3D Numerical Simulation of Non-Primary Wave Generation from Mountain Wave Breaking over Europe

In this paper, we simulate a breaking mountain wave event over central Europe and investigate the subsequent generation, propagation, spectra, and momentum deposition of non-primary 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 500-1000 km. The breaking generates a broad range of non-primary waves with horizontal scales ranging from the mountain wave instability scales (20-300km), to multiples of the mountain wave packet scale (420km+) and phase speeds from 40-150 m/s in the lower thermosphere. The non-primary 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 non-primary waves and local wave forcing which generates even more non-primary waves. The tidal winds also influence the spectra of non-primary waves that reach the thermosphere. The non-primary 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 110-130 km altitude. The non-primary 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 non-primary waves at relatively small-scales which arise from the wave breaking processes.