重力波在中层大气温度波导中的传播模式研究

本文给出了重力波在中层大气温度波导中的导制传播模型,并在此模型的基础上详细讨论了重力波部分导制传播下的对称模式与非对称模式,导出了不同模式下相应的特征函数和色散方程,进一步用离散的方法对两类色散方程进行了求解;同时还利用二维全隐欧拉格式（FICE）对重力波在温度波导中的传播进行了模拟,模拟的结果也成功地展现了对称与非对称两种传播模式.研究表明,下边界的扰动能量在向上传播进入波导区域后被俘获,形成导制传播.不同周期的初始扰动,在波导内均会形成对称与非对称形式两种模式的导制传播,由于两者的行进速度不一致,最终会引起两种不同模式的分离.数值模拟中重力波的水平行进速度与线性模型预测值非常接近.波导中不同模式下重力波的水平波长与初始扰动的水平波长非常一致,然而波导区域内重力波的频率与初始扰动的频率无关,频率不同的初始扰动会激发出相同频率的重力波对称与非对称导制传播模式.这表明在确定的温度波导中,水平波数才是决定重力波传播特性的决定因素.进一步的分析显示,初始扰动的水平波数-频率分布越接近完全导制传播的色散关系时,温度波导中更易于生成以该种模式部分导制传播的重力波.     

重力波波包在向上传播过程中的破碎

采用二维全隐欧拉（FICE）格式对具有高斯分布的重力波波包在可压大气中传播时的饱和过程进行数值模拟和分析．数值计算结果表明,波振幅首先随高度增加而增加,但当波振幅接近于线性不稳定性给出的阈值时,不再增加,重力波波包达到饱和进而破碎．破碎出现的高度（86．50km）比线性理论预言的结果（84．59km）要高一些,并且一般都在波包的下游出现．波破碎过程能使波能量在空间重新分配,并对重力波能量有明显的耗散作用．并且波破碎会使波相关能量传输方向偏离线性射线理论的射线路径．     

用传输函数构建的大气重力波传播理论模式

本文根据考虑大气热传导和黏滞的重力波复色散关系,采用传输函数的概念,基于重力波的线性理论,构建了用于研究对流层内重力波激发源与电离层响应之间的传输函数数值模式.在相空间中讨论了传输函数振幅的分布特性,并以地面单位脉冲源为例,分析了从地面到300 km高空的响应,得到了物理量的时空分布特征.结果表明：（1）对内重力波的传播而言,大气相当于一个滤波器,只有波动周期在15～30 min,水平波长在200～450 km之间的重力波扰动才最容易到达300 km电离层高度;（2）电离层的响应主要在与地面的激发源之间相隔较远的水平距离上发生;（3）黏滞和热传导系数在低层对上传重力波的影响较小,随着高度的增加它们对重力波的影响越来越大;（4）在低层计算的波动频率与Row理论的计算结果比较一致,然而到了高层却相差较大.     

耗散大气中水平不均匀风场对内重力波传播的影响

采用包括耗散的射线跟踪方法，计算了在水平不均匀风场作用下，不同尺度重力波从对流层直至220km观测高度的传播，结果表明，垂直于重力波传播方向的风以及风剪切能够引起波射线的折射，从而导致重力波明显偏离初始传播方向.在强顺风场作用下，由于风场引起的捕获，大量重力波不能传播到观测高度.由于风场引起的多普勒频移，小周期的重力波在弱顺风条件下能够传播到观测高度.由于反射作用，强逆风场不支持周期低于约18min的较高频重力波的传播.而在弱逆风作用下，大部分中尺度范围重力波都能够传播到观测高度.本文统计了武汉电离层观象台的TID观测数据随热层风场的分布，统计结果与模拟结果符合较好.     

利用COSMIC RO数据分析青藏高原平流层重力波活动特征

本文利用2006年5月至2013年4月COSMIC干温廓线数据,提取了青藏高原地区大气重力波势能,以此研究了青藏高原大气重力波势能的分布频率模型和大气重力波活动的时空变化特征,并进一步分析了高原大气重力波活动与高原地形、风速和高原大陆热辐射之间的相关性.青藏高原地区大气重力波势能的分布频率服从对数生长分布;青藏高原地区大气重力波在16~18 km和28~31 km高度较活跃,而在20~26 km高度较平静;高原大陆边缘各季节重力波活动均较活跃,而高原大陆上空大气重力波活动呈明显季节性变化,其在冬春季节较活跃,在夏秋季节较平静;2010年冬季青藏高原大气重力波活动异常平静;各季节整个高原上空大气重力波活跃度有随大气高度升高而降低的趋势,高原上低层大气重力波向高层传播会发生耗散作用.地形与风速是影响青藏高原大气重力波活动的重要因素.地形主要影响平流层底部的重力波活动;纬向风比经向风对该地区平流层大气重力波活动的影响大,纬向风总体上会促进高原大气重力波活动.青藏高原大陆热辐射对高原大气的加热作用是导致青藏高原大气重力波活动呈季节性变化的重要因素.     

中层大气重力波的全球分布特征

从2002年1月到2009年12月的SABER温度剖面数据提取了可以反映重力波活动的垂直尺度2~10 km的中尺度温度扰动,分析了全球中层大气重力波的分布.重力波扰动在夏季和冬季明显强于春季和秋季,而冬季与夏季相比,在70 km以下的高度夏季弱于冬季,在70 km以上夏季比冬季要强.从全球重力波分布来看,较大值分布在冬季半球和25°N到25°S的热带范围,其中热带范围重力波的峰值随着高度向北移动,而在南半球高纬度地区重力波扰动较大值位于极区涡流的边缘.热带范围的扰动沿着经度方向有明显的变化,这是由风过滤、地形和波动等因素共同作用的结果.重力波扰动强度随高度变化,在25~30 km处呈现下降趋势,而超过42 km后又逐渐递增.对比8年平均的重力波在不同高度的强弱分布,可以看到,在较低高度,重力波的强弱明显与地形有关,而在较高高度,重力波的分布与地形的关系变得不明显.这说明重力波的形成与地形有显著相关性,但在传播过程中重力波的分布会随高度出现明显的变化.     

青藏高原上空UTLS区域一次地形重力波过程中的物质上传

利用美国航空航天局MERRA(Modern-Era Retrospective Analysis for Research and Applications)再分析资料和MODIS(Moderate-Resolution Imaging Spectroradiometer)卫星资料以及欧洲气象中心ECMWF-Interim(European Centre for Medium-Range Weather Forecasts)再分析资料,分析了发生于青藏高原北侧上空的一次地形重力波事件,并使用中尺度预报模式WRF-ARW.V3.0(Weather Research and Forecasting model,V3.0)对其进行了数值模拟.在此基础上,诊断分析了此次地形重力波在UTLS(Upper Troposphere and Lower Stratosphere)区域造成的物质和能量垂直传输特征.分析结果表明这一中尺度地形重力波信号的水平波长约为600km,与地形扰动水平尺度接近,重力波在对流层中传播的垂直波长约为3km,在垂直方向上随着高度的增加呈现出由东向西倾斜的结构特征.此次地形重力波上传进入平流层并在150hPa附近破碎,波破碎后动量通量在短时间内发生了强烈的衰减,重力波携带的能量在破碎高度附近释放.重力波破碎的同时垂直方向湍流混合变得异常强烈,湍流交换系数可在短时间内增加到背景值的8倍以上,剧烈湍流混合过程导致了对流层上层的空气进入平流层,使下平流层空气出现了位势涡度和臭氧的低值区,在浮力频率的垂直剖面中也可以看到由于地形重力波过程造成的平流层下层浮力频率异常低值区.     

夜光云内小尺度重力波对冰晶粒径谱的影响规律研究

本文利用AIM卫星搭载的CIPS云图反照率和冰晶粒径数据,从中提取了2007/08南半球和2008年北半球共6489个小尺度重力波活动(波长5~150km范围)个例,对重力波区域与背景云层冰晶粒径谱进行对比分析,从而研究重力波对冰晶平均半径和谱宽的影响规律.结果表明,北半球重力波区域冰晶的平均半径和谱宽分别比背景云层小2.5nm和6.1nm,南半球则分别减小1.1nm和7.9nm.在随纬度的分布上,小于80°时,南北半球的平均半径扰动值均为负值,绝对值随纬度增大而减小,而大于80°时,负扰动转变为正扰动,且绝对值增加;谱宽扰动的绝对值也随着纬度增加而减小,但均为负值.在季节内随时间的分布上,南北半球重力波对冰晶平均半径和谱宽的扰动在始末阶段以负值为主,且绝对值较大,而在中期阶段正负值相当,且绝对值较小.这一特征与重力波引起冰晶粒径变化的振幅在纬度和时间上的分布趋势一致.重力波的波长均随纬度升高而减小,在季节的始末阶段较大,中期小,且南半球的平均波长和变化幅度都要明显大于北半球的,粒径扰动振幅随波长的变化率为南半球0.207nm·km-1,北半球的0.163nm·km-1.根据分析推断,重力波自身的扰动振幅应与其影响区域内的谱参数相对于背景云层的变化量有直接关系,且振幅越大,平均半径和谱宽的负扰动就越大.     

非均质矿体地震模型的衍射、转换波及次生波研究

采用超声地震模型、数学模型正演试验和矿井试验观测研究了１＜ｋα＜１０范围非均质体散射波场中，衍射波和转换波的运动学和动力学特征．上述方法获得了一致的结果．研究中发现一种次生震源及次生波，其在今后地震勘探中可能有一定的应用价值．     

北极地区低平流层惯性重力波的观测研究

南极地区重力波活动有大量报道,相对而言,北极地区重力波的研究还很少.本文利用极区Ny-Alesund站点（78.9°N,11.9°E）无线电探空仪从2012年4月1日到2017年3月31日共5年的观测数据,统计分析了北极地区低平流层惯性重力波的特征.观测显示,月平均纬向风在20 km以下盛行东向风,再随着高度增加,逐渐呈现出半年振荡现象.对流层顶高度在5~13 km范围内变化,其月平均高度显示出年循环,最高出现在夏季,约为10 km,最低出现在冬季,约为8.5 km.对流层和低平流层月平均温度都显示出明显的年周期变化,这与中低纬度观测结果有所不同.结合Lomb-Scargle谱分析和矢端曲线方法,估算了准单色惯性重力波参数.个例研究表明,低平流层惯性重力波呈现出远离源区的自由传播性质.统计结果显示,惯性重力波的水平和垂直波长分别集中在50~450 km和1~4 km范围内,本征频率集中在1~2.5倍惯性频率间,这些值都比中低纬度观测值稍小.垂直方向本征相速度主要集中在-0.3~0 m·s^-1,而纬向和经向本征相速度集中在-40~40 m·s^-1之间.在5年的观测中,大约91.5%的惯性重力波向上传播.在冬季和早春,由于极地平流层极涡活动,激发出向下传播的惯性重力波,因此,向下传播的比例上升到相应月份的20%左右.由于低层大气盛行的东向风的滤波效应,低平流层大部分惯性重力波向西传播.波能量呈现出明显的年周期变化,最大值在冬季、最小值在夏季,与北半球中低纬度观测结果一致,表明北半球重力波活动普遍冬季强、夏季弱.

     

空间尘埃等离子体中的重力波特性

建立尘埃等离子体中重力波的基本方程,推导尘埃等离子体中重力波的色散关系,分析地球极区中间层顶处尘埃等离子体层中的重力波特性,研究了重力波在电子密度垂直分层的尘埃等离子体中的反射. 结果表明尘埃等离子体改变了通常大气中的重力内波的色散关系,限制了小水平波数重力内波的传播,改变了波的能量特性,减小了重力波在不均匀大气中垂直向上传播时振幅的增长;在尘埃等离子体中传播时重力波可被电子密度垂直分层的结构反射而导致波能量的集中, 它产生的湍动所导致的空间电子密度的不均匀性分布是极区上空PMSEs的可能机制.     

The nonlinear effects on the characteristics of gravity wave packets: dispersion and polarization relations

By analyzing the results of the numerical simulations of nonlinear propagation of three Gaussian gravity-wave packets in isothermal atmosphere individually, the nonlinear effects on the characteristics of gravity waves are studied quantitatively. The analyses show that during the nonlinear propagation of gravity wave packets the mean flows are accelerated and the vertical wavelengths show clear reduction due to nonlinearity. On the other hand, though nonlinear effects exist, the time variations of the frequencies of gravity wave packets are close to those derived from the dispersion relation and the amplitude and phase relations of wave-associated disturbance components are consistent with the predictions of the polarization relation of gravity waves. This indicates that the dispersion and polarization relations based on the linear gravity wave theory can be applied extensively in the nonlinear region.     

Frequency shift of acoustic gravity waves in a stratified,isothermal, turbulent atmosphere

We study the modification of the frequency of small amplitude acoustic gravity waves which propagate in an isothermal turbulent atmosphere that is stratified by a homogeneous gravitational field. Using a Green's function method, the dispersion relation for the frequency of the waves is formulated as an integral eigenvalue equation and it is solved by perturbation techniques. We draw the following main conclusions: (a) for an arbitrary turbulent correlation spectrum the dispersion relation has a root with a negative imaginary part in the unphysical Riemann sheet of the dispersion function, leading to wave attenuation, much in the same way as it happens for Landau damping; (b) the real part of this root differs from the frequency of an acoustic gravity wave propagating in a nonturbulent medium and, for all forms of the turbulent correlation spectrum, the absolute value of this difference increases if gravity increases; (c) for a Gaussian turbulent correlation spectrum, this difference is always positive; (d) conversely if this frequency difference is known from observations, the auto-correlation function of the temperature fluctuations can be calculated through a simple inversion formula.     

Observations indicating parametric instabilities in internal gravity waves at thermospheric heights

Abstract

Theoretical studies predict a parametric instability of finite-amplitude internal gravity waves which hitherto has been observed only in laboratory experiments. The occurrence of this process in the atmosphere is of basic interest because finite-amplitude gravity waves, which are almost ubiquitous especially at upper atmospheric heights, would produce unstable flows even at large Richardson numbers. Maximum entropy power spectra of a strong internal gravity wave in the thermosphere, which was generated by a volcanic eruption and detected on records of the Doppler shift of high-frequency radio waves, in fact show good agreement with the spectra of synthetic Doppler records obtained from a calculated unstable gravity wave. The frequencies and wavenumbers observed in the gravity wave domain satisfy in particular the theoretically predicted resonance conditions. The observed Doppler records also show two significant lines in the acoustic domain which probably result from a nonlinear interaction with the basic gravity wave. It is suggested that acoustic double peaks, which are commonly observed in high-frequency Doppler spectra in the presence of nearby thunderstorms, represent parametric instabilities of internal gravity waves generated by penetrative cumulus convection.     

损耗大气中重力波的非线性相互作用理论

基于中高层大气重力波动力学是由非线性过程和损耗过程共同决定的物理思想,本文采用弱非线性相互作用近似,推导出损耗大气中重力波的非线性相互作用方程.这组方程是研究固定相位和随机相位重力波相互作用问题的出发点.通过引入平均振幅,我们得到了损耗情况下离散重力波的三波相互作用方程,该方程描述了重力波波包非线性时空演变的规律.作为该方程的一个具体应用,我们考虑了由于波-波相互作用产生的不稳定性.当一大尺度大振幅的主重力波通过大气传播时,非线性相互作用可能导致两个次级波振幅随时间指数增长.由于分子损耗和频率失配,主波的振幅必须大于一个阈值,这种指数增长才可能出现.共振条件满足时,阈值变为最小.频率失配还会导致次级波本征频率发生改变,改变的大小是频率失配值的一半.     

Acoustic gravity wave growth and damping in convecting plasma

The propagation of acoustic gravity waves through steadily convecting plasma in the thermosphere has been analysed theoretically. The growth and damping rates of internal gravity waves due to the feedback effects of wave-modulated Joule heating and Laplace forcing have been calculated. It is found that large convection flow velocities lead to the growth of large-scale internal gravity waves, whilst small- and medium-scale waves are heavily damped, under similar conditions. It has also been shown that wave growth is favoured for waves travelling against the plasma flow direction. The effects of critical coupling when wave phase speeds match the plasma flow speed have also been investigated. The results of these calculations are discussed in the context of the atmospheric energy budget and thermosphere-ionosphere coupling.     

The influence of background winds and attenuation on the propagation of atmospheric gravity waves

The influence of background winds and energy attenuation on the propagation of atmospheric gravity waves is numerically analyzed. The gravity waves, both in the internal and ducted forms, are included through employing ray-tracing method and full-wave solution method. Background winds with different directions may cause ray paths of internal gravity waves to be horizontally prolonged, vertically steepened, reflected or critically coupled, all of which change the accumulation of energy attenuation along ray paths. Only the penetrating waves propagating against winds can easily reach the ionospheric height with less energy attenuation. The propagation status of gravity waves with different periods and phase speeds is classified into the cut-off region, the reflected region and the propagating region. All the three regions are influenced significantly by winds. The area of the reflected region reduces when gravity waves propagate in the same direction of winds and expands when propagating against wind. In propagating region, the horizontal attenuation distances of gravity waves increase and the arrival heights decrease when winds blow in the same direction of gravity waves, while the attenuation distances decrease and the arrival heights increase when gravity waves propagate against winds. The results for ducted gravity waves show that the influence of winds on waves of lower atmospheric modes is not noticeable for they propagate mainly under mesosphere, where the wind field is relatively weak. However, strong winds at thermospheric height lead to considerable changes of dispersion relation and attenuation distance of upper atmospheric modes. Winds against the wave propagating direction support long-distance propagation of G mode, while the attenuation distances decrease when winds blow in the same direction of the wave. The distribution of TIDs observed by HF Doppler array at Wuhan is compared with the simulation of internal gravity waves. The observation of TIDs shows agreement with our numerical calculations.     

Geophysical flows with anisotropic turbulence and dispersive waves: flows with stable stratification

The quasi-normal scale elimination (QNSE) is an analytical spectral theory of turbulence based upon a successive ensemble averaging of the velocity and temperature modes over the smallest scales of motion and calculating corresponding eddy viscosity and eddy diffusivity. By extending the process of successive ensemble averaging to the turbulence macroscale one eliminates all fluctuating scales and arrives at models analogous to the conventional Reynolds stress closures. The scale dependency embedded in the QNSE method reflects contributions from different processes on different scales. Two of the most important processes in stably stratified turbulence, internal wave propagation and flow anisotropization, are explicitly accounted for in the QNSE formalism. For relatively weak stratification, the theory becomes amenable to analytical processing revealing just how increasing stratification modifies the flow field via growing anisotropy and gravity wave radiation. The QNSE theory yields the dispersion relation for internal waves in the presence of turbulence and provides a theoretical reasoning for the Gargett et al. (J Phys Oceanogr 11:1258–1271, 1981) scaling of the vertical shear spectrum. In addition, it shows that the internal wave breaking and flow anisotropization void the notion of the critical Richardson number at which turbulence is fully suppressed. The isopycnal and diapycnal viscosities and diffusivities can be expressed in the form of the Richardson diffusion laws thus providing a theoretical framework for the Okubo dispersion diagrams. Transitions in the spectral slopes can be associated with the turbulence- and wave-dominated ranges and have direct implications for the transport processes. We show that only quasi-isotropic, turbulence-dominated scales contribute to the diapycnal diffusivity. On larger, buoyancy dominated scales, the diapycnal diffusivity becomes scale independent. This result underscores the well-known fact that waves can only transfer momentum but not a scalar and sheds a new light upon the Ellison–Britter–Osborn mixing model. It also provides a general framework for separation of the effects of turbulence and waves even if they act on the same spatial and temporal scales. The QNSE theory-based turbulence models have been tested in various applications and demonstrated reliable performance. It is suggested that these models present a viable alternative to conventional Reynolds stress closures.     

Nonlinear waves in rotating magnetohydrodynamics

We consider an electrically conducting fluid in rotating cylindrical coordinates in which the Elsasser and magnetic Reynolds numbers are assumed to be large while the Rossby number is assumed to vanish in an appropriate limit. This may be taken as a simple model for the Earth's outer core. Fully nonlinear waves dominated by the nonlinear Lorentz forces are studied using the method of geometric optics (essentially WKB). These waves are assumed to be of the form of an asymptotic series expanded about ambient magnetic and velocity fields which vanish on the equatorial plane. They take the form of short wave, slowly varying wave trains. The first-order approximation is sinusoidal and basically the same as in the linear problem, with a dispersion relation modified by the appearance of mean terms. These mean terms, as well the undetermined amplitude functions, are found by suppressing secular terms in a “fast” variable in the second-order approximation. The interaction of the mean terms with the dispersion relation is the primary cause of behaviors which differ from the linear case. In particular, new singularities appear in the wave amplitude functions and an initial value problem results in a singularity in one of the mean terms which propagates through the fluid. The singularities corresponding to the linear ones are shown to develop when the corresponding waves propagate toward the equatorial plane.