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

Abstract

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.

Multi‐Step Vertical Coupling During the January 2017 Sudden Stratospheric Warming

“…Indeed, GW-mean flow interaction in the winter thermosphere is mainly due to GW-tidal interaction. The crucial role of the tides in controlling GW instability and mean flow effects is also well known for the MLT (e.g., Becker, 2017;Heale et al, 2022;Liu et al, 2014;Ortland & Alexander, 2006;Senf & Achatz, 2011). It is beyond the scope of this paper to explore the GW-tidal interactions in the thermosphere in greater detail.…”

Characteristics of Gravity Wave Horizontal Phase Velocity Spectra in the Mesosphere Over the Antarctic Stations, Syowa and Davis

“…Heale et al. (2022) classified secondary GWs into three scale types of secondary GWs: GWs with horizontal wavelengths smaller than, similar to, and longer than a horizontal wavelength of a primary GW. Because the horizontal wavelength of D2 wave (∼88 km) is ∼3 times smaller than that of the jet GWs (∼241 km), the D2 wave could be corresponding to the smaller‐scale secondary GWs in Heale et al.…”

“…(2022). This type of GW is generated by localized forcing, whose scales are associated with instability and vortex scales, produced by a primary GW breaking (Chun & Kim, 2008; Franke & Robinson 1990; Heale et al., 2020, 2022; Snively & Pasko, 2003; Song et al., 2003; Vadas & Becker, 2019; Zhou et al., 2002.). Therefore, the jet GWs possibly caused instabilities with scales similar to the D2 wave's horizontal wavelength (∼88 km).…”

“…Making the assumption that the southeastward propagating GW in the AIRS data had ∼3-hr/29-min ground-based period, it can be asserted that the wave saturated at ∼45-50 km; therefore, the GWs with ∼100 ms −1 southeastward phase velocity at Davis (D2 in Figure 7) could be secondary GWs caused by breaking of the tropospheric jet GWs. Heale et al (2022) classified secondary GWs into three scale types of secondary GWs: GWs with horizontal wavelengths smaller than, similar to, and longer than a horizontal wavelength of a primary GW. Because the horizontal wavelength of D2 wave (∼88 km) is ∼3 times smaller than that of the jet GWs (∼241 km), the D2 wave could be corresponding to the smaller-scale secondary GWs in Heale et al (2022).…”

“…Heale et al (2022) classified secondary GWs into three scale types of secondary GWs: GWs with horizontal wavelengths smaller than, similar to, and longer than a horizontal wavelength of a primary GW. Because the horizontal wavelength of D2 wave (∼88 km) is ∼3 times smaller than that of the jet GWs (∼241 km), the D2 wave could be corresponding to the smaller-scale secondary GWs in Heale et al (2022). This type of GW is generated by localized forcing, whose scales are associated with instability and vortex scales, produced by a primary GW breaking (Chun & Kim, 2008;Franke & Robinson 1990;Heale et al, 2020Heale et al, , 2022Snively & Pasko, 2003;Song et al, 2003;Vadas & Becker, 2019;Zhou et al, 2002.).…”