Windfarms interact in complex ways with the atmospheric boundary layer. An important aspect that has gained recent attention is wind-farm blockage, i.e. the slow-down of the flow tens of kilometers upstream of the farm. An extensive database of large-eddy simulations is discussed, parametrizing blockage under different atmospheric conditions, as well as the associated excitation of gravity waves in the capping inversion and free atmosphere above the wind farm. Next to this, the development and validation (against LES) of fast engineering models that parametrize these effects is also presented.