3D Numerical Simulation of Secondary Wave Generation From Mountain Wave Breaking Over Europe

In this paper, we simulate an observed mountain wave event over central Europe and investigate the subsequent generation, propagation, phase speeds and spatial scales, and momentum deposition of secondary waves under three different tidal wind conditions. We find the mountain wave breaks just below the lowest critical level in the mesosphere. As the mountain wave breaks, it extends outwards along the phases and fluid associated with the breaking flows downstream of its original location by 500–1,000 km. The breaking generates a broad range of secondary waves with horizontal scales ranging from the mountain wave instability scales (20–300 km), to multiples of the mountain wave packet scale (420 km+) and phase speeds from 40 to 150 m/s in the lower thermosphere. The secondary wave morphology consists of semi-concentric patterns with wave propagation generally opposing the local tidal winds in the mesosphere. Shears in the tidal winds cause breaking of the secondary waves and local wave forcing which generates even more secondary waves. The tidal winds also influence the dominant wavelengths and phase speeds of secondary waves that reach the thermosphere. The secondary waves that reach the thermosphere deposit their energy and momentum over a broad area of the thermosphere, mostly eastward of the source and concentrated between 110 and 130 km altitude. The secondary wave forcing is significant and will likely be very important for the dynamics of the thermosphere. A large portion of this forcing comes from nonlinearly generated secondary waves at relatively small-scales which arise from the wave breaking processes.