Directional Filtering Due to Mesospheric Wind Shear on the Propagation of Acoustic-gravity Waves

Gravity waves with periods close to the Brunt-V(a|¨)is(a|¨)l(a|¨) period of the upper troposphere are often observed at mesopause altitudes as short period,quasi-monochromatic waves.The assumption that these short period waves originate in the troposphere may be problematic because their upward propagation to the mesosphere and lower thermosphere region could be significantly impeded due to an extended region of strong evanescence above the stratopause.To reconcile this apparent paradox,an alternative explanation is proposed in this paper.The inclusion of mean winds and their vertical shears is sufficient to allow certain short period waves to remain internal above the stratopause and to propagate efficiently to higher altitudes.A time-dependent numerical model is used to demonstrate the feasibility of this and to determine the circumstances under which the mesospheric wind shears play a role in the removal and directional filtering of short period gravity waves. Finally this paper concludes that the combination of the height-dependent mean winds and the mean temperature structure probably explains the existence of short period,quasi-monochromatic structures observed in airglow images of mesopause region.

Numerical Simulation of Acoustic-Gravity Waves Propagation in a Heterogeneous Earth-Atmosphere Model with Wind in the Atmosphere

A numerical-analytical solution for seismic and acoustic-gravity waves propagation is applied to a heterogeneous “Earth-Atmosphere” model. Seismic wave propagation in an elastic half-space is described by a system of first order dynamic equations of elasticity theory. Propagation of acoustic-gravity waves in the atmosphere is described by the linearized Navier-Stokes equations with the wind. The algorithm proposed is based on the integral Laguerre transform with respect to time, the finite integral Fourier transform along the spatial coordinate with the finite difference solution of the reduced problem.

Study on generation mechanism of anomalous acoustic-gravity waves before the 2011 Beijing earthquake(M_L=3.0)

Observation results of abnormal acoustic-gravity waves before a Beijing earthquake （ML =3.0） are presented. During this period, abnormalities of earth surface tilt variations were also recorded. The cross-correlations between the both values are high, which reach maximal values of 0.5 in the area close to the epicenter. The correlations decrease with increasing distances from the epicenter. It was proposed that generation of the anomalous waves may be associated with the pressure and wind perturbations in the air flow caused by slowly shaking mountains during slow surface motion preceding the earthquake in Beijing. Based on the wind velocity data taken from a 350 m meteorological tower in Beijing, the propagation of ducted acoustic-gravity waves in a two-layer model of the atmosphere was numerically simulated. It is shown that characteristic periods, amplitudes and velocities of the simulated phases which were assumed from non-stationary air flow relative to mountains are approximate to the observed phases. A consistency between the simulated results and observation data indicates that a slow surface motion may be a possible source of the anomalous acoustic-gravity waves observed prior to the earthquake.

A COMPUTER MODEL OF DERIVATIVE METHOD FOR POINT SOURCE GRAVITY WAVES IN ATMOSPHERE

Analytical Solutions of point source gravity wave s in an isothermal atmosphere are given.A computer model based on the analytical solutions is developed for point source gravity waves in a horizontally and isothermally stratified atmosphere.The numerical calculation of the propagation of point source gravity waves in a tealistic atmosphere then becomes feasible. The propagation of point source acoustic waves in the atmosphere can also be numerically calculated using the same model.