above, below, or in the middle of the turbine rotor swept area. We find that
the wakes recover relatively fast when the LLJ is above the turbines. This is
due to the high turbulence below the LLJ and the downward vertical entrainment
created by the momentum deficit due to the wind farm power production. This
harvests the jet's energy and aids wake recovery. However, when the LLJ is
below the turbine rotor swept area, the wake recovery is very slow due to the
low atmospheric turbulence above the LLJ. The energy budget analysis reveals
that the entrainment fluxes are maximum and minimum when the LLJ is above and
in the middle of the turbine rotor swept area, respectively. Surprisingly, we
find that the negative shear creates a significant entrainment flux upward when
the LLJ is below the turbine rotor swept area. This facilitates energy
extraction from the jet, which is beneficial for the performance of downwind
turbines.
