Numerical Simulation of Mountain Waves over the Southern Andes. Part I: Mountain Wave and Secondary Wave Character, Evolutions, and Breaking

Abstract

This paper addresses the compressible nonlinear dynamics accompanying increasing mountain wave (MW) forcing over the southern Andes and propagation into the mesosphere and lower thermosphere (MLT) under winter conditions. A stretched grid provides very high resolution of the MW dynamics in a large computational domain. A slow increase of cross-mountain winds enables MWs to initially break in the mesosphere and extend to lower and higher altitudes thereafter. MW structure and breaking is strongly modulated by static mean and semidiurnal tide fields exhibiting a critical level at ~114 km for zonal MW propagation. Varying vertical group velocities for different zonal wavelengths λx yield initial breaking in the lee of the major Andes peaks for λx ~ 50 km, and extending significantly upstream for larger λx approaching the critical level at later times. The localized extent of the Andes terrain in latitude leads to “ship wave” responses above the individual peaks at earlier times, and a much larger ship-wave response at 100 km and above as the larger-scale MWs achieve large amplitudes. Other responses above regions of MW breaking include large-scale secondary gravity waves and acoustic waves that achieve very large amplitudes extending well into the thermosphere. MW breaking also causes momentum deposition that yields local decelerations initially, which merge and extend horizontally thereafter and persist throughout the event. Companion papers examine the associated momentum fluxes, mean-flow evolution, gravity wave–tidal interactions, and the MW instability dynamics and sources of secondary gravity waves and acoustic waves.

First studies of mesosphere and lower thermosphere dynamics using a multistatic specular meteor radar network over southern Patagonia

“…Hence, it is clear that the wave activity in the MLT region over the Patagonian sector is very strong. This is also consistent with previous studies based on satellite measurements (e.g., Ern et al, 2011), and numerical simulations (e.g., Lund et al, 2020).…”

Modeling Responses of Polar Mesospheric Clouds to Gravity Wave and Instability Dynamics and Induced Large‐Scale Motions

“…The resulting scheme is globally conservative for mass, momentum, total energy, and kinetic energy. Time advancement is achieved via a low-storage, third-order accurate Runge-Kutta scheme with a fixed time step of  Δ 0.8 t s. Additional details for CGCAM are provided by Dong et al (2020) and Lund et al (2020).…”

Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54°S, 36°W) and comparison to WACCM simulations

“…Crucially, these 2GWs (and higher order) appear to consist of concentric rings in modelling and observational studies (Kogure et al, 2020;Lund et al, 2020;Fritts et al, 2021) that radiate in all directions, except for the direction exactly perpendicular to the original breaking GW. This could explain why we do not see a strong zonal tendency for GW variance during winter above 85 km, as the GW directions are more evenly distributed in azimuth.…”

“…South Georgia is located near to the global GW "hot spot" of activity in the stratosphere of the southern Andes and Antarctic Peninsula (Hindley et al, 2015;Hoffmann et al, 2013;Hindley et al, 2020), and is also an intense source of wintertime GW activity itself (Hoffmann et al, 2014;Hindley et al, 2021). Further, recent observations and modelling have indicated significant generation and propagation of 2GWs in the mesosphere and thermosphere in the region (Becker and Vadas, 2018;Kogure et al, 2020;Lund et al, 2020;Fritts et al, 2021).…”