Time evolution of the normalized modal energies as computed with Eq. (33). The initial drop in energy is due to the arrival of waves at the lower C-PML from the source.The energy of these downward propagating waves is plotted as the dot-dash line. The second drop in the energy corresponds to the first arrivals at the upper C-PML (displayed with dashed lines). Modes with large horizontal wave numbers and weakly reflected waves arrive gradually at the boundaries at later times, resulting in an extended energy decay.Both upward and downward propagating wave packets are efficiently absorbed by the CPMLs.