On The Characteristics Of Gravity Waves Generated By Two Typical Convective Systems

In this thesis,gravity waves generated by mei-yu front and tropical cyclone(TC),which are both typical convective systems,are investigated based on high-resolution numerical model.By conducting idealized simulations and sensitivity experiments,using the methods of wave spectral and energy spectral analyses,this thesis studies the generation mechanism and spectral characteristics of gravity waves,and the effects of moisture on gravity-wave spectra in the mei-yu front system;compares the spectra of gravity waves generated by mei-yu front and TC;examines the effects of tropospheric vertical wind shear on gravity waves generated by TCs;and analyzes the energy spectra of TC and the role of gravity waves.The main conclusions are as follows:Two classes of gravity waves are generated successively during the evolution of the mei-yu front.The first class of waves,which is fanlike waves excited by the dynamical evolution of the front,propagates vertically from the troposphere to the lower stratosphere and has a coherent wave structure.The second class of waves,which belongs to convective gravity waves excited by the prefrontal moist convection,is vertically trapped in the troposphere and vertically propagating upstream and downstream in the lower stratosphere.Quantitative diagnosis suggests that the mechanical oscillator mechanism dominates the generation of the convective waves in the lower stratosphere.The convective gravity waves result in net northward momentum fluxes and drag forcing in the stratosphere,and the drag obviously intensifies with height.With the increase of initial moisture content,the momentum fluxes and drag forcing intensify and extend to smaller spatial-temporal scales.However,the change of moisture does not alter the structure of phase-speed spectra of momentum fluxes,but modifies their intensity within a certain phase-speed range.Although the amplitude of fanlike waves excited by the front is relatively weak,their energy distributes intensively in the spectral space,which makes them couple with the convective waves effectively in slow phase speeds and leads to the weakening of the wave packets.By comparison,it is found that convective systems with different structures will generate gravity waves with distinctly different spectral characteristics.As a quasi-two-demensional linear system,the mei-yu front appears as a line wave source and the generated waves in the lower stratosphere propagate mainly across the front;while as a vortex-like system,the TC appears as a point wave source and the generated waves show isotropic strongly,which propagate outwards from the storm center equally in all horizontal directions.Tropospheric vertical wind shear remarkably influences gravity waves generated in the TCs.Without the shear,gravity-wave momentum fluxes in the lower stratosphere are distributed uniformly over a broad spectral range,with fluxes in opposite directions showing symmetry.Under westerly shear,the wave activity leads to net westward fluxes and drag forcing.The active fluxes extend to shorter horizontal wavelengths,and contract to longer periods and slower phase speeds.The investigation on energy spectra of a TC shows that,during the mature period of the TC,the horizontal kinetic energy(HKE)and available potential energy(APE)spectra both exhibit a quasi –5/3 slope for wavelengths below 500 km.The HKE spectra display an arc-like shape in the troposphere and a quasi-linear shape in the lower stratosphere,while the APE spectra exhibit consistent quasi-linear shape at all heights.The spectral budgets of APE are leaded by diabatic and energy conversion processes,of which the diabatic effect is the most important source of APE,and the APE is then converted to other forms of energy by the buoyancy effect.The conversion tends to towards HKE at larger scales and towards gravitational potential energy of total moist species at smaller scales.The vertically propagating gravity waves influence the formation and maintenance of HKE spectra not only by transporting energy vertically,but also by modifying the intensity and direction of energy cascade.