The propagation of inertia-gravity waves and their influence on mean zonal flow,part one: the propagation of inertia-gravity waves

Inertia-gravity waves play an increasingly important role in the middle atmosphere dynamics. As a result, more attention has been paid to the study of inertia-gravity waves, especially to the middle atmosphere gravity waves. This paper presents some aspects of inertia-gravity waves with emphasis on the propagation. Two methods are used here, namely, geometric optical method and physical optical method. We can see from the study that inertia-gravity waves are similar to planetary waves in some respects and they are different from planetary waves in others.     

The effects of zonal flow on nonlinear Rossby waves

In this paper, we using phase plane method have derived the stability criteria of linear and nonlinear Rossby waves under the conditions of semi-geostrophic approximation and have gotten the solutions and geostrophic vorticity of corresponding solitary Rossby waves. It is pointed out that the wave stability is connected with the distri-bution of zonal flow and when the zonal flow is different the solitary wave trough or ridge is formed.     

球面斜压大气中上传行星波与纬向平均气流的相互作用

本文指出在球面斜压大气中的行星尺度扰动与纬向平均气流相互作用中必须考虑非地转风分量对位涡度南北方向输送的作用,从而证明了球面大气中行星尺度运动的波作用守恒,并求得球面大气中的行星尺度扰动与纬向平均气流相互作用的欧拉平均运动方程组。 本义利用1979年2月的实际天气观测资料分析了北半球一次对流层阻塞形势建立与平流层爆发性增温过程的行星波与纬向平均气流的相互作用。从分析结果可以看到:由于行星波向上传播,使得在高纬度对流层中、上层出现行星波的波作用通量(也称E-P通量)辐合,使得西风气流减速,因而在高纬度对流层中、上层形成阻塞形势。并且,随着行星波的继续向上传播,使得平流层下、中层的基本气流相继减速并出现东风,因而在平流层下、中层出现爆发性增温。然而,由于高纬度对流层低层出现东风,行星波不能往上传播,在上层波的E-P通量矢量变成辐散,气流由东风又变成西风。     

Storm tracks and zonal mean flow variability: a comparison between observed and simulated data

A statistical technique is used to analyze the relation between monthly mean zonal flow and storm tracks activity in the observations and numerical simulations (ECHAM4 model). The singular value decomposition technique (SVD) has been used to correlate storm tracks and monthly mean wintertime anomaly fields. The analysis has been performed on data from January 1980 to December 1989 (NMC analyses) and on an ensemble of AGCM simulations with prescribed SST for the same period, separately in the Euro-Atlantic and Pacific sectors. We found good correlation between storm tracks activity and zonal flow in both regions. In both data and simulations the dominant SVD modes show that the storm tracks spatial displacement is in conjunction with jet shifts in the same direction. Our analysis suggests that the model is highly sensitive to the equatorial ocean forcing. Although the model produces an excessive response to El Niño and La Niña phases, it shows good capability of simulating the dynamical connection between storm tracks and jet.     

40—50天的纬向基流低频振荡及其失稳效应

初步的资料分析表明,大气运动的40—50天的低频振荡不仅存在于热带对流层,而且也存在于中高纬对流层及平流层.在温度及高度场的功率谱中,40—50天的振荡比双周振荡更为显著. 用球面二层模型对对流层冬季纬向振荡基流所作的稳定性分析表明,小扰动的增长随基流振荡而加强,经向温度梯度40—50天的振荡对波数3—8的波动能产生相当可观的失稳效应。对更长周期的基流振荡,增长率的频率响应出现“饱和”现象. 对平流层的分析表明,平流层定态纬向基流是动力稳定的,而纬向流的振荡分量及非纬向定态扰动则是两个可引起基态失稳的因子.     

Occurrence and features of ducted modes of internal gravity waves over western Europe and their influence on microwave propagation

Several years of intense investigation of internal gravity waves propagating in low-level ducts in the atmosphere over a rather flat region not influenced by land-sea circulation or mountains are reported. This study gives a rather complete picture of the occurrence, features, and of some of the excitation conditions of these waves. Simultaneous direct measurements made on a meteorological tower, 250 m high, using temperature, vector wind, and sonic anemometer sensors at different altitudes are reported along with indirect measurements using an array of microbarographs with separations over a range of more than 100 km. The study also included microwave propagation links crossing over the meteorological site. Individual wave crests some tens of kilometers long could be followed over a path of more than 100 km. Because of the variability and non-stationary conditions of the lower atmosphere, a direct correlation between a propagating internal wave group and the fading pattern of EM propagation could be found in only a few cases.It is interesting to note Goldie's references to Kelvin and Helmholtz in this regard.     

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 verti-cal shear of mean flow is to some extent weakened by thermal forcing.     

Effect of tropospheric temperature change on the zonal mean circulation and SH winter extratropical cyclones

This study aims to understand the mechanisms which cause an overall reduction of SH extratropical cyclone activity with a slight increase in the high latitudes in a warmer climate simulated in general circulation models (GCMs) with increasing CO₂. For this purpose, we conducted idealized model experiments by forcing warm temperature anomalies to the areas where climate change models exhibit local maximum warming—the tropics in the upper troposphere and the polar regions in the lower troposphere—simultaneously and separately. The Melbourne University atmospheric GCM (R21) coupled with prescribed SST was utilized for the experiments. Our results demonstrate that the reduction of SH extratropical cyclone frequency and depth in the midlatitudes but the slight increase in the high latitudes suggested in climate change models result essentially from the tropical upper tropospheric warming. With this tropical warming, the enhanced static stability which decreases baroclinicity in the low and midlatitudes turns out to be a major contributor to the decrease of cyclone activity equatorward of 45°S whereas the increased meridional temperature gradient in the high latitudes seems an important mechanism for the increase of cyclone activity over 50°–60°S.     

The observed generation and breaking of atmospheric internal gravity waves over the ocean

Observations are presented for internal gravity waves and their breaking at a height of 23.5 m over the ocean in surface-based inversions which are formed because of the advection of warm air over cold water. The spectral and cospectral analyses of velocity and temperature fluctuations were made to establish the characteristic features of the waves. Flow visualization photographs of smoke released during the breaking of a wave are also presented. Comparison between the turbulent energies present during and after breaking of a wave indicates enormous mixing and dispersion occurring during breaking.This research was performed under Contract No. EY-76-C-02-0016 with the United States Energy Research and Development Administration.     

The influence of sea waves on the wind profile

From wind profile and wave measurements performed during the JONSWAP II experiment, relations between the dimensionless profile slope and the significant wave height are derived. It is shown that the wind profile is distorted by the waves especially in the vicinity of the water surface. The wave influence on the profile seems to be restricted to heights below about three wave heights. Above this level, the dimensionless profile slope is an approximately constant value corresponding to a drag coefficient of about 1.15 × 10^–3.     

Atmospheric lee waves in the Aegean Sea and their possible influence on the sea surface

Unusual satellite images of the Aegean Sea, in both the visible and infrared ranges, are discussed. Alternating bright and dark features downwind of islands suggest the presence of atmospheric lee waves. However, close examination of these features indicates that the observed signal is likely the signature of the influence of the lee waves on the sea surface rather than the signature of the lee waves themselves through atmospheric effects.     

Structures of dynamically unstable shear flows and their implications for shallow internal gravity waves part II: Nonlinear response

Summary The nonlinear response of a dynamically unstable shear flow with critical level to an initial temperature anomaly is investigated using a nonlinear numerical model. Both nonconstant and constant shear profiles of the basic flow are considered. Effects of the solid lower boundary on the dynamically unstable, nonlinear flow are also studied. It is found that in a dynamically unstable, linear flow with a hyperbolic tangent wind profile, the updraft is tilted upshear. The result in consistent with that of a linear stability model (LC). The upshear tilt can be explained by the Orr mechanism (1907) and the energy argument proposed by LC. In a dynamically unstable, nonlinear flow, the updrafts produced by a sinusoidal initial temperature perturbation are stronger in the lower layer and are more compact and located further apart compared to the corresponding linear flow. In addition, the perturbed wave energy is slightly smaller than the linear case. It is found that the growth rate is smaller during the early stage and much larger during the later stage. For a localized initial temperature perturbation in a dynamically unstable flow, a stronger updraft with two compensated downdrafts are produced. Gravity waves are produced in a dynamically stable flow with both a hyperbolic tangent wind profile and a linear wind profile. For a linear shear flow with Richardson number less than 1/4, the disturbance grows in the early stage and then decays algebraically at later times, similar to that found in other linear theoretical studies. The influence of the solid lower boundary is to suppress the shear instability in a nonlinear flow with a hyperbolic tangent wind profile ofRi<1/4.With 9 Figures     

The influence of atmospheric stratification on the growth of water waves

The atmospheric surface layer over sea has a density stratification which varies with moisture content and air/sea temperature difference. This influences the growth of water waves. To study the effect quantitatively, the Reynolds equations are solved numerically. For given wind speed and surface roughness, wave growth is found to be more rapid in unstably stratified conditions than in stable conditions. This is due to an increase in turbulence, primarily caused by an increase of mixing length.Under the assumption of a Charnock relation between surface roughness and friction velocity, it is found that for large inverse wave age (u _*/c>0.07), the effect of stratification on wave growth is weell described by Monin-Obukhov scaling of the friction velocity. For smaller values ofu _*/c, Monin-Obukhov scaling overpredicts.The effect on duration-limited wave growth is studied with the third-generation WAM surface wave model driven by 10 m winds. Effects of stratification on the significant wave height are found to be of the order of 10%. The results are comparable to those of a recent reanalysis of field measurements, although the measured stratification effect is somewhat stronger. Implementation of a stratification-dependent growth in wave models is recommended, as it can lead to small but significant improvements in wave forecasts when accurate air and sea temperatures are available.     

北太平洋纬向热力差对长江中下游梅雨的影响

近29年长江中下游入梅期、梅雨量和出梅期各指数与前期各月的北太平洋海温场的相关普查,显示出各梅雨指数首先和当年初夏北太平洋海温场的纬向热力差有关.凡北太平洋中、低纬海温距平场西正(负)东负(正),则倾向于加强(减弱〕初夏东亚—东太平洋的海陆热力梯度,纬向垂直环流偏强(弱);长江中下游入梅早(迟),梅雨量大(小).此种海温场的纬向热力差距可分别追溯至1—2季前以至一年前;即上年初夏以来,北太平洋反气旋式海洋环流东半环的冷洋流趋向偏强(弱),当年冬春西半环的暖洋流也偏强(弱),则随后的入梅提早(推迟),梅雨量     

Estimation of critical levels of climate change influence on the natural terrestrial ecosystems on the territory of Russia

Developed are the axiomatics and criteria for estimating the critical levels of climate change influence on the natural terrestrial ecosystems based on the revelation of key climate-dependent environmental elements and model analysis of their variations. Developed is an empirical statistical vegetation model for the territory of Russia considering 15 vegetation zones including five ones in the permafrost zone. The model was used to estimate the proximity of the climate impact on the natural terrestrial ecosystems to the critical level for several climate projections.     

南亚高压次季节尺度东西振荡对我国长江流域降水及水汽输送的影响

使用常规资料及HYSPLIT(Hybrid Single Particle Lagrangian Integrated Trajectory Model)模式,分析了早夏和晚夏期间南亚高压次季节尺度异常偏东事件和异常偏西事件发生时,我国长江流域降水及水汽输送异常特征。结果表明:早夏期间当南亚高压异常东伸时,西太平洋副热带高压可西伸至我国东南沿岸,此时,降水正异常场主要位于我国东部地区,表现为江淮区域降水异常偏多;而晚夏期间当南亚高压异常东伸时,西太平洋副热带高压的西脊点更偏东,降水正异常区位置比早夏偏西,表现为四川盆地的降水异常偏多。早夏期间,与南亚高压异常偏西事件相比,南亚高压异常偏东事件有利于更多的水汽从南方的水汽源地输送到我国江淮区域,加上此时江淮流域存在异常上升运动,动力和水汽条件共同使得江淮流域降水偏多。晚夏四川盆地地区有类似的结论。     

Measurements of flow over an elongated ridge and its thermal stability dependence: The mean field

Measurements of mean wind flow and turbulence parameters have been made over Cooper's Ridge, a 115 m high elongated ridge with low surface roughness. This paper describes measurements of the streamwise and vertical variations in the mean field for a variety of atmospheric stability conditions. In near-neutral conditions, the normalised speedup over the ridge compares well with measurements from Askervein (Mickleet al., 1988). The near-neutral results are also compared to an analytical flow model based on that of Huntet al. (1988a). Measured streamwise variations show less deceleration at the foot of the hill and slightly more acceleration at the crest of the hill than does the model. In non-neutral conditions, the speedup over the ridge reduces slightly in unstable conditions and increases by up to a factor of two in stable conditions. The model is modified to allow boundary-layer stability to change the upwind wind profile and the depths of the inner and middle layers. Such a modification is shown to describe the observations of speedup well in unstable and weakly stable conditions but to overestimate the speedup in moderate to strongly stable conditions. This disagreement can be traced to the model's overestimation of the upstream scaling velocity at the height of the middle layer through its use of a stable wind profile form which has greater shear than that of the observed profiles, in possible combination with the three-dimensionality of the ridge which would allow enhanced flow around, rather than over, the feature in more stable conditions.     

双台风相互作用及对它们移动的影响

本文根据20个(1956—1975年)夏半年(6—11月)西北太平洋的92对双台风个例,研究了双台风的相互旋转、相互靠近现象。并着重对双台风的移动、打转问题进行了分析。 发现二个台风的平均气压太高或太低时,相互旋转都趋向于变慢,这表明过去一些工作只考虑两台风相互作用的风力是不全面的。 当双台风相距10纬距以内时,相互旋转以气旋式占明显优势(>70％),因而可将它定为相互作用明显的距离。双台风中心连线12小时的平均转角θ与二台风距离r的经验关系式为 r=3.23+59.32 (1/θ)° 从相互作用对两个台风整个移动路径的影响来看,约有1/3的个例影响不明显。而对有影响的个例,其中约42％,相互作用的结果,只是二者的移速有变化,移向变化并不明显。 对西台风打转的情况,发现打转时间长短与东台风北上速度关系密切,相关系数达0.72,并求得了它们之间的回归方程。