Theoretical Analysis And Simulation Studies Of The Impact Of Atmospheric Waves On The Middle And Upper Atmosphere

Accurate modeling of the middle and upper atmosphere（including the stratosphere,mesosphere,and thermosphere）is a major objective of the development of whole atmosphere general circulation models（GCMs）,and an essential way to forecast near-space environment and space weather and climate.As a key medium connecting the lower atmosphere with the middle and upper atmosphere,atmospheric waves drive general circulation by transfering momentum and energy between atmospheric regions,and affect atmospheric compositions and temperature through mixing.Therefore,the study of atmospheric waves and their effects on the middle and upper atmosphere has been a hot topic for a long time.GCMs can generally resolve large scale atmospheric waves（such as planetary waves and tidal waves）,but cannot resolve small and medium scale atmospheric waves（such as gravity waves）.The influence of small and medium scale atmospheric waves on the middle and upper atmosphere is represented in the model through the parameterization scheme.However,the current whole atmosphere GCMs still have many defects in simulating the real middle and upper atmosphere,which largely depends on the simulation level of large-scale atmospheric waves and the parametric scheme design of small and medium-scale atmospheric waves.In particular,the current whole atmosphere GCMs do not have a comprehensive gravity wave parameterization scheme that takes into consideration of the highly viscous thermosphere.Based on the above problems,we first use model,observational and reanalysis data to study and compare the spatial patterns of winter stratopause temperature trends in the Northern Hemisphere and the contribution of quasi-stationary planetary waves to the trends（Chapter 3）.Secondly,a parameterization scheme of anisotropic gravity wave sources is developed in the Whole Atmosphere Community Climate Model Extension（WACCM-X）to reproduce the seasonal variation of the mesosphere zonal reversal circulation.On this basis,the inertial gravity wave parameterization scheme is introduced to reduce the strong stratospheric jet and cold-pole bias（Chapter 4）.Using the governing equations of gravity waves in the viscous environment,the relationship between the dispersion and polarization of gravity waves in the viscous environment is derived,and the influence of molecular viscosity and thermal conduction on the propagation and dissipation of gravity waves in the thermosphere is obtained（Chapter 5）.Based on the viscous dispersion relationship,a new gravity wave parameterization scheme is formulated.A column model is developed to test the parameterization scheme from the troposphere to thermosphere（Chapter 6）.Finally,this new thermospheric gravity wave parameterization scheme is implemented in the WACCM-X model.This is the first time a gravity wave parameterization scheme applicable for the thermosphere is incorporated in WACCM-X.It will greatly improve the simulation results of general circulation and chemical distribution in the theremosphere（Chapter 7）.The main results of the dissertation are as follows:（1）Our analysis reveals a zonally asymmetric temperature trend pattern near the northern mid-to-high latitude stratopause during January,and this zonally asymmetric temperature trend pattern underwent an evident transition around the2000s.From 1980 to 2001,there was a cooling trend in the western hemisphere and a warming trend in the eastern hemisphere.In contrast,a reversed zonally asymmetric temperature trend pattern existed in the east-west direction from 2001 to 2020.Although the warming trends are statistically insignificant,they contrasted with the overall cooling trend in the upper stratosphere due to ozone depletion and an increase in well-mixed greenhouse gases in recent decades.The zonally asymmetric temperature trends were induced by the transition in the intensity of quasi-stationary planetary wavenumber 1（wave 1）near the stratopause.The increasing（decreasing）trend of the intensity of wave 1 enhanced（reduced）its meridional temperature advection near the stratopause before（after）the 2000s,consequently,a zonally asymmetric temperature trend pattern exists in the east-west direction near the stratopause.The transition in the intensity of the stratospheric wave 1 around the2000s is most likely caused by the transition in the intensity of wave 1 activity in the troposphere.The result of the reanalysis data is similar to that of the satellite observation,which can better show the shape of the zonal asymmetric temperature trend near the stratosphere,but the model data does not show the obvious shape of the zonal asymmetric temperature trend,which may be caused by the difference between the model and the reanalysis data in the analysis of quasi-stationary planetary wave 1.（2）Seasonal variations in the mesosphere zonal wind reversal（i.e.,the altitude of the westward wind reversal in the winter hemisphere is higher than that of the easterward wind reversal in the summer hemisphere）have been observed by various satellites and radars,but this seasonal feature is often not well captured in whole atmosphere GCMs with current gravity wave parameterization schemes.We introduce an anisotropic gravity wave source spectrum with weak westward momentum flux and strong eastward momentum flux to reproduce this seasonal feature in WACCM-X.The simulation results show that after the introduction of anisotropic gravity wave parameterization scheme,the altitdue of the westerward wind reversal in the mesosphere is up to 0.0001 h Pa（about 95 km）,and the altitude of the easterward wind reversal in the mesosphere is between 0.01 and 0.001 h Pa（about80 km）.Furthermore,additional stratospheric forcing is needed to control the large winter stratospheric zonal wind and alleviate the“cold-pole”problem in the southern winter.This is accomplished by the application of an inertial gravity wave parameterization scheme.With these changes,the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension（WACCM-X）can produce zonal mean zonal wind that is in better agreement with climatology from the stratosphere to the mesosphere and low thermosphere（MLT）,including the seasonal variation of the zonal wind reversal.（3）Using the governing equations of gravity wave-acoustic wave in the viscous environment,the gravity-acoustic wave dispersion and polarization relations under the effects of molecular damping（including molecular viscosity and thermal conduction）on gravity waves are derived.Based on the viscous dispersion relation,the propagation and dissipation parameters of gravity waves in thermosphere are obtained by an iterative method,including vertical wavelength,dissipation rate and dissipation height.The results show that with the exponentially increasing molecular viscosity and thermal conduction with height in the thermosphere,the dissipation rate of gravity wave caused by molecular damping grows exponentially.Under the influence of molecular damping,the dissipation of gravity wave is sensitively dependent on the square of the vertical wavelength and the phase velocity（i.e.,the dissipation rate is inversely proportional to the vertical wavelength and the phase velocity）.Gravity waves with short vertical wavelength（tens of km）and slow phase velocity（tens of m/s）dissipate rapidly in the low thermosphere However,gravity waves with longer vertical wavelength（hundreds of km）and faster phase velocity（hundreds of m/s）can propagate to the middle-upper thermosphere.In addition,we further derive the formulas of gravitational wave drag,dissipative heating and vertical diffusion coefficients in viscous environment.These formulas are the theoretical basis for the development of thermospheric gravity wave parameter scheme.（4）Combining the linear saturation theory in the middle and lower atmosphere with the molecular damping theory in the middle and upper atmosphere,a column model is developed to test the parameterization based on the formulation described above from the troposphere to the thermosphere.The simulation results show that the column model can well simulate the propagation and dissipation processes of parameterized gravity waves in the whole atmosphere due to wave breaking and molecular damping,and the model output,including gravity wave drag,dissipative heating and effective vertical diffusion coefficient.Under the effects of wave breaking,the gravity wave will dissipate discontinuously during upward propagation,and the dissipation is concentrated from the mesospherr to the lower thermosphere.Under the effects of molecular damping,with the exponential increase of molecular damping in the middle-upper atmosphere,the gravity wave dissipates continuously during upward propagation.Gravity waves with faster phase speed have stronger and higher dissipation intensity.In addition,we introduced wave sources in the lower thermosphere to represent secondary gravity waves.Based on high-resolution WACCM-X simulations,these waves are assumed to have high phase velocities and can thus propagate to higher altitudes.Moreover,the dissipation of gravity wave drags and effective vertical diffusion coefficient in the thermosphere is significantly larger than that in the mesosphere The simulation results of the column model demonstrate significant impacts of the gravity wave and the importance of thermospheric gravity wave parameterization scheme of the WACCM-X model.（5）We have implemented this thermospheric gravity wave parameterization scheme in the WACCM-X（including the effects of molecular viscosity and thermal conduction on gravity waves,the secondary gravity wave sources and time-efficient for thermospheric gravity waves）.This is the first time a gravity wave parameterization scheme that is applicable for the thermosphere is incorporated in WACCM-X.With this scheme,WACCM-X can capture gravity wave dissipation effect in the thermosphere（including zonal drag,meridional drag and effective vertical diffusion coefficient）.Furthermore,local time（LT）dependence of thermospheric gravity wave is represented in the model,with LT 5 to 10 hour in the Northern Hemisphere in winter and LT 3 to 8 hour in the Southern Hemisphere in winter.For the monthly mean simulation of WACCM-X,the latitudinal/longitudinal drag is stronger in the middle and high latitudes at about 200 to 400 km altitude,mainly in the east/south direction（up to hundreds of m/s/day）,while the intensity of gravity wave drag at low latitudes is weak.The propagation and dissipation characteristics of these gravity waves are consistent with the results of high-resolution model simulation.Considering effects of thermospheric gravity wave drags,the zonal wind in the thermosphere becomes more eastward,and the northward wind in the summer hemisphere strengthens,while the northward wind in the winter hemisphere weakens.The gravity wave drag also modifies the mean thermospheric circulation,in particular weakens the polarward/downward circulation in the winter hemisphere.This in turn leads to changes in the thermospheric composition,as seen most evidently in decrease of∑/₂.In addition,we introduce effective vertical diffusion coefficient into WACCM-X.Consequently,the global averaged effective vertical diffusion coefficient more than 36 m²/s.The effective diffusion coefficient can accelerate the mixing of chemical compositions and further reduce∑/₂.As a result,the maximum∑/₂ decreased from 2.2 to 1.6,which is closer to observations.Therefore,the introduction of the thermospheric gravity wave parameterization scheme in WACCM-X model greatly improves the representation of the dynamical and compositional structure of the thermosphere.