Vertical velocity u_z in 2D test model, red is downflow and blue upflow with a color scale set to  ± 50 km s^-1, as a function of time and height at the position x = 5 Mm in our 2D test model (see text). The disturbance clearly visible is a fast mode wave that originates due to a slight imbalance in the Lorentz force in the transition region in the initial state. The wave propagates fairly cleanly through the upper boundary, with a reflection of some 5%.