On The Characteristics Of Stratospheric Gravity Waves Generated By An Eastward-Moving Southwest Vortex

The activity of gravity waves plays an important role in determining the dynamics,thermodynamics,and circulation structure of the atmosphere,and is one of the most important dynamical processes,in especially the lower stratosphere.In this dissertation,the characteristics of gravity waves in the lower stratosphere are investigated by selecting an eastward-moving southwest vortex process accompanied by persistent heavy rainfall during the Meiyu period in June 2011.The work of this paper is divided into two parts:the first part is focused on numerical study.Firstly,numerical simulation of the eastward-moving southwest vortex process is performed by using the WRF（Weather Research and Forecasting）model.The process is divided into three stages as development,mature and decay stages according to the intensity and location of the southwest vortex.Secondly,the fundamental features and spectral parameters,such as the spatial distribution,propagation direction,momentum flux and drag forcing,the frequency-wavenumber and phase velocity spectrum distribution,as well as the rotational kinetic energy spectrum and divergent kinetic energy spectrum,are diagnosed and analyzed,and the differences of the characteristics of stratospheric gravity waves in the three stages are revealed.Finally,the sensitivity experiments on the orography and latent heating are designed,and the wave source and generation mechanism of the gravity waves generated by the southwest vortex are revealed through comparative analyses.The second part is focused on the theory research.Firstly,the analytical solutions of the convectively-generated stratospheric gravity wave is obtained under the lower boundary condition of forced oscillation by using the two-dimensional,non-hydrostatic,linear equations based on Boussinesq approximation.Secondly,the analytical solutions are discussed,the fundamental features of the analytical solutions under different background environments are presented,and the conditions for the resonance between the stratospheric gravity waves and convective forcing are proposed.Finally,the analytical solution is numerically calculated by using the actual southwest vortex environmental and spectral parameters.The theoretical calculation and the numerical simulation results are compared and analyzed,and the dynamical explanation of the convective gravity waves is given.The main innovative conclusions are summarized as follows:（1）The stratospheric gravity waves appear on the east and south sides of the southwest vortex,which are distributed in the shape of semicircular bands.It can be divided into two branches according to the structure.One branch is on the east side of the low vortex,which propagates to the northeast.The other branch is on the south side of the vortex,which spreads eastward.The amplitude of the waves increases with the intensification of the southwest vortex and precipitation.（2）The wave sources of the gravity waves generated by the southwest vortex are convection and latent heating caused by the precipitation.Moreover,the gravity waves generated by the convection have a wider range of frequencies and wavenumbers,namely,small-scale and high-frequency gravity waves can be excited.（3）During the three stages of the eastward movement of the southwest vortex,the momentum flux of the gravity waves is selective to the magnitude of the phase velocity and the propagation direction of the waves.The total momentum flux propagates eastward and northward,and the large values mainly appear in the range of medium and low phase velocities less than 30 m s^-1.In the development stage,the peak value of northward-propagating momentum flux is larger.In the mature stage,the peak values of momentum flux propagating eastward and northward are comparable.In the weakening stage,momentum flux is mainly manifested as eastward propagation.（4）The analytical solution of the stratospheric gravity waves generated by convective forcing is obtained,and it is found that the vertical wavenumber of the gravity waves decreases with the increase of the frequency and wavenumber of the forced oscillation.The resonance condition is derived,and the dynamical explanation of the stratospheric gravity waves generated by the southwest vortex is given.