Propagation Of Gravity Wave Packets Research Articles

Numerical simulation of the long‐range propagation of gravity wave packets at high latitudes

AbstractWe use a 2‐D, nonlinear, time‐dependent numerical model to simulate the propagation of wave packets under average high latitude, winter conditions. We investigate the ability of waves to propagate large horizontal distances, depending on their direction of propagation relative to the average modeled ambient winds. Wave sources were specified to represent the following: (1) the most common wave parameters inferred from observations of Nielsen et al. (2009) (18 km λx, 7.5 min period), (2) waves consistent with the average phase speed observed (40 m/s) but outlying horizontal wavelength and period values (40 km λx, 17 min period), and (3) waves which would be subject to strong ducting as suggested by Snively et al. (2013) (25 km λx, 6.7 min period). We find that wave energy density was sustained over large horizontal distances for waves ducted in the stratosphere. Waves traveling against winds in the upper stratosphere/lower mesosphere are more likely to be effectively ducted in the stratosphere and travel large horizontal distances, while waves which escape in the form of leakage are more likely to be freely propagating above 80 km altitude. Waves propagating principally in the direction of the stratopause winds are subject to weaker stratospheric ducting and thus increased leakage of wave energy density from the stratosphere. However, these waves are more likely to be subject to reflection and ducting at altitudes above 80 km based upon the average winds chosen. The wave periods that persist at late times in both the stratosphere and the mesosphere and lower thermosphere (MLT) range from 6.8 to 8 min for cases (1) and (3). Shorter‐period waves tend to become trapped in the stratosphere, while longer‐period waves can dissipate in the thermosphere with little reflection or trapping. It is suggested that the most common scenario is of partial ducting, where waves are observed in the airglow after they leak out of the stratosphere, especially at large horizontal distances from the source. Stratospheric ducting and associated leakage can contribute to a periodic and horizontally distributed forcing of the MLT.

Propagation of gravity wave packet near critical level

A couple of two-dimensional linear and fully nonlinear numerical models for compressible atmosphere are used to numerically study the propagation of the gravity wave packet into a mean wind shear. For a linear propagation wave packet, the critical level interactions are in good agreement with the linear critical level theory. The dynamically and convectively unstable regions are formed due to the critical level interaction of a finite-amplitude wave packet, but they would not break. The free exchange of potential energy with kinetic energy in the background atmosphere at rest ceases after entering the mean wind shear. However, it still goes on in the nonlinear propagation. It is shown that the nonlinear effects modify the mean flow markedly, reduce the momentum and energy propagation velocity and drop the elevation of the critical level. The gravity wave packet becomes unstable and breaks down into smaller scales in some regions. It expends much more kinetic energy than potential energy in the early phase of the breakdown. This means that the wave breakdown sets up due to the action of the shear instability rather than a convective one.

A numerical study on global propagations and amplitude growths of large‐scale gravity wave packets

By using a full‐implicit‐continuous‐Eulerian (FICE) scheme and taking a set of basic atmospheric motion equations in spherical coordinates as governing equations, a fully nonlinear numerical model for the dynamics of the middle and upper atmosphere is established to numerically study the nonlinear global propagations and amplitude growths of large‐scale gravity wave packets. The simulation results show that the newly established model can successfully exhibit the essential characteristics of the global propagations of gravity waves: wave energies propagate upward along the ray paths derived from the linear gravity wave theory, and wave disturbance amplitudes increase with the increasing heights. During the upward propagation of gravity wave packets, there occurs evident enhancement of background wind along the propagation direction. Moreover, our simulation demonstrates that Earth rotation has little influences on the wave energy propagations and amplitude growths. However, the primary nonlinear curvature terms ( tan and − tanθ) play an important role in the growths of gravity wave amplitudes, which causes a evident latitudinal dependence of wave amplitude growth. In the Northern Hemisphere, with the same spatial scales and initial zonal disturbance amplitudes, the waves propagating in lower‐latitude regions have larger disturbance amplitudes.

A numerical study of propagation characteristics of gravity wave packets propagating in a dissipative atmosphere

By using a two‐dimensional, full‐implicit‐continuous‐Eulerian (FICE) scheme, we simulated the nonlinear propagation and evolution of gravity wave packets in a compressible, nonisothermal and dissipative atmosphere. The numerical results show that when an upgoing gravity wave packet is generated in the lower mesosphere, it can propagate along its ray path until it reaches lower thermosphere. However, upon reaching the lower thermosphere, the wave packet and associated energy propagate almost horizontally, which departs obviously from the prediction of linear gravity wave theory under WKB approximation in the nondissipative case. Further discussion indicates that the influences of nonlinearity and background temperature are not strong enough to restrict completely the upward energy propagation of the wave packet and that the influence of constant molecular viscosity on the characteristics (energy propagation path and wave parameters) of gravity waves is insignificant. It is the vertical inhomogeneity of molecular viscosity that causes the restriction of upward energy propagation of the gravity wave packet. Moreover, throughout propagation, the dominant vertical wavelength of the wave packet decreases with time, as it is affected by the joint actions of nonlinearity, background temperature, and dissipation. These results indicate that the molecular viscosity, especially the vertical inhomogeneity of molecular viscosity, plays an important role in the nonlinear propagation of gravity wave packets.

A numerical simulation of nonlinear propagation of gravity wave packet in three-dimension compressible atmosphere

By using FICE scheme, a numerical simulation of nonlinear propagation of gravity wave packet in three-dimension compressible atmosphere is presented. The whole nonlinear propagation process of the gravity wave packet is shown; the basic characteristics of nonlinear propagation and the influence of the ambient winds on the propagation are analyzed. The results show that FICE scheme can be extended in three-dimension by which the calculation is steady and kept for a long time; the increase of wave amplitude is faster than the exponential increase according to the linear gravity theory; nonlinear propagation makes the horizontal perturbation velocity increase greatly which can lead to enhancement of the local ambient winds; the propagation path and the propagation velocity of energy are different from the results expected by the linear gravity waves theory, the nonlinearity causes the change in propagation characteristics of gravity wave; the ambient winds alter the propagation path and group velocity of gravity wave.

Behavior of gravity waves with limited amplitude in the vi-cinity of critical layer

By using the FICE scheme, a numerical simulation of three-dimensional nonlinear propagation of gravity wave packet in a wind-stratified atmosphere is presented. The whole nonlinear propagation process of the gravity wave packet is shown; the propagation behavior of gravity waves in the vicinity of critical layer is analyzed. The results show that gravity waves encounter the critical layer when propagating in the fair winds whose velocities increase with height, and the height of critical layer propagating nonlinearly is lower than that expected by the linear gravity waves theory; the amplitudes of gravity waves increase with height as a whole before gravity waves encounter the critical layer, but the increasing extent is smaller than the result given by the linear theory of gravity waves, while the amplitudes of gravity waves reduce when gravity waves meet the critical layer; the energy of wave decreases with height, especially at the critical layer; the vertical wavelength reduces with the height increasing, but it does not become zero.