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1.
准Λ型4能级系统具有电磁诱导吸收(EIA)和电磁诱导透明(EIT)两种特性.准Λ型4能级系统包括两个基态精细结构能级和两个激发态精细结构能级,除光学耦合场和探测场外,附加了一个射频场作用于两个激发态精细结构能级之间.若对此系统进行拓展,在两个基态精细结构能级之间引入一个驱动场,则构成4场作用下的闭合Λ型4能级系统.本文对新引进的驱动场的作用规律进行了重点研究.研究结果表明,当驱动场和射频场的Rabi频率满足不同关系时,系统呈现EIA或EIT两种不同特性,探测吸收曲线的整体轮廓也随之改变. 相似文献
2.
The spontaneous emission decay dynamics of a tripod configuration four-level atom driven by a single laser field is studied. Under different initial conditions, we discuss the effects of quantum interference and detuning of external driving field on atomic spontaneous emission properties. For the larger detuning, the interesting phenomena of the spectral line narrowing are found which stem from the contribution of external driving field. 相似文献
3.
The Mindanao Current (MC) bridges the North Pacific low-latitude western boundary current system region and the Indonesian Seas by supplying the North Pacific waters to the Indonesian Throughflow. Although the previous study speculated that the diapycnal mixing along the MC might be strong on the basis of the water mass analysis of the gridded climatologic dataset, the real spatial distribution of diapycnal mixing along the MC has remained to be clarified. We tackle this question here by applying a finescale parameterization to temperature and salinity profiles obtained using two rapid-sampling profiling Argo floats that drifted along the MC. The western boundary (WB) region close to the Mindanao Islands and the Sangihe Strait are the two mixing hotspots along the MC, with energy dissipation rate ε and diapycnal diffusivity Kρ enhanced up to?~?10–6 W kg?1 and?~?10–3 m2 s?1, respectively. Except for the above two mixing hotspots, the turbulent mixing along the MC is mostly weak, with ε and Kρ to be 10–11–10–9 W kg?1 and 10–6–10–5 m2 s?1, respectively. Strong mixing in the Sangihe Strait can be basically attributed to the existence of internal tides, whereas strong mixing in the WB region suggests the existence of internal lee waves. We also find that water mass transformation along the MC mainly occurs in the Sangihe Strait where the water masses are subjected to strong turbulent mixing during a long residence time.
相似文献4.
An experimental study of stratified mixing caused by internal solitary waves in a two-layered fluid system over variable seabed topography 总被引：4，自引：0，他引：4
Stratified mixing is observed in a wave flume on an internal solitary wave (ISW) of depression or elevation type propagating over a submarine ridge. The submarine ridges, which comprise the seabed topography, are either semicircular or triangular. Tests are performed in a series of combinations of submarine ridges with different heights and ISW in different amplitudes within a two-layer fluid system. When the thickness of the top layer is less than that of the lower layer (i.e., H1<H2), a depression-type ISW may produce a strong hydraulic jump with downwards motion and continuous eddy diffusion. During diffusion, the leading profile of the ISW transforms a wrapped vortex on the front face of the ridge, and a vortex separation at the apex of the ridge. Meanwhile, an elevation-type ISW causes a vortex in the lee of a submarine ridge, which resembles a surface solitary wave in terms of wave transmission process. The degree of wave-obstacle interaction is determined by energy loss, which is induced by submarine ridge blockage. The experiment results suggest that degree of blocking can be applied to classify various degrees of ISW-obstacle encounter in the stratified two-layer fluid system. 相似文献
5.
Behaviour of the optical properties of coloured dissolved organic matter under conservative mixing 总被引：3，自引：0，他引：3
The optical properties of coloured dissolved organic matter (CDOM) can be used, in some environments, to trace water masses and provide information about the dynamics of the dissolved organic fraction in natural waters. This work presents the results from a modelling exercise, laboratory experiment and field data, which describe the variations in the optical properties of CDOM during mixing. The exponential slope coefficient (S) is frequently used to characterise different CDOM pools; however, its behaviour during conservative mixing of two different CDOM types is often misunderstood. Identification of a theoretical conservative mixing line allows the rapid identification of non-conservative processes (e.g. in situ production, flocculation and degradation) acting on the pool during mixing. The results suggest that some of the patterns reported in the literature could purely be a result of conservative mixing rather than a product of non-conservative processes. 相似文献
6.
7.
Use of nautical radar as a wave monitoring instrument 总被引：2，自引：0，他引：2
Common marine X-Band radars can be used as a sensor to survey ocean wave fields. The wave field images provided by the radars are sampled and analysed by a wave monitoring system (called WaMoS II) developed by the German research institute GKSS. This measuring system can be mounted on a ship, on offshore stations or at coastal locations. The measurement is based on the backscatter of microwaves from the ocean surface, which is visible as ‘sea clutter' on the radar screen. From this observable sea clutter, a numerical analysis is carried out. The unambiguous directional wave spectrum, the surface currents and sea state parameters such as wave periods, wave lengths, and wave directions can be derived. To provide absolute wave heights, the response of the nautical radar must be calibrated. Similar to the wave height estimations for Synthetic Aperture Radars, the so-called ‘Signal to Noise Ratio' leads to the determination of the significant wave height (HS). In this paper, WaMoS II results are compared with directional buoy data to show the capabilities of nautical microwave radars for sea state measurements. 相似文献
8.
Turbulent mixing of water masses of different temperatures and salinities is an important process for both coastal and large-scale ocean circulation. It is, however, difficult to capture computationally. One of the reasons is that mixing in the ocean occurs at a wide range of complexity, with the Reynolds number reaching , or even higher.In this study, we continue to investigate whether large eddy simulation (LES) can be a reliable computational tool for stratified mixing in turbulent oceanic flows. LES is attractive because it can be times faster than a direct numerical simulation (DNS) of stratified mixing in turbulent flows. Before using the LES methodology to compute mixing in realistic oceanic flows, however, a careful assessment of the LES sensitivity with respect to Re needs to be performed first. The main objectives of this study are: (i) to investigate the performance of different LES models at high Re, such as those encountered in oceanic flows; and (ii) to study how mixing varies as a function of Re. To this end, as a benchmark we use the lock-exchange problem, which is described by unambigous and simple initial and boundary conditions. The background potential energy, which accurately quantifies irreversible mixing in an enclosed system, is used as the main criterion in a posteriori testing of LES.This study has two main achievements. The first is that we investigate the accuracy of six combinations of two different classes of LES models, namely eddy-viscosity and approximate deconvolution types, for 3×103Re3×104, for which DNS data is computed. We find that all LES models almost always provide significantly more accurate results than cases without LES models. Nevertheless, no single LES model that is persistently superior to others over this Re range could be identified. Then, an ensemble of the four best performing LES models is selected in order to estimate mixing taking place in this system at Re=105 and 106, for which DNS is presently not feasible. Thus the second achievement of this study is to quantify mixing taking place in this system over an Re range that changes by three orders of magnitude. We find that the background potential energy increases by about 67% when Re is increased from Re=103 to Re=106, within the computation period, with the most significant increase taking place from Re=3×103 to Re=105. 相似文献
9.
V. P. Kukharets O. G. Nalbandyan A. V. Shmakov 《Izvestiya Atmospheric and Oceanic Physics》2009,45(4):411-415
The vertical transport of passive additives in a medium in the presence of both turbulent and convective mixing is studied.
It is shown that, despite the fact that turbulent and convective fluxes are uncorrelated, no additive mechanism of mixing
exists. On the basis of a theoretical model and a comparison with model calculations, it is shown that, in the presence of
both turbulent and convective mixing in a medium, the effect of the interference of turbulent and convective transport mechanisms
takes place. This effect is manifested in the following way: the presence of convection decreases the coefficient of turbulent
transport and the presence of turbulence decreases the coefficient of convective transport; in this case, this effect depends
on the parameter of the inhomogeneity (extension) of convective fluxes. The relative contribution of each of the mechanisms
to the total transport is estimated. It is shown that the interference of turbulence and convection results in an anisotropy
of the coefficient of turbulent transport in an isotropic turbulent velocity field. The effect of the coefficient of molecular
diffusion is assumed to be negligibly small, and its effect is not taken into consideration when analyzing model results.
The interference of the turbulent and convective mechanisms of transport is explained by the inclusion of an additional mechanism
that decreases the distance of linear particle propagation. 相似文献
10.
海洋上层垂向混合在模式中发挥重要的作用,以往的研究表明垂向混合的不足使得模拟的海洋温度和混合层深度与观测存在显著偏差。前人提出一种修正方案,考虑波浪产生的垂向混合,将由表面风作用下产生的波浪这样一个实际物理过程的湍混合进行参数化,其结果被证实能够显著提高模式模拟和预报的准确性。本文首次将浪致混合引入海气耦合的古气候模式,基于末次冰盛期和工业革命前2种不同的气候条件,探究浪致混合在海气耦合模式中的作用。在不同气候背景下,由于风场强度的不同,导致末次冰盛期浪致混合的强度小于工业革命前,但2个气候时期都体现出中纬度混合强度最大的特点。将浪致混合加入到气候模式中,模拟结果表明：中纬度海域2个时期都出现海表面降温而次表层升温的现象,但末次冰盛期的表面降温强度弱于工业革命前状态;不同月份下的模拟结果显示,在南北半球的夏季,海洋表层温度的降温最为显著。中纬度海域海洋上混合层深度在年平均条件下2个气候背景时期都出现加深现象,但末次冰盛期的加深程度弱于工业革命前;不同月份下的模拟结果显示,在南北半球的冬季,混合层加深的变化达到极值。另一方面,在高纬度海域,末次冰盛期的海表面温度出现了显著升高,这是由于浪致混合导致海冰的减少进而引发海洋表层升温。最后将末次冰盛期的模拟结果与代用资料进对比,发现浪致混合使得72%的数据点模拟结果与代用数据的差异减少。 相似文献
11.
Toshimichi Ito Arata Kaneko Hiroyuki Tsubota Noriaki Gohda 《Journal of Oceanography》1994,50(4):465-477
The localized near-bottom water with silica content higher than that in the adjacent shelf water was observed to exist at the eastern margins of the East China Sea continental shelf. The core of the high silica water possessed the silica content corresponding to that in the Kuroshio at depths greater than on the shelf. The mixing analysis of water masses using temperature (T) and silica (Si) showed that the core water can be produced through the vertical mixing of intermediate water of the Kuroshio deeper than 100 m. This study provides us a conclusion that the intermediate water of the Kuroshio is strongly mixed on the shelf slope and then upwelled to form the ridge-like distribution of water masses with low temperature and high silica content at the shelf edge. 相似文献
12.
The mean available potential energy released by baroclinic instability into the meso-scale eddy field has to be dissipated in some way and Tandon and Garrett [Tandon, A., Garrett, C., 1996. On a recent parameterization of mesoscale eddies. J. Phys. Oceanogr. 26 (3), 406–416] suggested that this dissipation could ultimately involve irreversible mixing of buoyancy by molecular processes at the small-scale end of the turbulence cascade. We revisit this idea and argue that the presence of dissipation within the thermocline automatically requires that a component of the eddy flux associated with meso-scale eddies must be associated with irreversible mixing of buoyancy within the thermocline. We offer a parameterisation of the implied diapycnal diffusivity based on (i) the dissipation rate for eddy kinetic energy given by the meso-scale eddy closure of Eden and Greatbatch [Eden, C., Greatbatch, R.J., 2008. Towards a meso-scale eddy closure. Ocean Modell. 20, 223–239.] and (ii) a fixed mixing efficiency. The implied eddy-induced diapycnal diffusivity (κ) is implemented in a coarse resolution model of the North Atlantic. In contrast to the vertical diffusivity given by a standard vertical mixing scheme, large lateral inhomogeneities can be found for κ in the interior of the ocean. In general, κ is large, i.e. up to o(10) cm2/s, near the western boundaries and almost vanishing in the interior of the ocean. 相似文献
13.
Generally, the sea-state (random waves) is best described by a wave spectrum. A number of statistical models for wave spectra has been well established and a sea-state can be specified. Once the specified sea-state is established, the corresponding model for wave forces acting on a single cylinder or a group of cylinders can be formulated. Since peak force is of more practical value, a multivariate or joint probability density function for wave forces has been developed for the peak force distribution of wave forces. This theoretical force model derives the tri-variate probability density function P(F, F′, F″), where F is the peak force defined by Morison equation. This model is of wide-band in nature and is tested by wave flume experiments. 相似文献
14.
Kazuya Ono Kay I. Ohshima Tokihiro Kono Motoyo Itoh Katsuro Katsumata Yuri N. Volkov Masaaki Wakatsuchi 《Journal of Oceanography》2007,63(2):281-291
Intensive CTD observations that resolve the mean and tidal components were done with a total of 129 casts in summer of 2001
at Bussol’ Strait. Based on these data and all the available historical data, we have revealed the outflow from Bussol’ Strait
to the Pacific and the significant diapycnal mixing in the strait. In the range 27.0−27.3σ
θ
, the water property in Bussol’ Strait is almost identical to that of the Kuril Basin Water (KBW). The KBW out of Bussol’
Strait forms a water mass front with the East Kamchatka Current Water (EKCW). This front also corresponds to the front of
the Oyashio Current. In the lower part of the intermediate layer (27.3−27.6σ
θ
), part of the water in the strait is characterized by lower temperature, lower salinity, and higher dissolved oxygen than
that of KBW and EKCW, which can be explained only by the diapycnal mixing. The strong diapycnal mixing in the strait can also
be shown by the density inversion, occurrence frequency of which corresponds well to the amplitude distribution of the diurnal
current. In the density range 26.7−26.8σ
θ
, the water in Bussol’ Strait has the lowest potential vorticity, suggesting that it is a source region of the low potential
vorticity water. Seasonal change of the water can reach up to a density of 26.8σ
θ
around Bussol’ Strait. This leads us to propose that the combination of winter convection and local tidal mixing leads to
effective ventilation of the intermediate layer. 相似文献
15.
The detection of a passive sonar target in the presence of ambient noise and a plane wave interference is discussed. Intuitively, such a detector consists of a spatial filter which nulls the interference, followed by a temporal filter. In this paper we study the role of the a priori knowledge of the spectrum of the interference and/or signal in improving detector performance. We develop three different generalized likelihood ratio test (GLRT) detectors, resulting from different cases of prior spectral information. We show that, for all cases of known/unknown source and/or interference power spectrum, the GLRT detectors are, as expected, null steering systems. The depth and shape of the null, as well as the postbeamforming temporal filter, are different and are functions of the a priori known spectrum. Under the assumption that all signals and noise are zero-mean Gaussian processes, we analyze the performance of the different detectors and we exploit their dependency on the array beampattern, as well as on the source and interference signal-to-noise ratio. This analysis serves to identify scenarios where the use of prior spectral information leads to significant performance improvement 相似文献
16.
Sherry M. Lippiatt Matthew T. Brown Maeve C. Lohan Kenneth W. Bruland 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(11):1091-1102
Mesoscale anticyclonic eddies in the Gulf of Alaska are an important mechanism for cross-shelf exchange of high iron, low nitrate coastal waters and low iron, high nitrate offshore waters. A Kenai eddy was sampled in September 2007, 8 months after formation. The subsurface eddy core layer contained reactive iron concentrations more than eight times greater than waters at the same depths outside the eddy. The subsurface core of the Kenai eddy (25.4≤σθ≤25.8) is suggested to be seasonally important as these waters can be brought to the surface with storm events and deep winter mixing. The deeper core layer (25.8≤σθ≤27.0) is suggested to be a source of iron to HNLC waters on a longer timescale, due to isopycnal mixing and eventual eddy relaxation. The subsurface and deeper core layers are important reservoirs of iron that can promote and sustain primary productivity over the lifetime of the Kenai eddy. In addition, dissolved and leachable particulate manganese are shown to be excellent tracers of eddy surface and subsurface waters, respectively. 相似文献
17.
Sensitivity studies with a new generalized coordinate ocean model are performed in order to compare the behavior of bottom boundary layers (BBLs) when terrain-following (sigma or combined sigma and z-level) or z-level vertical grids are used, but most other numerical aspects remain unchanged. The model uses a second-order turbulence closure scheme that provides surface and BBL mixing and results in a quite realistic climatology and deep water masses after 100 year simulations with a coarse resolution (1° × 1°) basin-scale terrain-following grid. However, with the same turbulence scheme but using a z-level grid, the model was unable to produce dense water masses in the deep ocean. The latter is a known problem for coarse resolution z-level models, unless they include highly empirical BBL schemes.A set of dense water overflow experiments with high-resolution grids (10 and 2.5 km) are used to investigate the influence of model parameters such as horizontal diffusivity, vertical mixing, horizontal resolution, and vertical resolution on the simulation of bottom layers for the different coordinate systems. Increasing horizontal diffusivity causes a thinner BBL and a bottom plume that extends further downslope in a sigma grid, but causes a thicker BBL and limited downslope plume extension in a z-level grid. A major difference in the behavior of the BBL in the two grids is due to the larger vertical mixing generated by the turbulence scheme over the step-like topography in the z-level grid, compared to a smaller vertical mixing and a more stably stratified BBL in the sigma grid. Therefore, the dense plume is able to maintain its water mass better and penetrates farther downslope in the sigma grid than in the z-level grid. Increasing horizontal and vertical resolution in the z-level grid converges the results toward those obtained by a much coarser resolution sigma coordinate grid, but some differences remain due to the basic differences in the mixing process in the BBL. 相似文献
18.
本文研究了全息存储实验中以波长532 nm的半导体激光器作为记录和读出光源,碱性品红作为光敏剂的丙烯酰胺基光致聚合物薄膜的光致光衍射现 象.它可以考虑为透过全息干涉条纹的原始入射光和反射光,与来自光聚物中未曝光的不均匀表面和内部的散射光四波混频的结果.根据简并四波混频理论中的位相匹配条件对双光束全息写入时产生的衍射光锥,以及单光束辐照复现时产生的衍射光锥现象分别做出了理论解释;并且利用全息散射理论结合三角学知识对入射光的入射角与衍射光锥的锥角的依赖关系进行了深入讨论. 相似文献
19.
The sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) occurs mainly at neap tides during summer, suggesting that tidal mixing is one of the essential factors regulating kyucho. In order to clarify the physical mechanisms responsible for the regulation of kyucho, we carry out non-hydrostatic three-dimensional numerical experiments allowing Kuroshio warm water to intrude into a strong tidal mixing region. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during neap (or spring) tides. The analysis of the dynamic balance off the east coast of the Bungo Channel shows that tidal residual currents generated by tidal flow interaction with complicated land configurations off the east coast of the Bungo Channel can also play an important role in regulating kyucho. In order to assess separately the effects of tidal mixing and tidal residual currents on kyucho, we incorporate the parameterized vertical mixing and tidal stresses into the numerical model instead of tidal currents. It is demonstrated that tidal mixing cannot by itself block the northward intrusion of Kuroshio warm water, and that an additional effect induced by tidal residual eddies equivalent to horizontal mixing is needed to regulate kyucho. This strongly suggests that the basin–ocean water exchange processes in areas with complicated land configurations can only be reproduced by taking into account the effects of tidal residual eddies on a 1-km scale in addition to tidal mixing effects evaluated by microstructure measurements. 相似文献
20.
Winfried Lampert 《新西兰海洋与淡水研究杂志》2013,47(3):483-490
Laboratory studies on cyanobacteria‐zooplankton interactions have largely focused on the inadequacy of cyanobacteria as a food source. Some features of cyanobacteria can be regarded as anti‐herbivore defences. Large colonies of Aphanizomenon, Anabaena and Microcystis cannot be handled by zooplankton, but do not interfere seriously with the filtering process. Small colonies and filaments, however, may cause severe inhibition of the feeding process by mechanical interference. This reduces zooplankton growth, reproduction, and survival. Copepods, rotifers, and Bosmina are less affected by mechanical disturbance than cladocerans. If ingested, some cyanobacteria may be poorly digested or may not provide essential nutrients. Some cyanobacteria are reported to be toxic to zooplankton. Several strains of Microcystis produce an endotoxin, but the amount of toxin produced differs among strains and with the condition of Microcystis. Zooplankton encountering toxic cells cease feeding. Some evidence of an extra‐cellular herbivore deterrent was found for Anabaena. Thus, cyanobacteria may either be individually protected or may create an environment that is unfavourable for efficient grazers (e.g., Daphnia). 相似文献