Numerical Simulation Of Gravity Wave Ducted Propagation And Wave-Flow Interaction In The Middle And Upper Atmosphere

Atmospheric gravity wave(GW)plays an important role in the momentum and energy transfer between different layers of the atmosphere.The ducted propagation of GW affects the energy transfer and coupling in the vertical and horizontal directions of the atmosphere.GW self-acceleration and breaking cascade packet energy to smaller scales of instability,turbulence and large-scale secondary gravity wave(SGW)and acoustic wave(AW),which also plays an important role in energy transfer between different layers of the atmosphere.These contents are the focus of this paper.Alough there are already mature linear theories to study the propagation characteristics of GW,the ducted propagation of GW is a complex singularity problem in mathematics.The GW self-acceleration and breaking are completely a nonlinear behavior,it is difficult to solve it by analytical methods.Through numerical simulation,these nonlinear problems are well understood to some extent.In this paper,we study the above problems based on two nonlinear compressible atmospheric models,which based on the full implicit continuous eulerian shceme(named as FICE)and finite volume(named as CGCAM),respectively.Both of these two models can switch between 2-D and 3-D models.Using the FICE model,we study the following two parts:1.Numerical simulation of the long horizontal distance propagations of atmospheric GW in a stratospheric thermal duct(Section 3).The numerical results show:(1)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.(2)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.(3)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.(4)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.2.Numerical simulation of the long horizontal distance propagations of atmospheric GW in a mesospheric Doppler duct(Section 4).The numerical results show:(1)The characteristics described above for the ducted propagation of the thermal duct are operative in the Doppler duct.(2)There are some differences in the characteristics of ducted propagation between the thermal duct and Doppler duct.In the thermal duct,the propagation velocity of the 0th wave mode is always the smallest,while in the Doppler duct,the propagation velocity of the 0th wave mode is always the largest.Regardless of whether in a thermal duct or a Doppler duct,a wave mode with the largest propagation velocity seems to be always slow in energy dissipation.When we observe the GW characteristics of thermal duct and Doppler duct in multiple stations,the observed GW characteristics may be different.For example,In the case 1 of the Doppler duct,the domain wave mode has lower frequency in the region close to the wave source,while it has a higher frequency in the region far from the wave source.This is exactly the opposite of what is obtained in the case 1 of the thermal duct.Using the CGCAM model,we study the following two parts:1.The exploration of self-acceleration dynamics to a GW packet localized in 2-D propagating in an idealized atmosphere at rest under mean solar conditions(Section 5).High resolution in the central 2.5-D domain enables the description of 3-D instability dynamics accounting for breaking,dissipation,and momentum deposition within the GW packet.2-D results describe responses to localized self-acceleration effects,including generation of SGW at larger scales able to propagate to much higher altitudes.2.5-D results exhibit instability forms consistent with previous 3-D simulations of instability dynamics and cause SGW generation and propagation at smaller spatial scales to weaken significantly compared to the 2-D results.SGW responses at larger scales are driven primarily by GW/mean-flow interactions arising at early stages of the self-acceleration dynamics prior to strong GW instabilities and dissipation.As a result,they exhibit similar responses in both the 2-D and 2.5-D simulations and readily propagate to high altitudes at large distances from the initial GW packet.2.The exploration of self-acceleration dynamics to a GW packet localized in 3-D propagating into tidal winds in the mesosphere and thermosphere(Section 6).As in the 2-D packet responses,3-D GW self-acceleration dynamics are found to be significant,and include 3-D GW phase distortions,stalled GW vertical propagation,local instabilities,and SGW and acoustic wave generation.Additional 3-D responses described here include refraction by tidal winds,localized 3-D instabilities,asymmetric SGW propagation,reduced SGW and AW responses at higher altitudes relative to 2-D responses,and forcing of transient,large-scale,3-D mean responses that may have implications for chemical and microphysical processes operating on longer time scales.