Abstract Large wind farms are expected to influence local and regional atmospheric circulations. Using a mesoscale parameterization of the effects of wind farms that includes a momentum sink and a wind speed–dependent source of turbulent kinetic energy, simulations were carried out to quantify the impact of a wind farm on an atmospheric boundary layer throughout a diurnal cycle. The presence of a wind farm covering 10 km × 10 km is found to have a significant impact on the local atmospheric flow and on regions up to 60 km downwind at night. Daytime convective conditions show little impact of the wind farm on wind speeds, as the momentum deficits generated by the wind farm rapidly mix through the depth of the boundary layer. At night, the stable layer within the rotor area inhibits turbulent mixing of the momentum deficit, leading to a shallower wake and a greater reduction in the wind speed within the wake. Although a low-level jet forms at altitudes within the rotor area in the hours before dawn, it is completely eliminated within the wind farm. At night, a maximum warming of 1 K is seen at the bottom of the rotor area. Near the surface, there is less warming (0.5 K). Downwind, the surface temperature perturbation is small, with a cooling of up to 0.3 K. Over the simulation period, the mean temperature change over the wind farm area at 2 m is a very slight warming (0.2 K). Mean temperature changes downwind are negligible. Other influences on turbulent kinetic energy, surface heat fluxes, and boundary layer height, are discussed.