Numerical Modeling of the Generation of Tertiary Gravity Waves in the Mesosphere and Thermosphere During Strong Mountain Wave Events Over the Southern Andes

Abstract

We investigate the effects on the mesosphere and thermosphere from a strong mountain wave (MW) event over the wintertime Southern Andes using a gravity wave (GW)‐resolving global circulation model. During this event, MWs break and attenuate at z∼50–80 km, thereby creating local body forces that generate large‐scale secondary GWs having concentric ring structure with horizontal wavelengths λH=500–2,000 km, horizontal phase speeds cH=70–100 m/s, and periods τr∼3–10 hr. These secondary GWs dissipate in the upper mesosphere and thermosphere, thereby creating local body forces. These forces have horizontal sizes of 180–800 km, depending on the constructive/destructive interference between wave packets and the overall sizes of the wave packets. The largest body force is at z=80–130 km, has an amplitude of ∼2,400 m/s/day, and is located ∼1,000 km east of the Southern Andes. This force excites medium‐ and large‐scale “tertiary GWs” having concentric ring structure, with λH increasing with radius from the centers of the rings. Near the Southern Andes, these tertiary GWs have cH=120–160 m/s, λH=350–2,000 km, and τr∼4–9 hr. Some of the larger‐λH tertiary GWs propagate to the west coast of Africa with very large phase speeds of cH≃420 m/s. The GW potential energy density increases exponentially at z∼95–115 km, decreases at z∼115–125 km where most of the secondary GWs dissipate, and increases again at z>125 km from the tertiary GWs. Thus, strong MW events result in the generation of medium‐ to large‐scale fast tertiary GWs in the mesosphere and thermosphere via this multistep vertical coupling mechanism.

Secondary Gravity Waves Generated by Breaking Mountain Waves Over Europe

“…At hr (Figures c and f), nonprimary waves, at a range of scales, are dominant in the thermosphere (above 100 km) with amplitudes of up to 250 K. The initial nonprimary waves, seen in Figures b and e, undergo further breaking and turbulence particularly at 130‐km altitude leading to further wave generation. Vadas and Becker () show a similar scenario in their GCM simulations of mountain wave breaking over the Andes. They find that primary mountain waves breaking between 60 and 80 km that generates secondary waves that are then dissipated around 110‐km altitude generate a further body force and produce tertiary waves.…”

“…The westward and eastward waves could simply be the emergence of “waves” that tunnel through the evanescent region and continue to propagate according to the local wind and shear as they emerge. However, it could also be the case that some of the waves are generated by further wave dissipation as described by Vadas and Becker (). In this mechanism, initial nonprimary waves are further dissipated or break (between 80 and 130 km, peaking at 110 km in their study), which generates body forces that produce further waves.…”

“…However, the situation is complicated by the fact that there is more than one level of breaking as can be seen in Figures c and f. Therefore, waves in the upper atmosphere could be generated by multiple levels of breaking and new wave production (Vadas & Becker, ). It is difficult to trace the origins of spectral components in the thermosphere directly to regions in the primary wave vicinity.…”

“…The latter mechanism is better suited to explaining the large wave scales dominant at 150‐km altitude; however, the periods are 16 and 22 min, respectively, which are much much shorter than the quasi‐stationary primary mountain wave. Vadas and Becker () a found that the dissipation of intersecting wave packets can lead to constructive and destructive forcing that generates waves with horizontal wavelengths smaller than the initial packet. It is also noted that very localized forcings (in either space or time) can produce a broad spectra of waves (Heale et al, , ), a prominent example of which are convective forcings from individual thunderstorm plumes (Alexander & Holton, ; Beres, ; Lane et al, ).…”

Challenges to Understanding the Earth's Ionosphere and Thermosphere

“…Gravity wave sources are variable and complex, including flow over mountain ridges, shear along wind jets, earthquakes and associated tsunamis, tropical convections, and frontal systems (Fritts & Alexander, 2003). Several possible pathways for gravity waves to reach the thermosphere and ionosphere are identified such as direct propagation and generation of secondary and tertiary waves in the mesosphere and thermosphere (e.g., Becker & Vadas, 2018;Vadas & Becker, 2019;Vadas & Liu, 2009), but all are sensitive to the background wind and temperature conditions. While most of these pathways are revealed in high resolution and\or regional models (e.g., Becker & Vadas, 2018;Heale et al, 2017;Miyoshi et al, 2014), their more global effects on the I-T are not directly included in models that do not have the necessary spatial resolution.…”