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1.
The stability analyses of the stream function at the upper atmosphere have been conducted using a global barotropic spectral
model with a view to examine the seasonal characteristics. The growing eigen modes are classified into three groups with periods
in the range of 3–8, 11–18 and 20–50 days. This study indicates that the growth and movement of all the three types of modes
are adequately sustained by the asymmetric basic flow. All the modes grow faster in summer than in winter. The meridional
shear of the basic flow is the most important source of growth of the perturbations. In the absence of the meridional wind
shear, the eigen modes grow slowly, in which case, the quasi-nonlinear triad interaction between the waves is identified to
contribute significantly to the growth of the modes. The robustness of the eigen modes is also examined in this study using
the barotropic model at different horizontal resolutions in the triangular truncation scheme. 相似文献
2.
A detailed barotropic, baroclinic and combined barotropic-baroclinic stability analysis has been carried out with mean monsoon
zonal currents over western India, eastern India and S.E. Asia. The lower and middle tropospheric zonal wind profiles over
western India are barotropically unstable. The structure and growth rate of these modes agree well with the observed features
of the midtropospheric cyclones. Similar profiles over eastern India and S.E. Asia, however, are barotropically stable. This
is attributed to weak horizontal shear, inherent to these profiles. The upper tropospheric profiles, on the other hand, are
barotropically unstable throughout the whole region. The features of these unstable modes agree with those of observed easterly
waves. The baroclinic and combined barotropic-baroclinic stability analyses show that the baroclinic effects are not important
in tropics.
Though the barotropic instability of the mean zonal current seems to be res ponsible for the initial growth of the mid-tropospheric
cyclones, neither barotropic nor baroclinic instability of the mean zonal current seem to explain the observed features of
the monsoon depressions. 相似文献
3.
The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation
(composite of 15 depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25° longitude
and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-north-east to south-south-west tilt in the
lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures
to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available
potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon
depressions and this possibly aids cumulus convection.
The profiles of potential vorticity indicate extremes (i) in the upper troposphere and (ii) at several midtropospheric levels
in the region of the monsoon trough indicating the possibility of combined barotropic-baroclinic instability. Using multi-level
quasi-geostrophic model and employing the eigen-value technique it is shown that the monsoon zonal current is notbaroclinically unstable. A barotropic stability analysis is also done for monsoon zonal current in the lower and middle tropospheres. It
yields very slowly growing unstable modes at lower tropospheric levels with wave lengths of 2500 km and 5000 km. 相似文献
4.
This paper deals with eddies generated by a shear flow instability in the tidal Potomac River, Maryland-Virginia, USA. An
evolution sequence of this robust phenomenon is synthesized using ASTER and Landsat visible imagery, and its growth from small-amplitude
waves into mature eddies is shown. Using feature tracking between two nearly contemporaneous ASTER and Landsat images, an,
along-river surface velocity profile is computed. Archived velocity and density profiles, taken during accelerating ebb at
locations up-river and down-river of the study area indicate the water column is relatively homogenous up-river, but resembles
a two-layer fluid down-river where estuarine stratification is present in the lower water column. Velocity variation with
depth in the upper water column is relatively small, deviating at mid depth from the near-surface value by only 9–15% at both
sites. We examined the horizontal stability properties of this flow by modeling it as inviscid and barotropic., and predicted
rapidly growing unstable waves with wavelengths close to the observed ones. Their proximity to the coast, as well as the precise
details of the coastline configuration, are shown to have a negligible effect in determining the observed unstable wavelengths. 相似文献
5.
V. Krishnakumar R. N. Keshavamurty S. V. Kasture 《Journal of Earth System Science》1992,101(2):123-152
The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects
on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic
(QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics
of the monsoon depressions using a nonlinear moist global spectral model.
Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric
modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared
to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging
from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important.
One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon
depression of the Bay of Bengal.
In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode
with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode
had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves.
Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating
distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model
was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed
a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed
that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the
generation of available potential energy. 相似文献
6.
由于室内剪切试验结果存在试样结构性扰动和尺寸效应等带来的不确定性问题,且以往的常规原位大型剪切试验是针对天然状态土体,不能获得饱和土体的强度参数,导致岩土结构稳定性分析和工程设计中大多采用反演方法求取强度参数,而很少直接运用这些试验参数.在本次试验方法研究中,试验仪器仍然采用常规的天然土体原位大型剪切试验仪器,针对土体饱和问题,设计了“制样后饱和法”和“饱和后制样法”2种浸水饱和装置,并采用这2套装置在甘肃省黑方台地区进行了7个饱和黄土原位大型剪切试验.结果表明,2种饱和方法都能够使土样饱和,“饱和后制样法”容易达到饱和,试验历时短,但对试样扰动大,甚至导致试样在制样时破坏,试验成功率低;“制样后饱和法”达到饱和历时稍长,对试样扰动小,试验成功率高.与原位天然大剪试验相比,粘聚力c值由44.65kPa下降至17.35kPa,φ值由14.18°降至11.95°;与室内饱和固结快剪相比,饱和黄土原位大剪的c值增加约4kPa,φ值增加3°左右. 相似文献
7.
Doppler sodar wind data for the boundary layer over Kharagpur obtained during MONTBLEX-1990 at a height interval of 30 m from
surface up to 1500 m have been analysed for the periods when intense synoptic scale disturbances from north Bay of Bengal
moved along the eastern end of the monsoon trough. The variation in the vertical wind profile in the lower boundary layer
over Kharagpur during the passage of synoptic scale disturbances has been discussed in the paper. The analysis indicates that
the mean winds over Kharagpur veered with height in the lower boundary layer near the surface suggesting divergence over Kharagpur
when the system lay south/southwest of the station. No such veering has been noticed when the centre of the system lay very
close to the station. 相似文献
8.
里德尔剪切的组合型式与走滑盆地组合型式的相似性 总被引：1，自引：0，他引：1
走滑断裂体系中经常发育里德尔剪切的断裂组合,世界上不同构造背景下与走滑断裂相关的盆地(走滑盆地)也很多见。因此里德尔剪切是地质构造研究中的重要方面。我们从三个方面对里德尔剪切的节理构造组合及盆地组合进行了对比:(1)里德尔剪切构造组合与比例尺无关。里德尔剪切带的(转换)拉张区与沉积盆地的分布区是相似的;(2)断裂的最大位移区与断陷盆地的沉积中心是一致的,断陷盆地长轴平行于断裂走向;(3)物理模拟试验及数学模拟试验都证实了走滑盆地的演化。基于上述认识,我们通过厘米级岩芯标本的观察,结合已发表的盆地资料,提出了6类与里德尔剪切有关的构造组合及断陷盆地组合。(1)雁列状构造及盆地组合:许多盆地发育雁列状构造。同时,与里德尔剪切相关的雁列状盆地的宽度与主剪切断裂的剪切位移呈正相关。(2)帚状或马尾状构造及盆地组合:二者在形态上相似,所以归为一类。成因上,马尾状构造及盆地主要发育在走滑断裂的拉张端部,而帚状构造或盆地反映走滑断裂的旋扭作用,可以在走滑断裂影响区域的任何部位。(3)串珠状构造及盆地组合:该类型的盆地主要是指释压盆地的组合,拉分盆地也可以形成串珠状盆地。(4)S状或Z状构造或盆地组合:左行走滑形成Z状构造或盆地,而右行走划形成S状构造或盆地。(5)多字型构造及盆地组合:是拉分盆地的典型组合,可以过渡到串珠状盆地。(6)复杂的网状构造及盆地组合:通常是由于分布型简单剪切的作用结果。以上盆地组合类型包括大型盆地内次级单元(次级盆地或更次级盆地)的组合,但不包括多成因、多期活动的构造及盆地。 相似文献
9.
R. Raghavarao R. Suhasini R. Sridharan B. V. Krishnamurthy O. P. Nagpal 《Journal of Earth System Science》1990,99(3):413-423
The equatorial wave campaign-II which formed a part of the Indian Middle Atmosphere Programme (IMAP), was conducted from SHAR
(13.7°N, 80.2°E) from 15 January to 28 February 1986. Winds were measured from ground to 60 km by means of high altitude balloon
and a meteorological rocket (RH-200), once everyday, for 45 days. The frequencies of the oscillations in the deviations of
the east-west component of the winds from its mean at each height with one kilometer interval were obtained by the maximum
entropy (ME) method and phases/amplitudes of these frequencies were determined by the least squares technique on the wind
variation time series. The ME method has the inherent advantage of providing periodicities up to 1.5 times the data length.
The height structure of the long period waves of > 23 day periodicities that have larger amplitudes nearly by a factor of
2 as compared to the medium (9 to 22 day) or shorter period (4 to 8 day) ones, reveal two height regions of enhanced amplitudes,
one in the troposphere and another in the upper stratosphere/lower mesosphere, that too, mostly in the regions of positive
(westerly increasing or easterly decreasing with height) wind shears. The waves are seen to be inhibited in the negative wind
shear regions. From the abrupt changes in the altitude variation of phase, the possible source region has been identified.
The vertical wavelengths have been estimated to be 34 km and 19 km in the troposphere and lower stratosphere respectively
and 8 km in the upper stratosphere and lower mesosphere. Around 56 km the wave amplitude is reduced to 1/4 of its value below,
while the vertical shear strength in the mean wind doubled up. The tropospheric waves are suggested to be Rossby waves of
extratropical origin penetrating to tropical latitudes. The stratospheric/mesospheric waves however appear to emanate from
a source around the stratopause. 相似文献
10.
AbstractCompaction driven fluid flow is inherently unstable such that an obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled waves of porosity, propagated by dilational deformation due to an effective pressure gradient within the wave. Viscous porosity waves have attracted attention as a mechanism for melt transport, but are also a mechanism for both the transport and trapping of fluids released by diagenetic and metamorphic reactions. We introduce a mathematical formulation applicable to compaction driven flow for the entire range of rheological behaviors realized in the lithosphere. We then examine three first-order factors that influence the character of fluid flow: (1) thermally activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For normal geothermal gradients, thermally activated creep stabilizes horizontal waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: (1) the vertical length scale for compaction driven flow is generally constrained by the activation energy for viscous deformation rather than the viscous compaction length, and (2) lateral fluid flow in viscous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inverted geothermal gradients stabilize vertically elongated waves or vertical channels. Decreasing temperature toward the earth’s surface can induce an abrupt transition from viscous to elastic deformation-propagated fluid flow. Below the transition, fluid flow is accomplished by short wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patterns vary strongly in space and time. Solitary porosity waves may nucleate in viscous, viscoplastic, and viscoelastic rheologies. The amplitude of these waves is effectively unlimited for physically realistic models with dependence of bulk viscosity on porosity. In the limit of zero initial connected porosity, arguably the only model relevant for melt extraction, travelling waves are only possible in a viscoelastic matrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes occur during propagation, the waves have the capacity to transmit geochemical signatures indefinitely. In addition to solitary waves, we find that periodic solutions to the compaction equations are common though previously unrecognized. The transition between the solutions depends on the pore volume carried by the wave and the Darcyian velocity of the background fluid flux. Periodic solutions are possible for all velocities, whereas solitary solutions require large volumes and low velocities. © Elsevier, Paris 相似文献
11.
Chloe Winant Arnoldo Valle-Levinson Aurelien Ponte Clinton Winant Guillermo Gutierrez-de-Velasco Kraig Winters 《Estuaries and Coasts》2014,37(6):1319-1328
Time series of current velocity profiles and thermistor chains were obtained throughout a cross-bay transect for ~90 days for the purpose of comparing observed wind-driven stratified flows to theory. This study concentrates on the synoptic scale wind and its influence on the bay’s circulation. The maximum water column stratification was 3–4 °C/m throughout the deployment and influenced wind-driven flows. Low-pass filtered flows showed more complicated structures than those expected from theory: a depth-dependent recirculating structure with the along-bay flow over one half of the transect moving in opposite direction to the other half. Analysis of complex empirical orthogonal functions indicated that the first six modes explained 80 % of the flow variability. Therefore, there was no predominantly energetic mode of variability. All modes exhibited a rich spatial structure with vertical and lateral variations. For all modes there was vertically sheared bidirectional flow, as expected from theory, with the largest eigenvector (mode value) asymmetrically influenced by Earth’s rotation and advection of momentum. 相似文献
12.
The characteristics of simulated air flow over Andaman Islands are studied with a two-dimensional version of the University
of Virginia meso scale model (UVMM). Using the observed synoptic data as initial conditions, 24 hr simulations are obtained
for a day each in April and November. These days are chosen to study the variations in the simulated flow pattern under different
synoptic conditions including precipitation effects. A large scale condensation scheme is employed to consider the effect
of latent heat release on the perturbations. The results show that the latent heat released by condensation strengthens the
intensity of perturbations and the topography accelerates the arrival of sea breeze by about an hour. The model-simulated
results, given in graphical form, are discussed and compared with available observations. 相似文献
13.
滚波(roll wores)是明渠均匀水流自由面失稳后发育的一系列波动现象.它的发生会带来诸多不利后果,如溢流、掺气、对河(渠)道水工建筑物的超负荷压力或应力等.滚波具有浅水长波的特征,因而利用一维浅水方程开展滚波问题的研究,取得了丰富的成果:如判定自由面稳定性的临界弗劳德数、演化方程的理论解及试验、数值研究等.构建能够更好地刻画滚波波动特征的理论模型,研究浑水、动床(包括床面变形)条件下滚波的形成及其演化规律,是今后滚波研究的一个重要方向. 相似文献
14.
ACTIVE DEFORMATION STYLE IN SOUTH-EASTERN AND NORTH MARGINS OF TIBETAN PLATEAU 相似文献
15.
16.
The low-frequency transport processes in a small, shallow coastal lagoon (Indian River Bay, Delaware) are examined based on a set of data derived from tide gauges, near-bottom current meter measurements, and drifter releases. The subtidal sea-level fluctuations in the interior of the lagoon are forced primarily by the coastal sea-level fluctuations off the mouth of the inlet, which connects the lagoon with the coastal ocean. The effect of local wind plays a secondary role in modifying the coastally forced sea level inside the lagoon. Given the continuity constraint which links sea-level fluctuations to the depth and laterally integrated barotropic transport, the coastal pumping effect would be expected to be the dominant factor in controlling the subtidal barotropic exchange within the bay. However, the dominance of the coastal pumping effect on the barotropic exchange does not readily translate into the dominance of this effect on the transport and distribution of waterborne material in the bay at subtidal frequencies. The observed nearbottom subtidal current fluctuations are not coherent with the coastal sea-level fluctuations. The observed current is also much stronger than the barotropic current inferred from the continuity constraint. This suggests the presence of a depth-dependent flow field, with current in the upper layer fluctuating in opposite direction to that at depth. Furthermore, the observed near-bottom current also shows significant spatial variability within the bay. As for the mean current, the residual flow field shows distinctly different patterns between the surface and the bottom. The residual current at the surface exhibits a consistent mean flow out of the bay. At the bottom, the residual current shows a mean flow into the estuary in the upper part of the lagoon and a spatially variable flow in the lower part of the lagoon. A competition between gravitational circulation and tidally rectified current may contribute to the observed vertical and horizontal variabilities in the residual flow field. 相似文献
17.
碳酸盐岩往往含有较多泥质夹层,多发育浅部岩溶裂隙,影响工程基础稳定性。文章依托广西合山市溯河矿区某道路浅部岩溶调查成果,分析了薄层灰岩岩溶宏观形态和垂直分布特征,按照溶蚀程度将其分为碎屑残留带、表层岩溶带和下部包气带;通过室内溶蚀试验,分析了矿物组成、CO2及水动力条件影响下的溶蚀规律;对比分析了薄层和厚层岩溶发育特征。结果表明:（1）薄层灰岩浅部岩溶发育程度与微地貌有关,通常在山谷和地下水强径流带溶蚀作用更为强烈;（2）薄层灰岩溶蚀优先开始于方解石,在岩石表面形成溶孔、粗糙程度增加,提供了侵蚀性溶液流动通道;（3）薄层灰岩岩溶较厚层灰岩岩溶规模小,分布较为均匀,由地表土体渗入地下岩体的地下水含有较多侵蚀性CO2水溶液,会促进岩溶作用的进行;（4）薄层灰岩浅部岩溶发育分布模型为:发育均匀的微小溶蚀裂隙在溶蚀下渗过程中遇到软弱夹层时会沿水平向扩展并随进一步溶蚀作用被充填,垂向深部发展时溶蚀作用逐步消散。 相似文献
18.
19.
砂土中盾构隧道开挖面失稳土体三维形状分析 总被引：1，自引：0，他引：1
合理确定盾构隧道开挖面前方失稳土体的形状是开挖面极限支护压力计算及开挖面失稳风险评估的基础和难点。建立模拟盾构隧道开挖面失稳过程的数值模型,利用Handy拱效应理论,依据Mohr-Coulomb破坏准则,得出失稳土体破坏位置和形状的计算方法。研究结果表明,水平方向由于隧道左右两端的支承作用产生水平压力拱,使失稳土体存在一个极限边界,极限边界内垂直方向的土体移动产生悬链线形的最小主应力拱,失稳土体形状类似贝壳形。以南京地铁3号线浦珠路站－滨江路站区间盾构隧道工程作为算例,理论分析和数值模拟结果吻合较好。 相似文献
20.
Using a model that couples wind flow with the motion of sand particles under different atmospheric stability intensities,
this paper studied the effects of atmospheric stability on the trajectory and velocity of sand particles in the saltation
layer, and the duration before a steady state was achieved. The vertical velocity, horizontal distance, and the maximum height
of saltating sand particles increased with increasingly negative stability intensity under unstable conditions. The wind–sand
flow reached equilibrium more quickly with increasingly negative stability intensity under unstable conditions, but reached
equilibrium more slowly with increasing stability intensity under stable conditions. 相似文献