激波管内水汽相变滞后效应的实验研究

本文研究了激波管中稀疏波、反射稀疏波对水汽-空气混合气体作用的非定常瞬态相变过程。利用两种不同波长光束的消光方法对水汽相变过程进行了测试。结果表明:水汽非定常瞬态相变发生的温度远远低于出现于自然大气中的凝结或冻结温度，混合气体膨胀越强，水汽瞬态相变的温度越低。非定常瞬态膨胀时的压力迅速下降是水汽瞬态相变温度远远低于自然大气中水汽相变温度的主要原因。水汽瞬态相变过程中，生成的微滴尺度均匀，但微滴的数密度却随时间发生变化。     

Semidiurnal frequency of the fields of the Arctic ice rarefaction and compression from the MTVZA-GYa satellite radiometer data

The spatiotemporal variability is considered ofthe field of ice rarefaction and compression in the Arctic due to the passage of semidiurnal tidal waves. A simplified method is developed for identification of such fields on the maps of the scattering index (SI) of ice computed from the MTVZA-GYa radiometer data. It is demonstrated that the low and high values of ice SI are ob served at tidal rarefactions and compressions, respectively. The analysis of the maps of extreme values of SI observed in overlapping semidiurnal and diurnal MTVZA-GYa measurements corroborated the existence of semidiurnal periodicity of alternating fields of the Arctic ice rarefaction and compression and revealed no variability in ice SI in the areas where tidal wave phases converge (there the convergence amplitude is minimum).     

水汽和潜热释放对背风波影响的数值试验

利用中尺度数值模式ARPS进行了理想场的数值模拟,分析研究了水汽和潜热释放对大气层结稳定度的影响以及其在背风波的发展和演变过程中的作用,研究发现,潜热释放对大气层结分布的影响要远大于水汽对大气层结分布的直接影响,如果没有潜热的释放,水汽对背风波的发展和演变的作用非常小,而潜热释放可以使湿层结稳定度急剧下降,迅速破坏原有的层结分布,使这个区域出现非拦截的强烈的垂直运动,波动的崩溃更加迅速和明显。但需要说明的是在试验中,将数值模式里控制潜热释放的参数设为:0、1/2、2的假定情况,则在实际的大气运动过程中是不可能存在的。     

Water exchange in fjords induced by tidally generated internal lee waves

Tidal processes are examined that control the water exchange between two basins of the Trondheimsfjord through a narrow channel with sills. For this purpose, a non-hydrostatic numerical model based on the laterally averaged Reynolds equations in the Boussinesq approximation was developed. The model takes into account the real vertical fluid stratification, variable bottom topography and variable cross-section of the fjord. Numerical experiments were performed to investigate tidally generated internal waves and their influence on the water exchange.The model produces both baroclinic tides and tidally generated lee waves. It was found that, for the Skarnsund strait which connects the Middle Fjord and the Beitstadfjord, the internal tides generated over the Skarnsund sills are very weak. Their amplitudes do not exceed 1 m.The intense short internal waves, which are identified as unsteady lee waves, comprise the basic input of the total internal wave field. These waves are generated by tidal currents at sill breaks, are trapped by topography in the generation area and grow by continuing feedback into large-amplitude waves. As the tidal flow slackens, they move upstream as freely propagating waves.As essentially nonlinear responses, the lee waves cause a nonlinear water transport. The detailed analysis of the residual currents produced by unsteady lee waves (which are propagating in both directions from the Scarnsund sills) has shown, in particular, that the residual currents can reach values as high as 0.27 m s⁻¹.It was also found that such currents exert a considerable effect on the water exchange through the Skarnsund strait between the adjacent basins. This mechanism can play an important role in water renewal and formation of the Beitasdfjord waters.     

Numerical experiments on internal gravity waves in an accelerating shear flow

The influence of an accelerating shear flow on the propagation of an internal gravity wave in a continuously stratified fluid is studied by means of two-dimensional numerical simulations. These are motivated by earlier laboratory experiments [Thorpe, S.A. 1978b. On internal gravity waves in an accelerating shear flow, Vol. 88. J. Fluid Mech. pp. 623–639]. In these experiments the mean flow is an accelerated Couette flow and the mean density profile is linear. The laboratory experiments revealed the striking effect of the unsteady shear flow in the evolution of an internal gravity wave leading to the wave focusing in a region where the flow is extremum. This phenomenon is associated with the growth of small scale density fluctuations. As a result density overturns are sometimes observed. This behaviour is well reproduced by the numerical simulations. We provide insights on the flow dynamics in particular on the possible occurrence of wavebreaking. We show that the dynamics is characterized by two competitive mechanisms that is a damping of the wave and a local enhancement of its steepness leading sometimes to density overturns. The budget for the energy of the wave reveals that the initial damping of the wave results from wave-mean flow interactions. These interactions lead to the development of a fine scale vertical density structure which is associated with high vertical shear. We find that in some cases wavebreaking occurs as a result of shear instability. The value of the acceleration of the mean flow is very likely to influence the onset of the instability. The scaling laws of the wave evolution, in particular the rate of decrease of its energy, are determined. From these laws the lifetime of the wave is found as a function of the acceleration of the shear. It may be expected that, in the ocean, this development will result in the largest fluctuations derived from wave-flow interactions occurring where the mean flow in the wave direction is greatest. Waves travelling normal to a two-dimensional shear flow will be unchanged. Waves travelling parallel will be damped. This may have particular application at the continental shelf where flow, mainly parallel to the isobaths, will damp waves travelling along-slope, but allows waves travelling normal to the isobaths (e.g., directly across the shelf-break) to be transmitted without attenuation. Similar effects are expected for the evolution of a high frequency wave interacting with a lower frequency (e.g., near inertial) motion.     

Long internal waves in shear flows: topographic resonance and wave-induced global instability

Issues pertaining to a mechanism whereby long internal waves in shallow seas may give rise to enhanced rates of resuspension of sedimentary material are addressed. The proposed mechanism is intimately related to the creation of conditions in the bottom boundary layer which are favorable for flow separation and spontaneous onset of global instability. It is shown that long waves generated by topographic resonance and propagating upstream against the oncoming current, especially a sheared current, have a strong tendency to release a coherent, pulsating dynamics in their footprint. The passage-through-resonance problem for a sheared, stratified flow is considered, conditions for topographic resonance in the flow model are defined, and preliminary results for the unsteady dynamics in the boundary layer under the footprint of a long wave packet are presented.     

Extreme waves in the North Atlantic

Analyzed are the reasons of extreme waves with the height of more than 14 m formed in the North Atlantic in 2002–2011 and the evolution of atmospheric processes causing the extreme waves. It is revealed that the extreme waves in the North Atlantic during that period were mainly (in 81% of cases) formed under the influence of “explosive” cyclones. The height of waves in these cyclones can reach 20 m and more.     

Group velocity of anisotropie waves—Part II: Conservative properties

It has been argued in Part I that traditional expression of multidimensional group velocity used in meteorology is only applicable for isotropic waves. While for anisotropic waves, it cannot manifest propagation of waves group along the trajectory of a reference wave point, and varies with rotation of coordinates. The general mathematical ex-pression of group velocity which may be used also for anisotropic waves has been derived in Part I. It will be proved that the mean wave energy, momentum and wave action density are all conserved as a wave group propagates at the general group velocity. Since general group velocity represents the movement of a reference point in either isotropic or anisotropic wave trains, it may be used to define wave rays. The variations of wave parameters along the rays in a slowly varying environment are represented by ray-tracing equations. Using the general group velocity, we may de-rive the anisotropic ray-tracing equations, which give the traditional ray-tracing equations for isotropic waves.     

Planetary stationary waves in the atmosphere Part I: Orographic stationary waves

It is proposed that the orographic stationary waves are required by long-term balance of momentum in the at-mosphere with zonally asymmetric orographic forcing, This hypothesis may be confirmed successfully with the theo-retical model of geostrophic waves. In the Part I, we will explain the observed phase distributions of orographic sta-tionary waves at middle and high latitudes of the Northern Hemisphere, according to the long-term balance of zonal momentum over the stationary orographic forcing. It is revealed that the geographic distribution of stationary waves depends not only on local topgraphy but also on mean circulation fields and angular momentum flux in the atmos-phere. So these waves cannot be simulated by the models in a restricted area.     

Modulation of short waves by long waves

The amplitude, wavelength, and frequency of short waves in the presence of waves of a longer scale vary in a manner that is related in phase to the long-wave profile. The purpose of this study is to observe and quantify the change in the variance of short-wave slope that occurs as a result of the change in short-wave position along a coincident long wave, during the active generation of the short-wave field by wind. To this end, measurements of wave-slope time series are made in a laboratory environment where the long-scale waves are generated mechanically and the short scale are generated primarily by air flow. The frequency variation of the short waves, as measured along the long-wave profile, is described by considering the waves to be linearly advected by the longer waves. The peak-to-peak variation along the long-wave profile of the short-wave slope variance for a given frequency band is commonly found to be 10% of its mean value. The magnitude of the excursions become smaller as short-wave frequency increases, and larger as wind speed increases. The maximum value of the short-wave slope variance generally leads the long-wave profile curve by 45 ° to 180 °.     

The activity of convectively coupled equatorial waves in CMIP3 global climate models

This study evaluates the convectively coupled equatorial waves in ten coupled general circulation models (GCMs) in the twentieth century experiment from the Coupled Model Intercomparison Project phase 3 of the World Climate Research Programme. The antisymmetric bands in all GCMs are weaker than in observations, and the mixed Rossby-gravity (MRG) wave seems to be a mixture of the equatorial Rossby (ER) and tropical depression-type (TD-type) waves rather than a mixture of the ER and inertiogravity waves found in observations. The simulated TD-type wave is more organized than in observations with a quasilinear wavenumber–frequency relationship. In most GCMs, the two observed activity centers of the MRG and TD-type waves over the southern Indian Ocean and the southwestern Pacific cannot be separated; only one wave activity center is found over the Maritime Continent. The observed northwestward propagation of the TD-type wave over the western North Pacific is also not well simulated in the GCMs. The simulated active season of the MRG and TD-type waves over the northern hemisphere during the boreal summer and fall is much shorter than in observations. The models from CCSR utilizing the Pan and Randall scheme with the convection suppression simulate the realistic Kelvin wave activity with the maximum activity near the equator, while the wave activities filtered for the Kelvin wave in the other GCMs are similar to the extratropical Rossby wave with the maximum activity at higher latitudes. Likewise, only these two models produce a realistic seasonal cycle of the Kelvin wave activity.     

线性准地转模型中副热带感热加热强迫的定常波

通过求取定常线性准地转位涡模式的解析解,研究了感热加热强迫所激发的副热带定常波的结构特征,讨论了基本流、牛顿冷却及地面摩擦等对定常波振幅和位相的影响。结果表明,东风时定常波在垂直方向上表现为上、下层反位相的第一斜压结构,且地面系统远强于中高层;西风时定常波呈现出向上的传播特征,在高层,随着风速增大振幅随高度的升高有增大趋势。在近地层,东风时气旋（反气旋）主体位于加热西（东）侧;西风时气旋（反气旋）主体位于加热东（西）侧,近地层以上相反。此外,发现东、西风基本流的作用具有对称特征,这与潜热加热显著不同。研究结果还表明,牛顿冷却对定常波有重要影响,基本流越弱影响越显著。在静止大气中,感热加热强迫下无斯韦尔德鲁普（Sverdrup）解,考虑牛顿冷却时,感热强迫在热源范围内的近地层和中高层分别激发出气旋式和反气旋式环流,气旋中心位于加热中心略偏西的位置。在非静止大气中,牛顿冷却项使地面系统中心向上风方向移动,东风时向东移。牛顿冷却对高、低层系统均有削弱作用。地面摩擦则明显不同,它总会使低层系统减弱,高层系统增强。     

华北一次浓雾过程爆发性增强的微物理特征

基于华北雾-霾综合观测试验资料,分析了2011年12月4日河北涿州一次浓雾过程爆发性增强的微物理特征及形成机理。结果表明:此次浓雾过程除具有均压场、地面辐射降温、逆温层、静稳天气等特征外,还具有雾微物理过程出现爆发性增强的特征,10 min内,小雾滴浓度显著增加,含水量增大了3个量级,雾滴谱由15 μm拓宽到35 μm,能见度由500 m骤降至70 m。夜间地面长波辐射冷却效应导致近地层雾的形成,而近地层雾的形成反过来快速地增强了地面长波辐射冷却效应,促使大量小雾滴的形成和碰并过程的产生,这是一种正反馈效应;大量雾滴形成释放的潜热,促使雾体抬升和向下长波辐射增强,又使地面长波辐射冷却效应减弱,产生负反馈效应。相对于南京辐射雾过程,此次涿州浓雾的小雾滴粒子数浓度高,液态水含量明显偏小,这与华北高浓度气溶胶和弱水汽输送有关。     

On the problem of the Neva river flood waves identification

Based on the analysis of the measurements of hydrometeorological characteristics, the identification is corroborated of the Neva River flood waves as the baroclinic topographic waves. It is demonstrated that during the formation and maximum development of the most significant sea level rises in the Neva Bay, the stratification in the Gulf of Finland still remains pronounced despite the storm conditions. The baroclinic nature of the flood wave is indicated by the significant changes in the dispersion of currents with depth with their direction changing to the reverse one as it occurs in the first baroclinic mode wave. Directions of major axes of the standard deviation ellipses are oriented not along the isobaths as it should be in case of long gravity waves (being the longitudinal ones) but are extended across the bottom topography contours that is typical of gradient-vorticity waves assigned to the class of horizontal transverse waves.     

Some aspects of internal gravity waves in the multicell-type convective system

Summary Some aspects of internal gravity waves in the multicell-type convective system are examined using a linear theory and a nonlinear numerical model. The basic-state wind is assumed to increase linearly with height and then remain constant.In the theoretical part, the two-dimensional, linear, steady-state response of a stably stratified atmosphere to specified cooling representing the evaporative cooling of falling precipitation in the subcloud layer is analytically considered. It is shown that there exist an updraft on the upstream side of the cooling and a downdraft on the downstream side. As the wind shear increases enough, the magnitude of the updraft decreases. This is because a large portion of the specified cooling is used to compensate for the positive vorticity associated with the positive wind shear and accordingly the effective cooling necessary to produce perturbations is reduced.In the numerical part, a two-dimensional version of the ARPS (Advanced Regional Prediction System) that is a nonhydrostatic, compressible model with detailed physical processes is employed. Results from the dry simulation, in which the steady cooling is specified in the model, show that the simulated quasi-steady field resembles the linear, steady-state solution field because the nonlinearity factor of thermally-induced waves in this case is small. For the moist simulation, the quasi-steady perturbations obtained from the dry simulation are used as initial conditions. It is shown that gravity waces can effectively initiate convection even with small amplitude and that updraft at the head of the density current somewhat resembles the linear, steady-state response of a stably straified flow to the specified cooling. The updraft, that is, forced internal gravity waves, at the head of the density current is responsible for the initiation of consecutive convective cells that move downstream and develop as a main convective cell. This study suggests that internal gravity waves play a major role in the initiation of consecutive convective cells in the multicell-type convective system and hence in its maintenance.     

On the initiation of surface waves by turbulent shear flow

An analytical model is developed for the initial stage of surface wave generation at an air–water interface by a turbulent shear flow in either the air or in the water. The model treats the problem of wave growth departing from a flat interface and is relevant for small waves whose forcing is dominated by turbulent pressure fluctuations. The wave growth is predicted using the linearised and inviscid equations of motion, essentially following Phillips [Phillips, O.M., 1957. On the generation of waves by turbulent wind. J. Fluid Mech. 2, 417–445], but the pressure fluctuations that generate the waves are treated as unsteady and related to the turbulent velocity field using the rapid-distortion treatment of Durbin [Durbin, P.A., 1978. Rapid distortion theory of turbulent flows. PhD thesis, University of Cambridge]. This model, which assumes a constant mean shear rate Γ, can be viewed as the simplest representation of an oceanic or atmospheric boundary layer.For turbulent flows in the air and in the water producing pressure fluctuations of similar magnitude, the waves generated by turbulence in the water are found to be considerably steeper than those generated by turbulence in the air. For resonant waves, this is shown to be due to the shorter decorrelation time of turbulent pressure in the air (estimated as ∝ 1/Γ), because of the higher shear rate existing in the air flow, and due to the smaller length scale of the turbulence in the water. Non-resonant waves generated by turbulence in the water, although being somewhat gentler, are still steeper than resonant waves generated by turbulence in the air. Hence, it is suggested that turbulence in the water may have a more important role than previously thought in the initiation of the surface waves that are subsequently amplified by feedback instability mechanisms.     

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 in-clude 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.     

Relationships between Cloud Droplet Spectral Relative Dispersion and Entrainment Rate and Their Impacting Factors

Cloud microphysical properties are significantly affected by entrainment and mixing processes. However, it is unclear how the entrainment rate affects the relative dispersion of cloud droplet size distribution. Previously, the relationship between relative dispersion and entrainment rate was found to be positive or negative. To reconcile the contrasting relationships, the Explicit Mixing Parcel Model is used to determine the underlying mechanisms. When evaporation is dominated by small droplets, and the entrained environmental air is further saturated during mixing, the relationship is negative. However, when the evaporation of big droplets is dominant, the relationship is positive. Whether or not the cloud condensation nuclei are considered in the entrained environmental air is a key factor as condensation on the entrained condensation nuclei is the main source of small droplets. However, if cloud condensation nuclei are not entrained, the relationship is positive. If cloud condensation nuclei are entrained, the relationship is dependent on many other factors. High values of vertical velocity, relative humidity of environmental air, and liquid water content, and low values of droplet number concentration, are more likely to cause the negative relationship since new saturation is easier to achieve by evaporation of small droplets. Further, the signs of the relationship are not strongly affected by the turbulence dissipation rate, but the higher dissipation rate causes the positive relationship to be more significant for a larger entrainment rate. A conceptual model is proposed to reconcile the contrasting relationships. This work enhances the understanding of relative dispersion and lays a foundation for the quantification of entrainment-mixing mechanisms.     

The wavenumber-frequency characteristics of the tropical waves in an aqua-planet GCM

Based on the aqua-planet experiments, the wavenumber-frequency characteristics of tropical waves and their influencing factors in SST distribution and the convective parameterization scheme are investigated using the spectral atmospheric general circulation model （SAMIL）. Space-time spectral analysis is used to obtain the variance of convectively coupled tropical waves. In the Control experiment with maximum SST located at the equator the simulated tropical-wave behaviors are in agreement with those in observations and theoretical solutions. When the maximum SST is located at 5°N, the symmetric and antisymmetric waves are much weaker than those in the control experiment, suggesting that tropical wave activities are very sensitive to the SST distributions. Importantly, the variance maximum of Madden-Julian oscillation （MJO） is found to occur around 5°N, which suggests that the development of the MJO depends largely on the latitude of maximum SST. Furthermore, the seasonal variations of MJO may be mainly caused by the seasonal variations of the maximum SST. The experiment results with two different cumulus schemes the Manabe moist convective adjustment and Zhang-McFarlane （ZM） convective scheme, were also compared to examine the impacts of convective parameterization. Weakened variances of each individual tropical wave when the ZM scheme is used suggest that the ZM scheme is not favorable for the tropical wave activities. However, the wave characteristics are different when the ZM scheme is used in different models, which may imply that the simulated basic state is important to the meridional distributions of the waves. The MJO signals suggest that the parameterization scheme may have great influence on the strength, but have less direct impact on the MJO distribution. The frequency of the tropical waves may be associated with the moisture control of convection and the large-scale condensation scheme used in the model.     

巨盐核对云滴活化影响的数值模拟研究

利用包含云凝结核(CCN)与巨核(GCCN)的核化,云滴凝结和碰并增长的分档气块模式模拟研究了不同的CCN数浓度、上升气流速度、CCN中值半径以及云底温度等情况下GCCN对CCN活化的影响,结果表明,在水汽供应相对充足的情况下GCCN对云滴活化数浓度的影响并不明显;而当水汽供应相对不充足时,增加GCCN至1 cm-3的量级以上可以有效减少CCN的活化数浓度.在水汽供应不充分且其他条件相同的情况下,增大CCN的平均直径或是增加云底温度都可以使GCCN对云滴活化的抑制作用增强.对比分析不同的GCCN数浓度对清洁大气和污染大气云底以上300 m高度处粒子谱型的影响可以看出,在水汽供应不充分的条件下加入GCCN,初始时刻CCN的数浓度对GCCN产生的大云滴数目及云滴谱宽的影响较小.在水汽供应相对充足的情况下,GCCN对CCN活化基本没有抑制作用,但此时在高过饱和度峰值下生成的大量小云滴争食水汽,反而导致云滴群凝结增长速度小于水汽供应相对不充足的情况,此时加入的GCCN可以先活化形成大云滴.