A Numerical Study of Gravity Wave Propagation Characteristics in the Stratospheric Thermal Duct

Abstract

We developed a fully nonlinear, two-dimensional, numerical model to simulate the long horizontal distance propagations of atmospheric gravity waves in a stratospheric thermal duct. The numerical results show that after the wind disturbance excited by the initial wave forcing enters the duct completely, symmetric and antisymmetric modes can be identified in the horizontal direction. The frequency-wave number spectrum indicates that various modes with different spatial structures can exist simultaneously in the duct. The primary parameters (horizontal wavelength, vertical wavelength and wave frequency) of the ducted wave packets in a given thermal duct are mainly determined by the horizontal wavelength of the initial wave forcing. The mean ratio of the upward propagating energy to the downward propagating energy in the thermal duct was regarded as an approximation of the power standing wave ratio (PSWR). The time variation of PSWR indicates that the standing wave was formed with the propagation of the ducted waves. During the gravity waves ducted propagation, most wave energy is restricted to the ducting region and dissipates slowly with time. The higher the wave frequency (or the shorter the horizontal wavelength) of the initial wave forcing, the slower the wave energy dissipates.

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

“…These locations exhibit limited bright GW phases due to the weaker GW responses at the leading edges and the void formation where GW amplitudes are larger. Such features resemble a number of observations of “fronts” and “wall waves” in other modeling and PMC and airglow imaging studies (Dong et al., 2018; Fritts, Kaifler et al., 2020; Swenson et al., 1998; Shiokawa et al., 2003). A number of such features can likely be attributed to local momentum deposition at a ducting level (e.g., a highly structured environment such as a mesospheric inversion layer or Doppler duct due to oscillatory tidal or inertia‐GW winds) enhancing GW amplitudes and the potential for instabilities at its edges.…”

“…The potential for high‐frequency GWs arising from lower atmosphere sources to propagate into the MLT depends strongly on the intervening wind and temperature fields (e.g., Dong et al., 2018; Yiğit & Medvedev, 2015, 2017). Realistic GW environments are surely more structured than we have employed here, and will influence the forms and responses of SA dynamics in the MLT.…”

Self‐Acceleration and Instability of Gravity Wave Packets: 2. Two‐Dimensional Packet Propagation, Instability Dynamics, and Transient Flow Responses

“…GWs attaining large amplitudes can undergo breaking at lower and higher altitudes, whether or not a critical level is present (Doyle et al, ; Horinouchi et al, ; Lilly, ; Lilly & Kennedy, ). In contrast, small‐amplitude GWs having suitable propagation environments can largely avoid instabilities at lower altitudes and break instead in the MLT (Eckermann et al, ; Fritts, Laughman, et al, ) or propagate without apparent breaking well into the thermosphere and ionosphere for sufficiently large phase speeds (Azeem et al, ), or be trapped in the lower atmosphere (Dong et al, ).…”

A Numerical Study of Gravity Waves Propagation Characteristics in the Mesospheric Doppler Duct

“…Small‐amplitude GWs having suitable propagation environments can largely avoid becoming unstable at lower altitudes. They may instead break at higher altitudes, propagate without apparent breaking well into the thermosphere and ionosphere (TI) (Azeem et al., 2015), be blocked by the highly variable winds in the TI (Larsen, 2002), become trapped in thermal/Doppler ducts (Dong et al., 2018; Fritts et al., 2018a, 2018b; Hickey, 2001; Snively et al., 2007).…”

“…TI (Larsen, 2002), become trapped in thermal/Doppler ducts (Dong et al, 2018;Fritts et al, 2018aFritts et al, , 2018bHickey, 2001;Snively et al, 2007).…”