Nonlinear modulation of O₃ and CO induced by mountain waves in the upper troposphere and lower stratosphere during terrain-induced rotor experiment

We analyze and explain distributions of trace gases (O₃, CO) and potential temperature observed in aircraft measurements in the upper troposphere and lower stratosphere during the terrain-induced rotor experiment. These distributions show fluctuations induced by mountain waves, with phases and amplitudes that are modulated and correlations that reverse sign along legs of the aircraft at constant altitudes. It is demonstrated that the observed correlations and distributions of gas traces can be explained by reversible processes induced by interactions between mountain waves with different wavelengths evolving on top of a mean vertical profile of ozone that is perturbed by synoptic scale motion. A wave with a large wavelength displaces the air column, causing horizontal variations in the vertical mean gradients. The short waves evolving in these modulated gradients induce wave signatures in O3 and CO, with amplitudes and phase relationships that depend on the vertical gradients encountered along the path of the aircraft. The proposed explanation is confirmed by reconstructed tracer variations deduced under this dynamical process. This is further supported by nonlinear analytical calculations that use background mean profiles from the observations, where the tracer variations induced by mutual wave-wave interaction are investigated.