Spectral Analysis Of Tropical Cyclone Boundary Layer Based On Large-Eddy Simulation

The tropical cyclone boundary layer plays an important role in the generating and developing of tropical cyclone.In this paper,an ideal tropical cyclone experiment is conducted based on high resolution large-eddy simulation（LES）with high frequent output(1 min^-1).Holo-Hilbert spectral analysis（HHSA）is used to calculate the spectra of important variables in tropical cyclone boundary layer at temporal scale and spatial scale,and the possible modulated relationship between turbulence,small-scale vortices in the boundary layer and the larger scale processes of tropical cyclone is discussed.In this paper,the secondary eyewall formation process of tropical cyclone in LES experiment is analyzed,and the important role of the boundary layer in this process and the response of the boundary layer to it are discussed.In the process of secondary eyewall formation,there are horizontal convergence and water vapor convergence region at the inner side of the large radial inflow of the outer core region.The super gradient wind,which promotes the development of deep convection,is clearly showed in the front side of the inflow and horizontal convergence region.The results of HHSA of different locations in the tropical cyclone boundary layer at temporal scale shows that large-scale processes including the change of tropical cyclone intensity,the vortex Rossby waves,and vortex filamentation process play important roles in modulating turbulence and small-scale vortices.The Holo-Hilbert（HH）spectra of tangential wind speed are consistent at different locations and heights.This is not the case for radial wind speed spectra which exhibit a stronger signal of modulation by the propagation of vortex Rossby waves.In the HH spectra of the vertical motion,the relative contribution of modulation by the tropical cyclone intensity change decreases,while the effect of the low frequency modulation wave is stronger.The HH spectra of pseudo equivalent potential temperature show that the energy propagation of vortex Rossby waves may be one of the reasons for the recovery of the boundary layer.Besides,the spectra of total energy are more significant than the spectra of pseudo equivalent potential temperature,which reveals that total energy can be used as a better variable to reflect the thermodynamic changes in the tropical cyclone boundary layer.In this paper,the spectra of variables at spatial scale in the outer core region of tropical cyclone boundary layer are also analyzed by using HHSA.The results of spectral analysis are as follows.Firstly,for the large-eddy simulation experiment of ideal tropical cyclone with constant sea-level temperature,the spatial scale of the turbulence and small-scale vortices in tropical cyclone boundary layer is about 5kilometers.Secondly,the spectra at outer core region are different whether there is the secondary eyewall or not,which is mainly reflected in the difference of energy density and the difference of wavelength.Finally,the total energy has a better performance at the middle and lower layers of the tropical cyclone boundary layer in the results of spectral analysis.