Long-wavelength Cloud Rolls Over the Arctic Ice

Cloud bands observed over the Arctic sea ice are interpreted in terms of long-wavelength gravity-wave modes trapped between the ground and a vertically-thin over-reflecting layer in the mid-troposphere. The over-reflecting layer is neutrally-stratified and lies between the counter-streaming upper troposphere and lower troposphere in a strong high-pressure system. The Arctic cloud bands appear to share many features with the long-wavelength roll waves observed in more temperate latitudes.     

Resonant trapped gravity waves and turbulent patches in an inversion layer

Instability of resonant trapped gravity waves is proposed as the mechanism responsible for the creation of turbulent patches in the inversion capping a well developed convective layer. The plume circulation of the convective layer results in a periodic forcing at the base of the inversion layer; trapped gravity waves in resonance with such forcing are considered. Good agreement with experimental results is obtained.Now at the Istituto di Fisica della Università di Urbino Via S. Chiara 61100 Urbino (Ps.) Italy.     

The effect of moist layer location on the stability of trapped gravity waves in an almost saturated atmosphere

Summary The stability characteristics of trapped gravity waves, generated by an isothermal bounded tanh (z) velocity profile in the presence of a saturated finite layer, are studied. The saturated layer is introduced at different levels above the inlection point and the variations of moisture content, layer thickness and distance from the origin are examined. The growthyrates and phase speeds of the unstable modes are obtained by solving numerically the equations of motion in the linear, inviscid, Boussinesq limit, via the technique of Lalas and Einaudi (1976).It is shown that the presence of the saturated layer can significantly affect the stability characteristics of the waves. inereases in moisture, layer thickness and distance of the layer from the inflection point are found to amplify the wave response because the saturated layer behaves as a top boundtry to the shear flow. The presence of such effective boundaries is found to stabilize short wavelengths and destabilize longer wavelengths. Finally, an application of the results to a rainband case produces values of the wave parameters in good agreement with the observed ones.With II Figures     

A study of gravity waves in the planetary boundary layer by acoustic sounding

Gravity waves associated with stably stratified layers were observed in the planetary boundary layer at two locations in France. Using an array of three monostatic Doppler acoustic sounders, the wave speeds and directions were found. A quantitative study of the waves has been performed to determine their physical characteristics. Based on their dynamics, a classification into three types was possible. Most were found to be trapped beneath a critical level.     

三层模式中风场对背风波的影响

为了研究风场对背风波的影响,针对边界层附近为弱稳定层结的背风波,建立了一个三维三层的理论模型和线性计算模式,分析了各层中风速和风向的变化对背风波特征的影响,揭示了气流过孤立山脉产生背风波的有利风场条件。结果表明：背风波的波长、振幅等特征对各层风速和风向的变化具有相当的敏感性,波长随着低、高层风速的增大而增大,随着中层风速的增大先减小后增大;振幅随着低、中层风速的增大先增大后减小,随着高层风速的增大而增大。此外,风速和上下层风向切变的增大均使背风波的形态逐渐由横波型转为辐散型,但是上下层风向的切变对背风波形态的影响比风速更为显著。     

Cross-equatorial structures of equatorially trapped nonlinear Rossby waves

Analysis of the Sea Surface Height (SSH) from satellite altimeters has shown that equatorially trapped Rossby waves exhibit asymmetric cross-equatorial structures; their northern extrema are much larger in magnitude than their southern counterparts. Such asymmetry is inconsistent with the classical theory for the first baroclinic, first meridional equatorially trapped Rossby mode, which predicts that SSH and zonal velocity are symmetric in latitude and the meridional velocity is latitudinally antisymmetric (Matsuno, 1966). Chelton et al. (2003) attributed the observed asymmetry to the mean-shear-induced modifications of first meridional mode Rossby waves. The present paper examines nonlinear rectification of cross-equatorial wave structures in the presence of different zonal mean currents. Nonlinear traveling Rossby waves embedded in shears are calculated numerically in a 1.5-layer model. Nonlinearity is shown to increase the cross-equatorial asymmetry substantially making the northern extrema even more pronounced. However, nonlinearity only slightly increases the magnitude of the westward phase speed.     

Ondes de gravite interfaciales marginalement instables dans la couche limite planetaire

Wavelike motions within a strong morning inversion of the planetary boundary layer were investigated experimentally using two atmospheric research aircraft: an Aerocommander 280FL and a Cessna 206. The Aerocommander aircraft, instrumented for the measurement of rapid fluctuations of temperature, water vapour density and air velocities, was flown horizontally at different levels within the inversion layer in order to document adequate data on the wave motion. An example of such motions observed on 8 June, 1974 is described and analyzed in the present paper. The aircraft records obtained within the inversion layer at about 600 m above the ground show large fluctuations of the meteorological variables with well-defined amplitudes and wavelengths.Spectra and cross-spectra of temperature, water vapour density and air velocities were computed and analyzed to determine characteristics of gravity waves according to the method described by Metcalf (1975). These spectra exhibit a sharp maximum associated with high coherences at a particular wavelength. In this narrow spectral band, phase angles ±90 ° are obtained between vertical velocity and temperature as well as between vertical and horizontal velocities. These features suggest that observed motions are horizontally propagating trapped or evanescent waves. They enable us to estimate true wavelengths (500 m), wave vector azimuths, intrinsic frequencies and phase velocities of these waves. These results appear to be mutually consistent. Furthermore, it is possible to confirm these latter with the detailed vertical profiles of the boundary layer provided by the Cessna aircraft making spiral soundings. In this regard, the vertical structure of the Brunt-Väisälä frequency confirms that the waves are everywhere evanescent except within a thin highly stable layer between the diurnal mixed layer and the overlapping inversion. Moreover, examination of the wind profiles reveals that the interfacial vertical wind shear might be a relevant parameter reducing phase velocities. Such a conclusion is also supported by the observed wave vector directions which appear to be closely parallel to the wind shear vectors at the 600-m level. Additional confirmation is found by comparing the observed wavelengths to those predicted by applying the hydrodynamical stability model of Hazel (1972) to the measured profiles. Although the wind shear clearly plays a role in wave development, local heat flux and temperature variance values show that in this case the instability is only a marginal and sporadic event embedded in nearly neutral waves. Accordingly, it is argued that the observed motions are interfacial waves at the inversion base level, the amplitude and wave vector azimuth of which are controlled by the vertical wind shear.

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Ondes de gravite interfaciales marginalement instables dans la couche limite planetaire

Résumé Des ondes de gravité interfaciales progressives sont étudiées à l'aide de deux avions dans la couche limite planétaire. Les spectres des vitesses et de la température fournissent les directions et vitesses de propagation. La comparaison des résultats aux profils verticaux du vent et de la température montre que ces ondes sont marginalement instables sous l'effet du cisaillement vertical du vent.

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IOPG, 12, avenue des Landais - 63001 Clermont-Ferrand Cedex.

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Complexe Scientifique des Cézeaux BP 45 - 63170 Aubiere.     

Observations of Atmospheric Solitary Waves in the Urban Boundary Layer

The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s^-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.     

Possible impact of equatorially trapped waves on the tropical cyclone drift

The effect of equatorially trapped waves on the movement of tropical cyclones (TC) is studied numerically based on a two-dimensional barotropic model in a beta-plane approximation. According to recent studies, equatorially trapped waves contribute to the genesis of TCs. It is thus natural to assume that these waves affect also the movement of the TC. The effect of three types of equatorially trapped waves, namely Kelvin, Rossby, and n = 0 eastward inertio-Gravity (EIG) waves, on the TC trajectory is investigated with a focus on the sensitivity on some key physical parameters such as the wavenumber and wavespeed. Using a simple barotropic model forced by a prescribed baroclinic flow, the barotropic response to equatorially trapped waves is simulated for a period of 50 days, under various wave parameter configurations. This response is then used as a background flow where TC’s can evolve and propagate. TC-like flows are injected into this wavefield background at arbitrary times during the simulation, and the TC trajectories are tracked and recorded for 48h after the injection time. The resulting TC trajectory patterns with respect to the injection times and wave parameters appear to be stochastic and the mean paths and the associated standard deviations are calculated and reported here. The statistics are different for different wave types. Kelvin waves make shorter length of TC trajectories and small divergence of direction. On the contrary, Rossby waves cause rather dramatic changes in the TC path and yield longer trajectories. Meanwhile, TCs in EIG waves maintain fairly the same direction and typically have longer trajectories though less dramatic. A robustness test using a random forcing instead has also been conducted.

     

A numerical study on severe downslope windstorms occurred on 5 April 2005 at Gangneung and Yangyang, Korea

Severe downslope windstorms occurred on 5 April 2005 in the Taebaek Mountain Range, located in the eastern coast of Korea, are examined using the Weather Research and Forecasting (WRF) model. Strong winds are observed at Gangneung and Yangyang during two separate periods with a rapidly decreasing period in between. These downslope windstorms are reproduced in the simulation reasonably well, although the rapidly decreasing surface wind speed after the second windstorm could not be captured at Yangyang. It is found that the generation mechanisms of the downslope windstorms in these two periods are somewhat different. The severe wind in the first period is likely due to the reflection of the mountain waves from a critical level that locates near z = 8–9 km. Upward-propagating waves and reflected downward-propagating waves interact constructively in a duct between the critical level and the surface, resulting in strong surface wind. In the second period, the hydraulic-jump theory can be applied in that the wave breaking above the downstream induces a well-mixed region, and severe downslope wind is developed beneath this turbulent region as the streamlines descend along the downstream. Simultaneous lee wave structure is also reproduced during the second windstorm period. The sensitivity of the downslope wind speed to the change in the land-cover map showed that the absorption of trapped lee waves in the boundary layer reduces the downslope wind speed significantly after the second windstorm at Gangneung, improving the model performance, although with no significant impact at Yangyang.     

Propagation of coastal‐trapped waves under an ice cover in Hudson Bay*

Abstract

Three arrays of current‐meter moorings were deployed under landfast sea ice in southeast Hudson Bay for eight weeks in spring 1986. Spectral analysis shows low‐frequency signals with periods of 3 to 11 days. These signals are interpreted as being due to coastal‐trapped waves propagating cyclonically in Hudson Bay; their theoretical dispersion relations and corresponding modal structures are presented for winter stratification and are compared with observations. At a period of 3 days both the modified external Kelvin wave and higher mode continental shelf waves may be important in describing the observed low‐frequency variability, whereas at a period of 10 days the Kelvin wave appears to be the dominant mode. The generation mechanisms for these coastal trapped waves are also investigated. Two sources have been studied: the longshore atmospheric pressure gradient and the average atmospheric pressure over the ice cover in Hudson Bay. Coherence and phase analyses performed with time series of longshore current and atmospheric forcing data reveal that both the average atmospheric pressure and the longshore atmospheric pressure gradient are important in explaining the observed low‐frequency variability, without indicating which one is the most important.     

Coastal trapped waves,with application to the northeast pacific ocean

The dispersion relation is derived for long coastal trapped waves of sub‐inertial frequency that propagate along a single‐step continental shelf in a two‐layer fluid. When the internal (Rossby) deformation radius is smaller than the shelf width, we show that the dispersion relation can be factored exactly, giving two possible modes: i. an internal Kelvin wave modified by topography;

ii. a continental shelf wave modified by the stratification.

A detailed discussion of the eigen‐functions associated with each of these modes is presented. Then the shelf wave dispersion relation is plotted for parameters applicable to the Oregon‐Washington coast. Theoretical values for the periods and wavelengths predicted from these plots are shown to agree favorably with observed values for this region.     

Laboratory simulations of coastally trapped waves with rotation, topography and stratification

We describe observations of the generation and propagation of coastally trapped waves in the laboratory and their comparison with theory, over a range of values of several experimental parameters. The topography and stratification used consisted of a sloping continental shelf and vertical continental slope with three-layer stratification that could be approximated by an extended version of the Gill and Clarke model [Gill, A.E., Clarke, A., 1974. Wind-induced upwelling, coastal currents and sea level changes. Deep Sea Res. 21, 325–345]. The latter was modified to accommodate a central mixed layer, curved geometry, and friction on the shelf. This configuration represents coastal geometry with large Burger number. The experiments were successful in realizing coastally trapped waves that were consistent with the theoretical expectations. However, the waves propagated more slowly, and for narrow shelves were damped more rapidly than predicted by the theory. The first was attributed to: (i) the effect of stratification on fluid on the shelf, reducing the topographic Rossby wave effect; (ii) the parameterization of the viscosity. The second difference was attributed to the mechanism of generation: the paddle used did not always generate sinusoidal waves, and the subsequent dispersion resulted in a net loss of amplitude.     

Internal gravity waves in a stably stratified boundary layer

Often, a combination of waves and turbulence is present in the stably stratified atmospheric boundary layer. The presence of waves manifest itself in the vertical profiles of variances of fluctuations and in low-frequency contributions to the power spectra. In this paper we study internal waves by means of a linear stability analysis of the mean profiles in a stably stratified boundary layer and compare the results with observed vertical variance profiles of fluctuating wind and temperature along a 200 m mast. The linear stability analysis shows that the observed mean flow is unstable for disturbances in a certain frequency and wavenumber domain. These disturbances are expected to the detectable in the measurements. It is shown that indeed the calculated unstable frequencies are present in the observed spectra. Furthermore, the shape of the measured vertical variance profiles, which increase with height, is explained well by the calculated vertical structure of the amplitude of unstable Kelvin-Helmholtz waves, confirming the contribution of waves to the variances. Because turbulence and waves have quite distinct transport properties, estimates of diffusion from measurements of variances would strongly overestimate this diffusion. Therefore it is important to distinguish between them.     

Influence of internal gravity waves on sound propagation in the lower atmosphere

Summary The effects of internal waves on the propagation of acoustic pulses in the lower atmosphere were studied theoretically and by acoustic pulse sounding of the stable atmospheric boundary layer. Due to a control in the experiments of the stratification and time variations of meteorological parameters, such as wind speed, temperature and atmospheric pressure, we were able to observe the influence of the variations of these parameters on a pulse wave form, travel time and time duration. For the travel time and wind speed variations we obtained statistical characteristics (variances, frequency spectra and coherences) in the range of periods from 1 min to 1h and found several dominant periods, which are inherent to the trapped internal waves in the lower atmosphere. Using a nonlinear model of internal wave spectrum in the atmosphere described here we have made the calculations of variances, frequency spectra and structure functions of travel time fluctuations, which allowed us to interpret some of the observed data.     

Mesoscale Variability in the Summer Arctic Boundary Layer

Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.     

与季节内振荡-超级云团系统相关的赤道波的识别

文中重点研究与热带季节内振荡－超级云团系统相关联的赤道波的识别。考虑到赤道波的本质为截获在赤道地区的一般的气候平衡场附近的线性动力过程的固有模态,首先从原理上阐明了ＰＯＰｓ分析技术对于分离各类赤道波动的有效性,以后利用这种分析方法对日本静止气象卫星所观测到的黑体红外辐射温度资料进行了ＰＯＰｓ分析。结果表明,在热带季节内振荡－超级云团系统中,对应于４０ｄ振荡的模态基本表现为Ｋｅｌｖｉｎ波的结构;对应于１０ｄ左右的模态,亦为Ｋｅｌｖｉｎ波,而在２ｄ尺度上,则分别存在经向模态序号ｎ＝１和ｎ＝２的西进和东进惯性重力波。分析中还发现了西进的混合Ｒｏｓｓｂｙ重力波的存在。     

边界层Scorer参数对背风波波长的影响

利用一个中层Scorer参数较大,上、下层Scorer参数较小的3层背风波模型,讨论了边界层(下层)Scorer参数对背风波波长的影响。结果表明,当边界层Scorer参数减小时,波长增大,反之波长减小;当中高层的Scorer参数较小时,背风波波长对边界层的Scorer参数敏感。利用该结果结合观测事实分析后发现,由于地表加热的日变化影响了边界层的Scorer参数,故造成了背风波波长在夜间缩短,在午后增加;此外还发现,因夏季的Scorer参数要比冬季小,这可导致波长变化率在夏季比冬季更大。     

三维多层流过山产生的山地重力波研究

为了研究了三维多层气流过山产生的三维山地重力波和大气船波动力学理论,在本文中,改进了一个多层流过山的三维山地重力波的线性理论计算模式,分析了三维多层流过孤立山地产生的三维山地重力波和大气船舶波的物理机制及其表现特征。揭示了多层流过孤立地形产生发散模态和横波两种模态拦截背风波的气象条件,增强了人们对山地重力波动力学的理解和对山脉重力波及其相联系的山脉天气的预测能力。     

Why do 2-day waves propagate westward?

In observations, the 2-day waves, identified as the convectively coupled equatorial inertio-gravity (IG) waves, only propagate westward. To understand this feature, a simple theoretical model is presented for the convectively coupled equatorial waves (CCEWs). Under the assumption that the convective heating is proportional to the vertical velocity on the first baroclinic mode, the nonlinear governing equation for the meridional velocity of the CCEWs can be derived. The optimal method is used to obtain the dispersion relation from this nonlinear equation, and the results show that the deep convection can slow down the IG waves by decreasing the mean state static stability, but the key leading to the westward propagation of the IG waves is the full meridional variation of the sea surface temperature (SST). The warm SST trapped near the equator excites long westward propagating IG waves, whereas the warm SST trapped near the ITCZ centered at 10° N excites short westward propagating IG waves. This theoretical model provides a simple tool to study the CCEWs in understanding the tropical circulation.