Spectrograms of the acoustic pressure at the receiver position (400, 0, 1.2) m for a homogeneous atmosphere and a rigid ground (a), a homogeneous atmosphere and a grassy ground (b), and a moving atmosphere with a log wind profile and a grassy ground (c). Fixed white dashed line are added in order to facilitate the comparison. The levels are normalized by the maximum of pressure at the receiver for the first simulation (a).