GRAVITY WAVES ON THE AXIS OF LOW LEVEL JET AND THEIR INSTABILITY

Using the wind data at mountain stations, gravity waves on the axis of low level jet and their relation to rain clusters have been studied by means of the spectrum method. It is found that the spectrum densities of both wind and rainfall reach their maximums at the period of 8 h and the moduli between wind and rainfall also reach the maximum in that range. Both observation and theoretical study give the same conclusion that the unstable development of gravity waves depends not only on the atmospheric stratification but also on the vertical shear of the environmental wind. The appearencc of the jet-shape vertical profile of wind is favourable for the unstable development of gravity waves. The lower the height of the axis, the larger the growth rate of amplitude.     

Analysis on the Interaction between Turbulence and Secondary Circulation of the Surface Layer at Jinta Oasis in Summer

The kinetic energy variations of mean flow and turbulence at three levels in the surface layer were calculated by using eddy covariance data from observations at Jinta oasis in 2005 summer.It is found that when the mean horizontal flow was stronger,the turbulent kinetic energy was increased at all levels,as well as the downward mean wind at the middle level.Since the mean vertical flow on the top and bottom were both negligible at that time,there was a secondary circulation with convergence in the upper half and divergence in the lower half of the column.After consideration of energy conversion,it was found that the interaction between turbulence and the secondary circulation caused the intensification of each other.The interaction reflected positive feedback between turbulence and the vertical shear of the mean flow.Turbulent sensible and latent heat flux anomaly were also analyzed.The results show that in both daytime and at night,when the surface layer turbulence was intensified as a result of strengthened mean flow,the sensible heat flux was decreased while the latent heat flux was increased.Both anomalous fluxes contributed to the cold island effect and the moisture island effect of the oasis.     

Investigating the Dominant Source for the Generation of Gravity Waves during Indian Summer Monsoon Using Ground-based Measurements

Over the tropics,convection,wind shear(i.e.,vertical and horizontal shear of wind and/or geostrophic adjustment comprising spontaneous imbalance in jet streams) and topography are the major sources for the generation of gravity waves.During the summer monsoon season(June-August) over the Indian subcontinent,convection and wind shear coexist.To determine the dominant source of gravity waves during monsoon season,an experiment was conducted using mesosphere-stratosphere-troposphere(MST) radar situated at Gadanki(13.5 N,79.2 E),a tropical observatory in the southern part of the Indian subcontinent.MST radar was operated continuously for 72 h to capture high-frequency gravity waves.During this time,a radiosonde was released every 6 h in addition to the regular launch(once daily to study low-frequency gravity waves) throughout the season.These two data sets were utilized effectively to characterize the jet stream and the associated gravity waves.Data available from collocated instruments along with satellite-based brightness temperature(TBB) data were utilized to characterize the convection in and around Gadanki.Despite the presence of two major sources of gravity wave generation(i.e.,convection and wind shear) during the monsoon season,wind shear(both vertical shear and geostrophic adjustment) contributed the most to the generation of gravity waves on various scales.     

Non-Acceleration Theorem in a Primitive Equation System: I. Acceleration of Zonal Mean Flow

Non-acceleration theorem in a primitive equation system is developed to investigate the influences of waves on the mean flow variation against external forcing. Numerical results show that mechanical forcing overwhelms thermal forcing in maintaining the mean flow in which the internal mechanical forcing associated with horizontal eddy flux of momentum plays the most important roles. Both internal forcing and external forcing are shown to be active and at the first place for the mean flow variations, whereas the forcing-induced mean meridional circulation is passive and secondary. It is also shown that the effects on mean flow of external mechanical forcing are concentrated in the lower troposphere, whereas those due to wave-mean flow interaction are more important in the upper troposphere. These act together and result in the vertically easterly shear in low latitudes and westerly shear in mid-latitudes. This vertical shear of mean flow is to some extent weakened by thermal forcing.     

EFFECTS OF WIND SHEAR ON ATMOSPHERIC GRAVITY WAVE SPECTRUM

The effects of constant wind shear on atmospheric gravity wave spectrum are examined.At first a three-dimensional equilibrium spectral model of gravity waves is established in which wind shear rate β is implicit.Based on this model,the expressions for one-dimensional frequency spectrum of atmospheric gravity waves arederived in which β is explicit.Numerical results show that (1) if we assume that mean wind U(z)=βz(z represents the altitude) and the internal gravity wave spectrum at the altitude of U=0 (that is,z=0) isVanZandt one,then the effect of Doppler shifting due to mean wind may be ignored;(2) when Richardson numberJ(J=N~2/β~2,N is Brunt-V(?)is(?)l(?) frequency,and is equal to 10~(-2)s~(-1) in this paper) satisfies J≥10.0,theeffects ofwind shear arealso ignored;(3)for f~2(?)ω~2(?)N~2 (f is the inertia frequency,and f=10~(-4)s~(-1)in this paper,and co is the observed frequency),the wind shear only affects the spectral amplitude,and does notalter the spectral shape;and (4) as wind shear becomes strong,a part of wave potential energy turns into wavekinetic one,and a part of the vertical kinetic energy further turns into the horizontal one.     

EFFECTS OF TOPOGRAPHY ON PROPAGATION AND DEVELOPMENT OF INERTIA GRAVITY WAVE

Effects of topography on the propagation and development of inertia gravity waves areinvestigated by means of WKBJ method.The equation of wave action conservation is obtained.It isfound that the inertia gravity wave tends to propagate to the higher elevation area,meanwhile theamplitudes of the waves increase.While the inertia gravity waves propagate to a lower elevation area,their amplitudes decrease.     

THE MESOSCALE WAVES AND THE FORMATION OF POLYGONAL EYE WALL IN TYPHOONS

Mesoscale waves in typhoons were diagnosed by using a simulated typhoon data in this paper. Through analyzing the structure of the waves in typhoons, we found that the waves possess the mixed features of gravity inertial waves and vortex Rossby waves. On the one hand, positive geopotential height perturbation is corresponding to negative vorticity perturbation and anticyclonic circulation. At the same time, negative geopotential height perturbation is corresponding to positive vorticity perturbation and cyclonic circulation. The maximum perturbation occurs near the radius of the maximum wind in the typhoon. On the other hand, the mesoscale waves possess the features of strong convergence and divergence and ageostrophic wind. Finally, the authors presented a concept model to explain a linkage mechanism between the mesoscale waves and the formation of polygonal eye wall in the typhoon.     

THE SEMI-GEOSTROPHIC ADAPTATION PROCESS WITH TWO-LAYER BAROCLINIC MODEL IN LOW LATITUDE ATMOSPHERE

In this paper, the adaptation process in low latitude atmosphere is discussed by means of a two-layer baroclinic model on the equator β plane, showing that the adaptation process in low latitude is mainly dominated by the internal inertial gravity waves. The initial ageostrophic energy is dispersed by the internal inertial gravity waves, and as a result, the geostrophic motion is obtained in zonal direction while the ageostro-phic motion maintains in meridional direction, which can be called semi-geostrophic balance in barotropic model as well as semi-thermal-wind balance in baroclinic model. The vertical motion is determined both by the distribution of the initial vertical motion and that of the initial vertical motion tendency, but it is unrelated to the initial potential vorticity. Finally, the motion tends to be horizontal. The discussion of the physical mechanism of the semi-thermal-wind balance in low latitude atmosphere shows that the achievement of the semi-thermal-wind balance is due to the adjustment between the stream field and the temperature field through the horizontal convergence and divergence which is related to the vertical motion excited by the internal inertial gravity waves. The terminal adaptation state obtained shows that the adaptation direction between the mean temperature field and the shear flow field is determined by the ratio of the scale of the initial ageostrophic disturbance to the scale of one character scale related to the baroclinic Rossby radius of deformation. The shear stream field adapts to the mean temperature field when the ratio is greater than 1, and the mean temperature field adapts to the shear stream field when the ratio is smaller than 1.     

NONLINEAR WAVES IN BAROTROPIC MODEL

In this paper, from the system of equation describing a barotropic atmosphere using the method of Taylor expansion for the nonlinear terms, the periodic solutions of the nonlinear inertio-surface gravity waves and Rossby waves have been obtained.The finite-amplitude nonlinear inertio-surface gravity waves and Rossby waves with horizontal divergence satisfy all the KdV equation. The solutions are all the cnoidal function, i, e., the cnoidal waves which include the linear waves and form the solitary waves under certain conditions. For the finite-amplitude Rossby waves with horizontal divergence, we find the new dispersive relation including both the wave number and the amplitude parameter. In case of small amplitude it is reduced to the Yeh formula. It is shown that the larger the amplitude and width, the faster the finite-amplitude inertio-surface gravity waves and the slower the finite-amplitude Rossby waves with horizontal divergence propagate. The blocking or cut-off system in which the amplitude and width are large may be considered as Rossby solitary waves.     

DYNAMIC STUDY ON INFLUENCE OF INERTIAL GRAVITY WAVES INDUCED BY UNBALANCED FLOW ON MEIYU FRONT HEAVY RAIN*

The Meiyu front heavy rain process in 1-3 June 2000 is numerically simulated in this paper, and results are then analyzed to show the effects of geostrophic balance collapse,unbalanced flow occurrence,low level jet (LLJ) development,and gravity waves genesis and propagation on the rainstorm.Analyses indicate that the sudden northwest movement of subtropical high may destruct the local geostrophic balance,leading to an increase in the local pressure gradient and the occurrence of ageostrophic flow,and meanwhile the adjustment of circulation starts to build a new balance.During the process,an LLJ and gravity waves appear correspondingly.The dispersion of unbalanced energy through the divergence/convergence of the geostrophic departure winds, promotes the propagation of strong wind cores along the LLJ,and the dispersion direction is influenced by the steering flow and the moisture concentration area.The development of LLJ is one of important conditions,which induces the heavy rain especially in the left front part of the jet where the convergence and shear of winds occur.It is also found that the genesis of disturbance, meso-vortex,and meso-convective system provides a favorable condition for the rainstorm.The above results are clearly illustrated by the high spatial and temporal resolution simulation data from a mesoscale numerical model.     

一次暴风雪过程中的中尺度重力波特征及其影响

应用地面自动气象站观测资料、数字化多普勒天气雷达探测资料和WRFV2.2.1中尺度数值模拟资料,分析了中尺度重力波与基本气流的相互作用,以及重力波活动对暴雪和大风天气的重要影响。结果表明,在波导中传播的中尺度重力波能够与基本气流进行动量交换,使得对流层中上层4.5—8 km气层内的水平平均风速趋于均匀,形成斜穿整个对流层的饱和湿空气急流,即"湿急流"。在高空急流出口区激发的垂直向下传播的重力波,使基本气流的水平风速在垂直方向上出现了加速和减速的交替变化,水平风加速的气层,反射率增大;水平风减速的气层,反射率减小。随着波动下传及其随基本气流的移动,反射率回波强度沿高空风的方向(由西南向东北)出现周期性变化,回波带呈西北—东南走向,强回波中心之间为宽约40 km的弱回波区。重力波下传期间,当地面气压迅速下降时,东北风快速增长,风向有明显的改变,反射率强度开始减弱;气压脊线过后,反射率降低到最低点。地面大风中心出现在反射率回波强度周期性变化的地带,沿西南—东北方向间隔着分布。雷达探测表明,对流层低层风速在风向切变层上下边界对称相等,因此推测在重力波与切变层汇合的高度层存在垂直环流,由风切变层上下边界附近的西南气流和东北气流与受重力波影响形成的垂直方向上的上升和下沉气流共同组成。切变层上方的动量通过垂直环流的下沉支到达地面,强风中心对应着下沉气流,出现在降水回波开始减弱之际。     

The Relationship Between Abnormal Meiyu and Medium-Term Scale Wave Perturbation Energy Propagation Along the East Asian Subtropical Westerly Jet

The East Asian subtropical westerly jet(EASWJ) is one of the most important factors modulating the Meiyu rainfall in the Yangtze-Huaihe River Basin, China. This article analyzed periods of the medium-term EASWJ variation,wave packet distribution and energy propagation of Rossby waves along the EASWJ during Meiyu season, and investigated their possible influence on abnormal Meiyu rain. The results showed that during the medium-term scale atmospheric dynamic process, the evolution of the EASWJ in Meiyu season was mainly characterized by the changes of3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves. The strong perturbation wave packet and energy propagation of the 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves are mostly concentrated in the East Asian region of 90°-150°E, where the two wave trains of perturbation wave packets and wave-activity flux divergence coexist in zonal and meridional directions, and converge on the EASWJ. Besides, the wave trains of perturbation wave packet and wave-activity flux divergence in wet Meiyu years are more systematically westward than those in dry Meiyu years, and they are shown in the inverse phases between each other. In wet(dry) Meiyu year, the perturbation wave packet high-value area of the 10-15 d low-frequency variability is located between the Aral Sea and the Lake Balkhash(in the northeastern part of China), while over eastern China the wave-activity flux is convergent and strong(divergent and weak), and the high-level jets are strong and southward(weak and northward). Because of the coupling of high and low level atmosphere and high-level strong(weak) divergence on the south side of the jet over the Yangtze-Huaihe River Basin, the low-level southwest wind and vertically ascending motion are strengthened(weakened), which is(is not)conducive to precipitation increase in the Yangtze-Huaihe River Basin. These findings would help to better understand the impact mechanisms of the EASWJ activities on abnormal Meiyu from the perspective of medium-term scale Rossby wave energy propagation.     

乌鲁木齐暴雪天气的环流配置及中尺度系统特征*

利用1980-2018年ERA-5全球再分析资料，对汉江流域汉中地区典型暴雨发生前纬向风场变化及天气尺度瞬变波活动(Eliassen-Palm通量特征)进行分析。结果表明：瞬变波EP通量特征分析为汉江流域暴雨潜势预报提供一个有利的参考指标；暴雨发生前，33°N附近200hPa附近有纬向风减速中心，对应200hPa为EP通量辐散区，这种垂直分布模型是暴雨前期有利形势，随着纬向风减速趋势加强，EP通量辐散区扩展并加强，有利于暴雨的发生；7月较之8月，暴雨强度更强、暴雨范围更广、纬向风变化更明显、200hPa以上EP通量辐散更强。若简单地将盛夏暴雨整体进行研究，会影响对不同月份瞬变波活动及大气环流变化趋势的诊断，造成诊断偏差且难以准确反映瞬变波与暴雨的联系。因此在讨论盛夏季天气尺度瞬变波与对流层环流的相互作用时，应针对不同月份分开进行讨论。     

2008年1月末九江雨凇转大雪天气过程成因分析

对2008年1月25—29日九江市一次雨凇转大到暴雪天气过程的大气垂直结构进行了分析。结果表明,在雨凇和降雪区域中层有暖层,低层有冷层,且存在对称不稳定层结。垂直风切变明显,有利于对称不稳定能量积累和上升气流的形成。高、低空急流辐散、辐合相互垂直,引发的较强上升运动是雨雪天气得以维持的重要条件。东西风向切变使冷、暖气流在雨雪区域上空交汇,形成水平气旋性辐合,产生垂直运动,且与垂直风切变相互作用,促动上、下层动量传递,使高、低空急流维持。雨凇转大到暴雪时,暖层气温降低,垂直风切变向上增强,对称不稳定能量增大,风向切变辐合加大,上升气流和水汽输送加大。     

高空急流区内纬向基本气流加速与EP通量的关系

采用多时间尺度方法, 并假设纬向基本气流是正压的, 在不引入剩余环流的情况下, 推导出纬向基本气流局地变化与一种新形式Eliassen-Palm通量之间的关系, 这种Eliassen-Palm通量关系在形式上比较简单, 而且物理意义明确, 适合诊断分析高空急流区内纬向基本气流的加速与减速.理论分析表明, 正压纬向基本气流的局地变化由新形式Eliassen-Palm通量散度决定.作者利用该Eliassen-Palm通量关系对2002年12月一次寒潮过程中200 hPa等压面上纬向基本气流的发展演变进行了诊断分析.诊断结果表明, 在200 hPa纬向基本气流急流区内, 新形式Eliassen-Palm通量散度是影响纬向基本气流发展演变的重要因子, 并且纬向基本气流的加速与减速主要是由新形式Eliassen-Palm通量中扰动动量的经向输送造成的.     

The subcloud-layer eddy kinetic energy budget of a microburst-producing thunderstorm determined from jaws dual-Doppler measurements

Dual-Doppler data collected from 1646 to 1648 MDT on 14 July, 1982 in Colorado were employed to study the eddy kinetic energy budget in the subcloud layer of a microburst-producing thunderstorm during its mature stage. Each term in the budget equation was computed from the Doppler-derived winds and retrieved thermodynamic fields within the 10 by 10 km horizontal domain. Results show that in the atmospheric boundary layer (ABL) where the microburst dominates, the turbulent flow extracts energy from the mean flow in order for the microburst to maintain its strong diverging outflow at low levels. The vertical transport of eddy kinetic energy is predominantly downward in the low layer due to the organized downdrafts in the microburst area. The horizontal flux convergence (divergence) of eddy kinetic energy by the mean and eddying motions is approximately balanced by that of the vertical flux divergence (convergence). Similarly, the contributions from the pressure and buoyancy production terms are nearly in balance. As a result, a net change of the eddy kinetic energy generation in the subcloud layer is relatively small in comparison with the individual term in the budget equation.     

汉江流域盛夏暴雨与天气尺度瞬变波EP通量的可能联系

利用1980—2015年ERA-5全球再分析资料,对汉中地区典型暴雨发生前纬向风场变化及天气尺度瞬变波活动(Eliassen-Palm通量特征)进行分析。结果表明:瞬变波Eliassen-Palm(EP)通量特征分析为汉江流域暴雨潜势预报提供一个有利的参考指标;暴雨发生前,33°N附近200 h Pa有纬向风减速中心,对应200 hPa为EP通量辐散区,这种垂直分布模型是暴雨前期的有利形势,随着纬向风减速趋势加快,EP通量辐散区扩展并加强,有利于暴雨的发生;与8月相比,7月暴雨强度更强,暴雨范围更广,纬向风变化更明显,200 hPa的EP通量辐散更强。若简单地将盛夏暴雨整体进行研究,会影响对不同月份瞬变波活动及大气环流变化趋势的诊断,造成诊断偏差且难以准确反映瞬变波与暴雨的联系。因此在讨论盛夏季天气尺度瞬变波与对流层环流的相互作用时,应按月份讨论。     

台风麦莎的正压特征波动结构及其稳定性

利用中尺度WRF模式对2005年8月西北太平洋台风麦莎（Matsa）进行了精细的数值模拟。使用模式输出资料,对正压浅水方程组进行了数值差分计算,分析它在最大强度时刻的正压特征波动结构和稳定性。结果表明,台风麦莎内部包含有沿逆时针方向传播的重力惯性外波和涡旋Ross-by波,两种波动的结构和稳定性存在显著性差异。前者主要存在于台风外围,增长率随波长的减小而增加,台风外围的波动相速度为48．9～68．5m／s;后者主要位于距离台风中心200km内,表现为3波最不稳定,半径100km处相速度约为5m／s。此外,重力惯性外波的扰动风场与高度场基本相垂直,扰动散涡比值大于3倍,甚至达到10＾3倍,运动以辐合、辐散为主;涡旋Rossby波的扰动风场基本平行于高度场,扰动散涡比值为10＾-1～10＾-2,涡旋运动是其主要运动,与内螺旋雨带沿着切向圆周方向的传播具有密切关系。     

Analysis of the Characteristics of Inertia-Gravity Waves during an Orographic Precipitation Event

A numerical experiment was performed using the Weather Research and Forecasting(WRF) model to analyze the generation and propagation of inertia-gravity waves during an orographic rainstorm that occurred in the Sichuan area on 17 August 2014. To examine the spatial and temporal structures of the inertia-gravity waves and identify the wave types, three wavenumber-frequency spectral analysis methods(Fourier analysis, cross-spectral analysis, and wavelet cross-spectrum analysis)were applied. During the storm, inertia-gravity waves appeared at heights of 10–14 km, with periods of 80–100 min and wavelengths of 40–50 km. These waves were generated over a mountain and propagated eastward at an average speed of 15–20 m s~(-1). Meanwhile, comparison between the reconstructed inertia-gravity waves and accumulated precipitation showed there was a mutual promotion process between them. The Richardson number and Scorer parameter were used to demonstrate that the eastward-moving inertia-gravity waves were trapped in an effective atmospheric ducting zone with favorable reflector and critical level conditions, which were the primary causes of the long lives of the waves. Finally, numerical experiments to test the sensitivity to terrain and diabatic heating were conducted, and the results suggested a cooperative effect of terrain and diabatic heating contributed to the propagation and enhancement of the waves.     

低空急流和非均匀层结条件下重力惯性波的传播与发展

本文从基流背景下线性化Boussinesq方程组出发,采用WKBJ方法,首先求得重力惯性波的广义波作用量变化方程及波参数随波包的变化方程,讨论了基流切变对重力惯性波传播和发展的影响;然后用Runge-Kutta方法讨论了不同层结(含降水影响)下重力惯性波的传播路径。得到了降水强度大、静力稳定度小的区域对重力惯性波有“吸引”作用,因而引起重力惯性波能量的集中,进而触发和强化对流天气的结论。