Articles de revues : « Bruit – Stabilité »

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Boukraa, Mohamed Aziz, Lorenzo Audibert, Marcella Bonazzoli, Houssem Haddar et Denis Vautrin. « Imagerie d’interface barrage-fondation par inversion de forme d'onde complète ». E3S Web of Conferences 504 (2024) : 04002. http://dx.doi.org/10.1051/e3sconf/202450404002.

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Dans le cadre de l’étude de la stabilité des barrages, la connaissance de l’interface entre le barrage et la roche revêt une grande importance. Le recours à des techniques géophysiques peut apporter des informations complémentaires par rapport aux mesures géotechniques. Nous proposons ici une méthode de traitement des mesures sismiques, l’objectif étant d'obtenir une image de l'interface entre le béton du barrage et le rocher de la fondation avec une résolution métrique. Il s’agit d’une technique de type « Full Waveform Inversion » avec optimisation de forme. Des résultats numériques utilisant des mesures synthétiques montrent la capacité de la méthode à retrouver l'interface avec une précision satisfaisante, pour un nombre limité de points de mesure et en présence de bruit.

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Dolas, Prakash M., et Karanam Kishore Kumar. « Retrieval of static stability parameter from the radiosonde/rawinsonde ascent rate profiles : a wavelet approach ». Annales Geophysicae 27, n^o 2 (2 février 2009) : 547–53. http://dx.doi.org/10.5194/angeo-27-547-2009.

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Abstract. In the present communication a novel method is presented to derive the altitude profile of Brunt-Väisälä period from the ascent rate profile of sounding balloons. The basic premise of the present method is that the oscillations in the ascent rate of the balloon will have the signature of Brunt-Väisälä frequency, which can be retrieved by using sophisticated spectral tools. We employ wavelet transforms to arrive at the Brunt-Väisälä period profile. Comparison of retrieved Brunt-Väisälä periods with the values derived from the temperature data available from the same radiosonde ascent shows good agreement. Retrieving the atmospheric temperature from the height profile of Brunt-Väisälä period is also discussed in the present communication. We have shown that it is possible to estimate the Brunt-Väisälä period and temperature profiles from the rawinsonde ascent rate data alone where temperature sounding is not available.

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Anderson, R., D. H. Jones et G. H. Gudmundsson. « Halley Research Station, Antarctica : calving risks and monitoring strategies ». Natural Hazards and Earth System Sciences 14, n^o 4 (17 avril 2014) : 917–27. http://dx.doi.org/10.5194/nhess-14-917-2014.

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Abstract. The British Antarctic Survey's Halley Research Station is located on the Brunt Ice Shelf, Antarctica, where it is potentially vulnerable to calving events. Existing historical records show that the Brunt Ice Shelf is currently extended further into the Weddell Sea than it was before its last large calving event, so a new calving event may be overdue. We describe three different possible future scenarios for a large-scale calving event on Brunt Ice Shelf. We conclude that currently the most threatening scenario for the Halley Research Station is a calving event on the neighbouring Stancomb-Wills Glacier Tongue, with subsequent detrimental consequences for the stability of the Brunt Ice Shelf. Based on available data, we suggest an increasing likelihood of this scenario occurring after 2020. We furthermore describe ongoing monitoring efforts aimed at giving advanced warning of an imminent calving event.

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Anderson, R., D. H. Jones et G. H. Gudmundsson. « Halley Research Station, Antarctica : calving risks and monitoring strategies ». Natural Hazards and Earth System Sciences Discussions 1, n^o 6 (5 novembre 2013) : 6227–56. http://dx.doi.org/10.5194/nhessd-1-6227-2013.

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Abstract. The British Antarctic Survey's Halley Research Station is located on the Brunt Ice Shelf, Antarctica, where it is potentially vulnerable to calving events. Existing historical records show that the Brunt Ice Shelf is currently extended further into the Weddell Sea than it was before its last large calving event, so a new calving event may be overdue. We describe three different possible future scenarios for a large-scale calving event on Brunt Ice Shelf, and conclude that the currently most threatening scenario for the Halley Research Station is a calving event on the neighbouring Stancomb–Wills Glacier Tongue, with subsequent detrimental consequences for the stability of the Brunt Ice Shelf. Based on available data, we suggest an increasing likelihood of this scenario occurring after 2020. We furthermore describe ongoing monitoring efforts aimed at giving advanced warning of an imminent calving event.

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BILLANT, PAUL. « Zigzag instability of vortex pairs in stratified and rotating fluids. Part 1. General stability equations. » Journal of Fluid Mechanics 660 (21 juillet 2010) : 354–95. http://dx.doi.org/10.1017/s0022112010002818.

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In stratified and rotating fluids, pairs of columnar vertical vortices are subjected to three-dimensional bending instabilities known as the zigzag instability or as the tall-column instability in the quasi-geostrophic limit. This paper presents a general asymptotic theory for these instabilities. The equations governing the interactions between the strain and the slow bending waves of each vortex column in stratified and rotating fluids are derived for long vertical wavelength and when the two vortices are well separated, i.e. when the radii R of the vortex cores are small compared to the vortex separation distance b. These equations have the same form as those obtained for vortex filaments in homogeneous fluids except that the expressions of the mutual-induction and self-induction functions are different. A key difference is that the sign of the self-induction function is reversed compared to homogeneous fluids when the fluid is strongly stratified: |max| < N (where N is the Brunt–Väisälä frequency and max the maximum angular velocity of the vortex) for any vortex profile and magnitude of the planetary rotation. Physically, this means that slow bending waves of a vortex rotate in the same direction as the flow inside the vortex when the fluid is stratified-rotating in contrast to homogeneous fluids. When the stratification is weaker, i.e. |max| > N, the self-induction function is complex because the bending waves are damped by a viscous critical layer at the radial location where the angular velocity of the vortex is equal to the Brunt–Väisälä frequency.In contrast to previous theories, which apply only to strongly stratified non-rotating fluids, the present theory is valid for any planetary rotation rate and when the strain is smaller than the Brunt–Väisälä frequency: Γ/(2πb2) ≪ N, where Γ is the vortex circulation. Since the strain is small, this condition is met across a wide range of stratification: from weakly to strongly stratified fluids. The theory is further generalized formally to any basic flow made of an arbitrary number of vortices in stratified and rotating fluids. Viscous and diffusive effects are also taken into account at leading order in Reynolds number when there is no critical layer. In Part 2 (Billant et al., J. Fluid Mech., 2010, doi:10.1017/S002211201000282X), the stability of vortex pairs will be investigated using the present theory and the predictions will be shown to be in very good agreement with the results of direct numerical stability analyses. The existence of the zigzag instability and the distinctive stability properties of vortex pairs in stratified and rotating fluids compared to homogeneous fluids will be demonstrated to originate from the sign reversal of the self-induction function.

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Reinecke, Patrick A., et Dale R. Durran. « Estimating Topographic Blocking Using a Froude Number When the Static Stability Is Nonuniform ». Journal of the Atmospheric Sciences 65, n^o 3 (1 mars 2008) : 1035–48. http://dx.doi.org/10.1175/2007jas2100.1.

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Abstract A parameter widely used to predict topographic flow blocking is the nondimensional mountain height or, synonymously, the inverse Froude number. Predictions using this parameter are based on the morphology of flows with uniform upstream static stability and wind speed, which rarely occur in the real world. The appropriateness of applying this theory in the presence of nontrivial background stability is therefore investigated using a numerical model. Two methods were considered to estimate the low-level stability, averaging the Brunt–Väisälä frequency below the crest and using the bulk change in θ between the ground and crest level. No single best method emerged for estimating the upstream static stability and thereby mapping the simulations with inversions onto the set of solutions with constant stratification. Instead, the best method depended on the application at hand. To predict the onset of flow stagnation, averaging the low-level stability worked best, while to predict low-level flow diversion the bulk estimate of low-level stability was most appropriate. These results are consistent across a range of inversion thicknesses and strengths. In addition, it is shown that variations in static stability above the mountain crest have little impact on flow blocking.

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Zaire, Bonnie, et Laurène Jouve. « Dipolar stability in spherical simulations : The impact of an inner stable zone ». Proceedings of the International Astronomical Union 15, S354 (juin 2019) : 185–88. http://dx.doi.org/10.1017/s1743921319009803.

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AbstractMagnetic fields vary in complexity for different stars. The stability of dipolar magnetic fields is known to depend on different quantities, e.g., the stellar rotation, the stratification, and the intensity of convective motions. Here, we study the dipolar stability in a system with an inner stable zone. We present preliminary results of dynamo simulations using the Rayleigh number as a control parameter. The stiffness of the stable zone is accordingly varied to keep a constant ratio of the Brunt-Väisälä frequency to the angular velocity. Similarly to the completely convective spherical shell, we find that a transition exists between a regime where the magnetic field is dipolar to a multipolar regime when the Rossby number is increased. The value of the Rossby number at the transition is very close to the one of the fully convective case.

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Pérez, I. A., M. L. Sánchez, M. Á. García et B. de Torre. « Boundary layer structure and stability classification validated with CO<sub>2</sub> ; concentrations over the Northern Spanish Plateau ». Annales Geophysicae 27, n^o 1 (21 janvier 2009) : 339–49. http://dx.doi.org/10.5194/angeo-27-339-2009.

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Abstract. A description of the lower boundary layer is vital to enhance our understanding of dispersion processes. In this paper, Radio Acoustic Sounding System sodar measurements obtained over three years were used to calculate the Brunt-Väisälä frequency and the Monin-Obukhov length. The Brunt-Väisälä frequency enabled investigation of the structure of this layer. At night, several layers were noticeable and the maximum was observed at the first level, 40 m, whereas during the day, it was present at about 320 m. The Monin-Obukhov length was calculated with the four first levels measured, 40–100 m, by an original iterative method and used to establish four stability classes: drainage, extremely stable, stable and unstable. Wind speed and temperature median profiles linked to these classes were also presented. Wind speeds were the lowest, but temperatures were the highest and inversions were intense at night in drainage situations. However, unstable situations were linked to high wind speeds and superadiabatic temperature profiles. Detrended CO2 concentrations were used to determine the goodness of the classification proposed evidencing values which under drainage at night in spring were nearly 28 ppm higher than those corresponding to unstable situations. Finally, atmosphere structure was presented for the proposed stability classes and related with wind speed profiles. Under extremely stable situations, low level jets were coupled to the surface, with median wind speeds below 8 m s−1 and cores occasionally at 120 m. However, jets were uncoupled in stable situations, wind speed medians were higher than 11 m s−1 and their core heights were around 200 m.

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Kumar, Naresh, M. Mohapatra et B. P. Yadav. « Boundary Layer Impact on Mountain Waves across Western Ghats of India ». Mapana - Journal of Sciences 10, n^o 1 (30 juin 2011) : 25–31. http://dx.doi.org/10.12723/mjs.18.3.

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A two- layer model has been developed assuming a steady, non- rotating and frictionless flow of vertically unbounded Boussinesq fluid across 2-D profile of Western Ghats of India. Further, it is assumed that lower layer is inviscid with neutral stability (N=0) and upper layer has constant static stability with respect to height, given by N= Constant (where N is Brunt- Väisälä frequency). The analytical expressions for vertical velocities and mountain drag have been derived across Western Ghats of India. It is found that vertical velocities are dependent on the characteristics like size and height of ridge as well as plateau region to the eastern side of the Western Ghats. However, the mountain drag is not dependent on the above characteristics of the plateau region to the east of Western Ghats confirming the earlier findings.

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Candelier, Julien, Stéphane Le Dizès et Christophe Millet. « Inviscid instability of a stably stratified compressible boundary layer on an inclined surface ». Journal of Fluid Mechanics 694 (2 février 2012) : 524–39. http://dx.doi.org/10.1017/jfm.2012.7.

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AbstractThe three-dimensional stability of an inflection-free boundary layer flow of length scale$L$and maximum velocity${U}_{0} $in a stably stratified and compressible fluid of constant Brunt–Väisälä frequency$N$, sound speed${c}_{s} $and stratification length$H$is examined in an inviscid framework. The shear plane of the boundary layer is assumed to be inclined at an angle$\theta $with respect to the vertical direction of stratification. The stability analysis is performed using both numerical and theoretical methods for all the values of$\theta $and Froude number$F= {U}_{0} / (LN)$. When non-Boussinesq and compressible effects are negligible ($L/ H\ll 1$and${U}_{0} / {c}_{s} \ll 1$), the boundary layer flow is found to be unstable for any$F$as soon as$\theta \not = 0$. Compressible and non-Boussinesq effects are considered in the strongly stratified limit: they are shown to have no influence on the stability properties of an inclined boundary layer (when$F/ \sin \theta \ll 1$). In this limit, the instability is associated with the emission of internal-acoustic waves.

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Hooper, D. A., J. Arvelius et K. Stebel. « Retrieval of atmospheric static stability from MST radar return signal power ». Annales Geophysicae 22, n^o 11 (29 novembre 2004) : 3781–88. http://dx.doi.org/10.5194/angeo-22-3781-2004.

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Abstract. An empirical technique for retrieving profiles of the square of the Brunt-Väisälä frequency, ωB2, from MST radar return signal power is presented. The validity of the technique, which is applied over the altitude range 1.0-15.7km, is limited to those altitudes at which the humidity contributions to the mean vertical gradient of generalised potential refractive index, M, can be ignored. Although this is commonly assumed to be the case above the first few kilometres of the atmosphere, it is shown that humidity contributions can be significant right up to the tropopause level. In specific circumstances, however, the technique is valid over large sections of the troposphere. Comparisons of radar- and (balloon-borne) radiosonde-derived ωB2 profiles are typically quantitatively and qualitatively well matched. However, the horizontal separation between the radar and the radiosondes (which were launched at the radar site) increases with increasing altitude. Under conditions of mountain wave activity, which can be highly localised, large discrepancies can occur at lower-stratospheric altitudes. This demonstrates the fact that radiosonde observations cannot necessarily be assumed to be representative of the atmosphere above the launch site.

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Ielo, Ileana, Giulia Rando, Fausta Giacobello, Silvia Sfameni, Angela Castellano, Maurilio Galletta, Dario Drommi, Giuseppe Rosace et Maria Rosaria Plutino. « Synthesis, Chemical–Physical Characterization, and Biomedical Applications of Functional Gold Nanoparticles : A Review ». Molecules 26, n^o 19 (26 septembre 2021) : 5823. http://dx.doi.org/10.3390/molecules26195823.

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Relevant properties of gold nanoparticles, such as stability and biocompatibility, together with their peculiar optical and electronic behavior, make them excellent candidates for medical and biological applications. This review describes the different approaches to the synthesis, surface modification, and characterization of gold nanoparticles (AuNPs) related to increasing their stability and available features useful for employment as drug delivery systems or in hyperthermia and photothermal therapy. The synthetic methods reported span from the well-known Turkevich synthesis, reduction with NaBH4 with or without citrate, seeding growth, ascorbic acid-based, green synthesis, and Brust–Schiffrin methods. Furthermore, the nanosized functionalization of the AuNP surface brought about the formation of self-assembled monolayers through the employment of polymer coatings as capping agents covalently bonded to the nanoparticles. The most common chemical–physical characterization techniques to determine the size, shape and surface coverage of AuNPs are described underlining the structure–activity correlation in the frame of their applications in the biomedical and biotechnology sectors.

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MacDonald, Michael, et João Teixeira. « Scaling Behavior of a Turbulent Kinetic Energy Closure Scheme for the Stably Stratified Atmosphere : A Steady-State Analysis ». Journal of the Atmospheric Sciences 77, n^o 9 (1 septembre 2020) : 3161–70. http://dx.doi.org/10.1175/jas-d-19-0332.1.

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Abstract We present a turbulent kinetic energy (TKE) closure scheme for the stably stratified atmosphere in which the mixing lengths for momentum and heat are not parameterized in the same manner. The key difference is that, while the mixing length for heat tends toward the stability independent mixing length for momentum in neutrally stratified conditions, it tends toward one based on the Brunt–Väisälä time scale and square root of the TKE in the limit of large stability. This enables a unique steady-state solution for TKE to be obtained, which we demonstrate would otherwise be impossible if the mixing lengths were the same. Despite the model’s relative simplicity, it is shown to perform reasonably well with observational data from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) using commonly employed model constants. Analyzing the scaling behavior of the nondimensional velocity and potential temperature gradients, or of the stability (correction) functions, reveals that for large stability the present model scales in the same manner as the first-order operational scheme of Viterbo et al. Alternatively, it appears as a blend of two cases of the TKE closure scheme of Baas et al. Critically, because a unique steady-state TKE can be obtained, the present model avoids the nonphysical behavior identified in one of the cases of Baas et al.

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Maddox, Emily M., et Gretchen L. Mullendore. « Determination of Best Tropopause Definition for Convective Transport Studies ». Journal of the Atmospheric Sciences 75, n^o 10 (octobre 2018) : 3433–46. http://dx.doi.org/10.1175/jas-d-18-0032.1.

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An idealized three-dimensional cloud-resolving model is used to investigate the sensitivity of cross-tropopause convective mass transport to tropopause definition. A simulation is conducted to encompass the growth and decay cycle of a supercell thunderstorm, with a focus on irreversible transport above the tropopause. Five previously published tropopause definitions are evaluated: World Meteorological Organization (WMO) temperature lapse rate, potential vorticity, static stability, vertical curvature of the Brunt–Väisälä frequency, and stratospheric tracer concentration. By analyzing the behavior of different definitions both during and after active convection, we are able to define “best” choices for tropopause definitions as those that return to states most closely matching the preconvective environment. Potential vorticity and stratospheric tracer concentration are shown to perform poorly when analyzing deep convection. The WMO thermal tropopause and static stability definitions are found to perform the best, providing similar tropopause placement and quantities of irreversible mass transport. This investigation highlights the challenges of defining a tropopause in the vicinity of deep convection and demonstrates the need to clearly communicate calculation methods and threshold choices in the literature.

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LESSHAFFT, L., B. HALL, E. MEIBURG et B. KNELLER. « Deep-water sediment wave formation : linear stability analysis of coupled flow/bed interaction ». Journal of Fluid Mechanics 680 (18 mai 2011) : 435–58. http://dx.doi.org/10.1017/jfm.2011.171.

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A linear stability analysis is carried out for the interaction of an erodible sediment bed with a sediment-laden, stratified flow above the bed, such as a turbidity or bottom current. The fluid motion is described by the full, two-dimensional Navier–Stokes equations in the Boussinesq approximation, while erosion is modelled as a diffusive flux of particles from the bed into the fluid. The stability analysis shows the existence of both Tollmien–Schlichting and internal wave modes in the stratified boundary layer. For the internal wave mode, the stratified boundary layer acts as a wave duct, whose height can be determined analytically from the Brunt–Väisälä frequency criterion. Consistent with this criterion, distinct unstable perturbation wavenumber regimes exist for the internal wave mode, which are associated with different numbers of pressure extrema in the wall-normal direction. For representative turbidity current parameters, the analysis predicts unstable wavelengths that are consistent with field observations. As a key condition for instability to occur, the base flow velocity boundary layer needs to be thinner than the corresponding concentration boundary layer. For most of the unstable wavenumber ranges, the phase relations between the sediment bed deformation and the associated wall shear stress and concentration perturbations are such that the sediment waves migrate in the upstream direction, which again is consistent with field observations.

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Krasnodubets, L. A. « Dynamic measurements in the problems of operational oceanology in the study of the properties of the oceanic plate ». Monitoring systems of environment, n^o 1 (28 mars 2022) : 56–65. http://dx.doi.org/10.33075/2220-5861-2022-1-56-65.

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The results of a study of the possible application of the basic method of vertical profiling of the oceanic column as a stratified aquatic environment, proposed by the author for using in operational oceanology, are presented. The method provides real-time measurement of the density of sea water in situ as a func-tion of depth. In addition to seawater density profiles, the method allows generating data arrays on the distributions of the total and thermohaline vertical stability of water mass layers and the corresponding Väisälä-Brunt frequency profiles of thermohaline oscillations. Applications of the method for the con-struction of smart profiling algorithms, which significantly reduce the time of oceanographic stations, as well as synchronous profiling algorithms, are considered. The issues of optimization of processes of dy-namic measurements are investigated. The results of computer simulation are presented.

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Mansoor, Abdul. « Bearing the Brunt : The Effect of Terrorism on the Foreign Direct Investment in South Asian Association for Regional Cooperation (SAARC) Nations ». University of Wah Journal of Social Sciences 5, n^o 1 (8 juin 2022) : 167–86. http://dx.doi.org/10.56220/uwjss2022/0501/10.

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The current state of terrorism has posed serious challenges to macroeconomic environment stability by causing the dislodgment of foreign direct investment (FDI). This study aims to find the impact of terrorism along with other important policy variables such as FDI, Terrorism, law and order, Tariff, and Government regulation in the SAARC member nations, namely, Bangladesh, Bhutan, India, Nepal, Pakistan, Afghanistan, Maldives, and Sri Lanka. Utilizing a panel econometric estimation model on annual data from 1990-2019, the results of the study show a significant negative impact of terrorism and law and order situations in the SAARC countries. Whereas the magnitude of FDI is technology-driven in Pakistan, Bangladesh, and Afghanistan where there is a deep focus on the physical structural transformation. This empirically establishes the fact that terrorism is a serious threat to FDI and economic growth for the economies in this region. Key Words: FDI; Terrorism; Tariff, SAARC

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THUSI, Xolani, et Mandisi MATYANA. « PROFESSIONALISATION AS A TOOL TO STABILISE PUBLIC FINANCES : SOUTH AFRICAN LOCAL GOVERNMENT PERSPECTIVE ». Journal of Public Administration, Finance and Law 31 (2024) : 492–506. http://dx.doi.org/10.47743/jopafl-2024-31-35.

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Local government is closer to the citizens and serves as a vehicle for democracy and service delivery in South Africa. Municipalities in South Africa face a significant challenge when it comes to managing public finances to provide effective and efficient services. Corruption, financial mismanagement, wasteful and fruitless expenditure, irregular expenditures, governance failure, lack of consequence, management, insufficient skills, and capacity are among the challenges. These factors have delivered an enormous setback to service delivery, and citizens are bearing the brunt of the burden. The most difficult situation in local government is that municipalities' finances are deteriorating year after year. According to the Auditor General of South Africa (AGSA), "the audit outcomes were in a poor state at the end of the previous administration's term, and this state did not improve in 2021-22." It is critical for the local government sphere to promote financial sustainability because most South African citizens rely on government services because the economy no longer provides favourable economic benefits such as employment that citizens can leverage. The goal of this paper is to investigate how professionalising South African municipal public finances can help to stabilise public funds. The qualitative method was used, which included a review of secondary sources on the topic at hand.

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BILLANT, P., A. DELONCLE, J. M. CHOMAZ et P. OTHEGUY. « Zigzag instability of vortex pairs in stratified and rotating fluids. Part 2. Analytical and numerical analyses. » Journal of Fluid Mechanics 660 (21 juillet 2010) : 396–429. http://dx.doi.org/10.1017/s002211201000282x.

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The three-dimensional stability of vertical vortex pairs in stratified and rotating fluids is investigated using the analytical approach established in Part 1 and the predictions are compared to the results of previous direct numerical stability analyses for pairs of co-rotating equal-strength Lamb–Oseen vortices and to new numerical analyses for equal-strength counter-rotating vortex pairs. A very good agreement between theoretical and numerical results is generally found, thereby providing a comprehensive description of the zigzag instability. Co-rotating and counter-rotating vortex pairs are most unstable to the zigzag instability when the Froude number Fh = Γ/(2πR2N) (where Γ is the vortex circulation, R the vortex radius and N the Brunt–Väisälä frequency) is lower than unity independently of the Rossby number Ro = Γ/(4πR2Ωb) (Ωb is the planetary rotation rate). In this range, the maximum growth rate is proportional to the strain Γ/(2πb2) (b is the separation distance between the vortices) and is almost independent of Fh and Ro. The most amplified wavelength scales like Fhb when the Rossby number is large and like Fhb/|Ro| when |Ro| ≪ 1, in agreement with previous results. While the zigzag instability always bends equal-strength co-rotating vortex pairs in a symmetric way, the instability is only quasi-antisymmetric for finite Ro for equal-strength counter-rotating vortex pairs because the cyclonic vortex is less bent than the anticyclonic vortex. The theory is less accurate for co-rotating vortex pairs around Ro ≈ −2 because the bending waves rotate very slowly for long wavelength. The discrepancy can be fully resolved by taking into account higher-order three-dimensional effects.When Fh is increased above unity, the growth rate of the zigzag instability is strongly reduced because the bending waves of each vortex are damped by a critical layer at the radius where the angular velocity of the vortex is equal to the Brunt–Väisälä frequency. The zigzag instability, however, continues to exist and is dominant up to a critical Froude number, which mostly depends on the Rossby number. Above this threshold, equal-strength co-rotating vortex pairs are stable with respect to long-wavelength bending disturbances whereas equal-strength counter-rotating vortex pairs become unstable to a quasi-symmetric instability resembling the Crow instability in homogeneous fluids. However, its growth rate is lower than in homogeneous fluids because of the damping by the critical layer. The structure of the critical layer obtained in the computations is in excellent agreement with the theoretical solution.Physically, the different stability properties of vortex pairs in stratified and rotating fluids compared to homogeneous fluids are shown to come from the reversal of the direction of the self-induced motion of bent vortices.

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Тэргэл, Ш., Г. Баясгалан et С. Эрдэнэсүх. « Шороон шуурга болох үеийн агаар мандлын тогтворшлын параметрүүд ». Geographical Issues 20, n^o 1 (1 mai 2020) : 64–76. http://dx.doi.org/10.22353/.v20i1.878.

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Although many studies investigated the spatial distribution, duration and frequency of dust storm in Mongolia, there are a few studies were undertaken it in terms of physical process that underpinned the current study on the subject. The study examines whether it is possible to acquire the distribution of dust storms in a relatively simple way by estimating the atmospheric stability during a dust storm. In order to achieve the main purpose of the study, the atmospheric stability parameters such as Brunt-Vaisala frequency (N), Gradient Richardson Number (Ri), and Eady Growth Rate (σ_E) were evaluated on 3 different cases of heavy dust storms which are occurred from March to May 2020, covered almost all over Mongolia, using reanalysis data produced by the European Centre for Medium-Range Weather Forecasts and radiosonde data from “Dalanzadgad” an aerological station in Dalanzadgad, Umnugovi. Differential equations of N, Ri, and σ_E are solved using the finite difference method. The results of the study revealed that both Ri and σ_E stability parameters can be identified well the distribution of dust storm, while a linear relationship between a dust storm and N as well as the buoyancy component of Ri was not found. In addition, in the study, a simplified method to compute the Ri parameter based on power function was acquired by the wind shear. It was difficult to make statistical analyses in the results of the study based on few cases between March and May in 2020. Therefore, it is necessary to consider more cases in the future, to undertake statistical analysis.

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Pieri, Alexandre B., F. S. Godeferd, C. Cambon et A. Salhi. « Non-geostrophic instabilities of an equilibrium baroclinic state ». Journal of Fluid Mechanics 734 (14 octobre 2013) : 535–66. http://dx.doi.org/10.1017/jfm.2013.478.

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AbstractWe consider non-geostrophic homogeneous baroclinic turbulence without solid boundaries, and we focus on its energetics and dynamics. The homogeneous turbulent flow is therefore submitted to both uniform vertical shear $S$ and stable vertical stratification, parametrized by the Brunt–Väisälä frequency $N$, and placed in a rotating frame with Coriolis frequency $f$. Direct numerical simulations show that the threshold of baroclinic instability growth depends mostly on two dimensionless numbers, the gradient Richardson number $\mathit{Ri}= {N}^{2} / {S}^{2} $ and the Rossby number $\mathit{Ro}= S/ f$, whereas linear theory predicts a threshold that depends only on $\mathit{Ri}$. At high Rossby numbers the nonlinear limit is found to be $\mathit{Ri}= 0. 2$, while in the limit of low $\mathit{Ro}$ the linear stability bound $\mathit{Ri}= 1$ is recovered. We also express the stability results in terms of background potential vorticity, which is an important quantity in baroclinic flows. We show that the linear symmetric instability occurs from the presence of negative background potential vorticity. The possibility of simultaneous existence of symmetric and baroclinic instabilities is also investigated. The dominance of symmetric instability over baroclinic instability for $\mathit{Ri}\ll 1$ is confirmed by our direct numerical simulations, and we provide an improved understanding of the dynamics of the flow by exploring the details of energy transfers for moderate Richardson numbers.

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LE DIZÈS, S., et X. RIEDINGER. « The strato-rotational instability of Taylor–Couette and Keplerian flows ». Journal of Fluid Mechanics 660 (23 août 2010) : 147–61. http://dx.doi.org/10.1017/s0022112010002624.

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The linear inviscid stability of two families of centrifugally stable rotating flows in a stably stratified fluid of constant Brunt–Väisälä frequency N is analysed by using numerical and asymptotic methods. Both Taylor–Couette and Keplerian angular velocity profiles ΩTC = (1 − μ)/r2 + μ and ΩK = (1 − λ)/r2 + λ/r3/2 are considered between r = 1 (inner boundary) and r = d > 1 (outer boundary, or without boundary if d = ∞). The stability properties are obtained for flow parameters λ and μ ranging from 0 to +∞, and different values of d and N. The effect of the gap size is analysed first. By considering the potential flow (λ = μ = 0), we show how the instability associated with a mechanism of resonance for finite-gap changes into a radiative instability when d → ∞. Numerical results are compared with large axial wavenumber results and a very good agreement is obtained. For infinite gap (d = ∞), we show that the most unstable modes are obtained for large values of the azimuthal wavenumber for all λ and μ. We demonstrate that their properties can be captured by performing a local analysis near the inner cylinder in the limit of both large azimuthal and axial wavenumbers. The effect of the stratification is also analysed. We show that decreasing N is stabilizing. An asymptotic analysis for small N is also performed and shown to capture the properties of the most unstable mode of the potential flow in this limit.

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Miljkovic, Ljubomir, et Dragana Trnavac. « MACROECONOMIC IMPACT OF THE RUSSIA AND UKRAINE CONFLICT ON THE GROSS DOMESTIC PRODUCT OF SERBIA ». MEST Journal 11, n^o 2 (12 juillet 2023) : 46. http://dx.doi.org/10.12709/mest.11.11.02.06.

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<em>The conflict between Russia and Ukraine comes at a time when countries worldwide are still recovering from the destabilizing effects of the global COVID-19 pandemic, which has caused deep economic regression, significant loss of productivity, worsening inequality, planetary pressures, and in some cases, security challenges. The economic impact reverberated through multiple channels, including commodity and financial markets, trade, and migration links. At the same time, rising inflation is increasingly reducing the purchasing power of consumers. Such market tensions will reduce economic activity while accelerating inflation with the appearance of stagflation, where a general jump in prices and a decline in production characterize the state of the economy. The direct effects of this conflict lead to macroeconomic instability. This conflict causes a jump in global prices and affects economies worldwide. One can expect that developing markets and countries in the European region, especially the Western Balkan, will bear the brunt. On the side of macroeconomic stability, the effects differ depending on their exposure to the market of Russia and Ukraine. The Republic of Serbia does not have significant, direct trade ties with Ukraine, but the conflict faced Serbia with new challenges, considering Serbia's energy dependence on Russia, as well as the agricultural sector, where products, primarily fruit, have a significant share in total exports to Russia. The crucial question is in which direction the conflict will affect the macroeconomic stability of the Republic of Serbia.</em>

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Sherstyankin, P. P., et L. N. Kuimova. « Vertical stability and the Brunt-Väisäla frequency of deep natural waters by the example of Lake Baikal, Lake Tanganyika, and the World Ocean ». Doklady Earth Sciences 429, n^o 2 (décembre 2009) : 1553–58. http://dx.doi.org/10.1134/s1028334x09090293.

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Park, Junho, Paul Billant, Jong-Jin Baik et Jaemyeong Mango Seo. « Competition between the centrifugal and strato-rotational instabilities in the stratified Taylor–Couette flow ». Journal of Fluid Mechanics 840 (6 février 2018) : 5–24. http://dx.doi.org/10.1017/jfm.2018.15.

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The stably stratified Taylor–Couette flow is investigated experimentally and numerically through linear stability analysis. In the experiments, the stability threshold and flow regimes have been mapped over the ranges of outer and inner Reynolds numbers: $-2000<Re_{o}<2000$ and $0<Re_{i}<3000$, for the radius ratio $r_{i}/r_{o}=0.9$ and the Brunt–Väisälä frequency $N\approx 3.2~\text{rad}~\text{s}^{-1}$. The corresponding Froude numbers $F_{o}$ and $F_{i}$ are always much smaller than unity. Depending on $Re_{o}$ (or equivalently on the angular velocity ratio $\unicode[STIX]{x1D707}=\unicode[STIX]{x1D6FA}_{o}/\unicode[STIX]{x1D6FA}_{i}$), three different regimes have been identified above instability onset: a weakly non-axisymmetric mode with low azimuthal wavenumber $m=O(1)$ is observed for $Re_{o}<0$ ($\unicode[STIX]{x1D707}<0$), a highly non-axisymmetric mode with $m\sim 12$ occurs for $Re_{o}>840$ ($\unicode[STIX]{x1D707}>0.57$) while both modes are present simultaneously in the lower and upper parts of the flow for $0\leqslant Re_{o}\leqslant 840$ ($0\leqslant \unicode[STIX]{x1D707}\leqslant 0.57$). The destabilization of these primary modes and the transition to turbulence as $Re_{i}$ increases have been also studied. The linear stability analysis proves that the weakly non-axisymmetric mode is due to the centrifugal instability while the highly non-axisymmetric mode comes from the strato-rotational instability. These two instabilities can be clearly distinguished because of their distinct dominant azimuthal wavenumber and frequency, in agreement with the recent results of Park et al. (J. Fluid Mech., vol. 822, 2017, pp. 80–108). The stability threshold and the characteristics of the primary modes observed in the experiments are in very good agreement with the numerical predictions. Moreover, we show that the centrifugal and strato-rotational instabilities are observed simultaneously for $0\leqslant Re_{o}\leqslant 840$ in the lower and upper parts of the flow, respectively, because of the variations of the local Reynolds numbers along the vertical due to the salinity gradient.

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Lischer, Sarah Kenyon. « The Global Refugee Crisis : Regional Destabilization & ; Humanitarian Protection ». Daedalus 146, n^o 4 (octobre 2017) : 85–97. http://dx.doi.org/10.1162/daed_a_00461.

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In addition to being a tragic output of civil war, large-scale displacement crises often become enmeshed in the politics, security, and economics of the conflict. Refugee and internally displaced populations thus exacerbate concerns about regional destabilization. The Syrian refugee crisis, for example, is deeply entwined with civil and international conflict. Neighboring host states of Turkey, Jordan, and Lebanon bear the brunt of the crisis, while European states seek to prevent further encroachment by Middle Eastern asylum seekers. Policy-makers often mistakenly view host state security and refugee security as unrelated–or even opposing–factors. In reality, refugee protection and state stability are linked together; undermining one factor weakens the other. Policies to protect refugees, both physically and legally, reduce potential threats from the crisis and bolster state security. In general, risks of conflict are higher when refugees live in oppressive settings, lack legal income-generation options, and are denied education for their youth. The dangers related to the global refugee crisis interact with many other threats that emanate from civil wars and weak states, such as fragile governments, rebel and terrorist group activity, and religious or ethnic fragmentation.

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Lago-Peñas, Santiago, Xoaquín Fernández-Leiceaga et Alberto Vaquero-García. « Spanish fiscal decentralization : A successful (but still unfinished) process ». Environment and Planning C : Politics and Space 35, n^o 8 (20 avril 2017) : 1509–25. http://dx.doi.org/10.1177/2399654417704663.

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Both fiscal and political decentralization have been a worldwide trend in recent decades. Spain is probably the best example of this evolution, insofar as it became one of the most decentralized countries in the world in just over three decades, departing for a highly centralized institutional framework. This paper aims to conduct a detailed analysis of this process, focusing on the fiscal aspects. While we show the successful aspects, we also point out its shortcomings and failures. Those lessons from Spain can be very useful for centralized or low decentralized countries involved in designing institutional reforms to become more decentralized. Our second aim is to review the way in which the Spanish sub-central levels of government have responded to the so-called “Great recession”. Again, Spain is a good laboratory for what actually works and what does not in the area of stability and fiscal sustainability in decentralized states, for two reasons. First, it is by far the country which has suffered the brunt of the economic and financial crisis most acutely among those with a federal structure. And second, subsequent different solutions for tackling the fiscal crisis of sub-central governments have been tested.

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Khamisu Madachi, Isah. « EXPLORING THE CHALLENGES OF DELAYED MARRIAGE AMONG YOUTH IN TARAUNI L.G.A. » SPECTRUM Journal of Social Science 1, n^o 1 (2024) : 7–14. http://dx.doi.org/10.61552/sjss.2024.01.002.

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The study “Exploring the Challenges of Delayed Marriage Among Youth in Tarauni L.G.A.” investigates the challenges of delayed marriage among youth, and its detrimental effects within the Tarauni Local Government Area, Kano, Nigeria. Marriage, an important social institution, traditionally symbolizes adult responsibilities and societal integration. However, in recent times, delayed marriage has become prevalent, leading to societal stigmatization and psychological distress among the youth. This research adopts a qualitative approach, utilizing in-depth interviews with eight respondents, including community leaders, religious figures, and unmarried individuals, to understand the implications of delayed marriage. The findings reveal several challenges faced by individuals, such as increased vulnerability to masturbation and premarital sex, inferiority complex, risk of remaining childless, interpersonal relationship difficulties, and reduced fertility. Particularly, women bear the brunt of societal stigma and mental health instability. Respondents also revealed severe consequences like illegal abortions and the birth of children out of wedlock, which contravene cultural norms and exacerbate emotional and health issues. This research contributes to the broader discourse on the socio-economic and impacts of delayed marriage, offering insights for policymakers and community stakeholders to develop strategies that support youth in achieving marital and societal stability.

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Yuter, Sandra E., David A. Stark, Justin A. Crouch, M. Jordan Payne et Brian A. Colle. « The Impact of Varying Environmental Conditions on the Spatial and Temporal Patterns of Orographic Precipitation over the Pacific Northwest near Portland, Oregon ». Journal of Hydrometeorology 12, n^o 3 (1 juin 2011) : 329–51. http://dx.doi.org/10.1175/2010jhm1239.1.

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Abstract Operational radar data from three winter seasons (2003–06) in Portland, Oregon, in the U.S. Pacific Northwest are used to describe how orographic precipitation varies with cross-barrier wind speed, 0°C level height, and stability over the moderately wide (~50-km half-width) Cascade Mountain Range. Orographic enhancement is specified in terms of location, frequency, and relative intensity of the reflectivity (precipitation field). The typical storm for the region, as defined by the 25th to 75th percentile characteristics, is compared to storms with <25th and >75th percentile characteristics for a given variable. About half of Portland-region storms have a low-level wind direction within a relatively narrow azimuth range. This subset of storms is used to examine the sensitivity of orographic enhancement relative to other environmental variables. Cross-barrier wind speed has a stronger role in determining the magnitude of precipitation frequency than either 0°C level or stability. Cross-barrier wind speed and 0°C level height have separate but comparable roles in determining the frequency of relatively heavier precipitation. The increase in precipitation frequency with stronger cross-barrier wind speed is partially attributed to the higher occurrence of intermittent convective cells intersecting the slope. The area where inferred riming growth occurs over local peaks on the windward slope broadens upslope as the 0°C level height increases. In the Portland region, variations in the squared moist Brunt–Väisälä frequency yield smaller differences in the pattern and intensity of precipitation enhancement than either cross-barrier wind speed or 0°C level height.

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Ziulu, Emmanuel Nkete, Gratien Bambanota Mokonzi, Paul Masimango Vitamara et Augustin Awongi Issoy. « Ampleur des Disparités dans la Scolarisation Primaire et Secondaire en République Démocratique du Congo de 2006 à 2018 ». European Scientific Journal, ESJ 19, n^o 1 (31 janvier 2023) : 163. http://dx.doi.org/10.19044/esj.2023.v19n1p163.

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Cette étude analyse l’ampleur des disparités dans la scolarisation primaire et secondaire en République Démocratique du Congo au cours de la période de 2006 à 2018. Sur la base des données statistiques tirées des annuaires statistiques produits au niveau national, l’étude examine les disparités axées sur le taux de scolarisation et l’indice de parité. Elle analyse les disparités non seulement de manière synchronique, au cours de chaque année, mais aussi diachroniquement, en faisant ressortir leur évolution temporelle. L’étude exploite, pour cela, les indices de dispersion (l’étendue de variation totale et le coefficient de variation), le scattergram et le coefficient de corrélation Rho de Spearman. Elle montre que si la scolarisation primaire est quasi-universelle dans la plupart des provinces, la province du Katanga est la moins scolarisée de la RDC. Cependant, les disparités sont très faibles entre les provinces, entre la zone de conflit et celle de stabilité, entre les filles et les garçons. En revanche, en dehors de la ville de Kinshasa et de la province du Bandundu, aucune autre province n’a atteint au niveau du secondaire le taux de scolarisation de 50%. Les disparités de la scolarisation secondaire entre les provinces, entre la zone de conflit et la zone de stabilité, de même qu’entre les genres sont très prononcées et significatives. Par ailleurs, les disparités provinciales se sont faiblement réduites pour le taux brut de scolarisation aussi bien au primaire qu’au secondaire. Par contre, elles se sont significativement réduites pour l’indice de parité fille-garçon. Pour réduire les disparités, il s’avère important d’accroître l’offre de l’éducation au niveau du primaire et surtout au niveau du secondaire dans les provinces les plus défavorisées. This study analyzes the extent of disparities in primary and secondary schooling in the Democratic Republic of Congo during the period from 2006 to 2018. Based on statistical data drawn from statistical yearbooks produced at the national level, the study examines the disparities based on the enrollment rate and the parity index. It analyzes the disparities not only synchronically, during each year, but also diachronically, highlighting their temporal evolution. For this, the study uses the dispersion indices (the total range of variation and the coefficient of variation), the scattergram and Spearman's Rho correlation coefficient. It shows that while primary education is almost universal in most provinces, the province of Katanga is the least educated in the DRC. However, the disparities are very low between the provinces, between the conflict zone and the stability zone, between girls and boys. On the other hand, apart from the city of Kinshasa and the province of Bandundu, no other province has reached the secondary school enrollment rate of 50%. Disparities in secondary education between provinces, between the conflict zone and the stability zone, as well as between genders are very pronounced and significant. Furthermore, provincial disparities have narrowed slightly for the gross enrollment rate at both primary and secondary level. On the other hand, they are significantly reduced for the girl-boy parity index. To reduce disparities, it is important to increase the supply of education at the primary level and especially at the secondary level in the most disadvantaged provinces.

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Leukauf, Daniel, Alexander Gohm et Mathias W. Rotach. « Quantifying horizontal and vertical tracer mass fluxes in an idealized valley during daytime ». Atmospheric Chemistry and Physics 16, n^o 20 (21 octobre 2016) : 13049–66. http://dx.doi.org/10.5194/acp-16-13049-2016.

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Abstract. The transport and mixing of pollution during the daytime evolution of a valley boundary layer is studied in an idealized way. The goal is to quantify horizontal and vertical tracer mass fluxes between four different valley volumes: the convective boundary layer, the slope wind layer, the stable core, and the atmosphere above the valley. For this purpose, large eddy simulations (LES) are conducted with the Weather Research and Forecasting (WRF) model for a quasi-two-dimensional valley. The valley geometry consists of two slopes with constant slope angle and is homogeneous in the along-valley direction. The surface sensible heat flux is horizontally homogeneous and prescribed by a sine function. The initial sounding is characterized by an atmosphere at rest and a constant Brunt–Väisälä frequency. Various experiments are conducted for different combinations of surface heating amplitudes and initial stability conditions. A passive tracer is released with an arbitrary but constant rate at the valley floor and resulting tracer mass fluxes are evaluated between the aforementioned volumes.As a result of the surface heating, a convective boundary layer is established in the lower part of the valley with a stable layer on top – the so-called stable core. The height of the slope wind layer, as well as the wind speed within, decreases with height due to the vertically increasing stability. Hence, the mass flux within the slope wind layer decreases with height as well. Due to mass continuity, this along-slope mass flux convergence leads to a partial redirection of the flow from the slope wind layer towards the valley centre and the formation of a horizontal intrusion above the convective boundary layer. This intrusion is associated with a transport of tracer mass from the slope wind layer towards the valley centre. A strong static stability and/or weak forcing lead to large tracer mass fluxes associated with this phenomenon. The total export of tracer mass out of the valley atmosphere increases with decreasing stability and increasing forcing. The effects of initial stability and forcing can be combined to a single parameter, the breakup parameter B. An analytical function is presented that describes the exponential decrease of the percentage of exported tracer mass with increasing B. This study is limited by the idealization of the terrain shape, stratification, and forcing, but quantifies transport processes for a large range of forcing amplitudes and atmospheric stability.

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Mahdinia, Mani, Pedram Hassanzadeh, Philip S. Marcus et Chung-Hsiang Jiang. « Stability of three-dimensional Gaussian vortices in an unbounded, rotating, vertically stratified, Boussinesq flow : linear analysis ». Journal of Fluid Mechanics 824 (5 juillet 2017) : 97–134. http://dx.doi.org/10.1017/jfm.2017.303.

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The linear stability of three-dimensional vortices in rotating, stratified flows has been studied by analysing the non-hydrostatic inviscid Boussinesq equations. We have focused on a widely used model of geophysical and astrophysical vortices, which assumes an axisymmetric Gaussian structure for pressure anomalies in the horizontal and vertical directions. For a range of Rossby numbers ($-0.5<Ro<0.5$) and Burger numbers ($0.02<Bu<2.3$) relevant to observed long-lived vortices, the growth rate and spatial structure of the most unstable eigenmodes have been numerically calculated and presented as a function of $Ro{-}Bu$. We have found neutrally stable vortices only over a small region of the $Ro{-}Bu$ parameter space: cyclones with $Ro\sim 0.02{-}0.05$ and $Bu\sim 0.85{-}0.95$. However, we have also found that anticyclones in general have slower growth rates compared to cyclones. In particular, the growth rate of the most unstable eigenmode for anticyclones in a large region of the parameter space (e.g. $Ro<0$ and $0.5\lesssim Bu\lesssim 1.3$) is slower than 50 turnaround times of the vortex (which often corresponds to several years for ocean eddies). For cyclones, the region with such slow growth rates is confined to $0<Ro<0.1$ and $0.5\lesssim Bu\lesssim 1.3$. While most calculations have been done for $f/\bar{N}=0.1$ (where $f$ and $\bar{N}$ are the Coriolis and background Brunt–Väisälä frequencies), we have numerically verified and explained analytically, using non-dimensionalized equations, the insensitivity of the results to reducing $f/\bar{N}$ to the more ocean-relevant value of 0.01. The results of our stability analysis of Gaussian vortices both support and contradict the findings of earlier studies with QG or multilayer models or with other families of vortices. The results of this paper provide a stepping stone to study the more complicated problems of the stability of geophysical (e.g. those in the atmospheres of giant planets) and astrophysical vortices (in accretion disks).

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Qian, Wenhao, Tao Song, Mao Ye, Haiyan Zhang, Chun Feng, Guolin Lu et Xiaoyu Huang. « Graphene Oxide/Ferrocene-Containing Polymer/Gold Nanoparticle Triple Nanocomposite ». Nanomaterials 9, n^o 2 (25 février 2019) : 310. http://dx.doi.org/10.3390/nano9020310.

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A facile strategy to prepare GO-based nanocomposites with both gold nanoparticles (AuNPs) and ferrocene (Fc) moieties was developed. The surface of GO was modified with PFcMAss homopolymer by surface-initiated atom transfer radical polymerization of a new methacrylate monomer of 2-((2-(methacryloyloxy)ethyl)disulfanyl)ethyl ferrocene-carboxylate (FcMAss), consisting of disulfide as an anchoring group for stabilizing AuNPs and Fc group as an additional functionality. AuNPs with an average diameter of about 4.1 nm were formed in situ on the surface of PFcMAss-decorated GO (GO-PFcMAss) via Brust-Schiffrin method to give GO-PFcMAss-AuNPs multifunctional nanocomposites bearing GO, AuNPs and Fc groups. The obtained nanocomposites were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Since disulfide-containing polymers, rather than the commonly used thiol-containing compounds, were employed as ligands to stabilize AuNPs, much more stabilizing groups were attached onto the surface of GO, and thus more AuNPs were able to be introduced onto the surface of GO. Besides, polymeric chains on the surface of GO endowed GO-PFcMAss-AuNPs nanocomposites with excellent colloidal stability, and the usage of a disulfide group provides possibility to efficiently incorporate additional functionalities by easily modifying structure of disulfide-based monomer.

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Moiseenko, K. B. « On the role of the stratosphere in the process of overflow of mesoscale mountains ». Annales Geophysicae 23, n^o 11 (21 décembre 2005) : 3407–17. http://dx.doi.org/10.5194/angeo-23-3407-2005.

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Abstract. A 2-D, two- and three-layer stratified airflow over a mountain of arbitrary shape is considered on the assumptions that upstream wind velocity and static stability within each layer are constant (Long's model). The stratosphere is simulated by an infinitely deep upper layer with enhanced static stability. The analytical solution for the stream function, as well as first (linear) and second order approximations to the wave drag, are obtained in hydrostatic limit N1L/U0→∞, where N1 is the Brunt-Väsälä frequency in the troposphere, L is a characteristic length of the obstacle, and U0 is upstream velocity. The results of numerical computations show the principal role of long waves in the process of interaction between the model layers for a typical mesoscale mountains for which the hydrostatic approximation proves valid in a wide range of flow parameters, in accordance with the earlier conclusions of Klemp and Lilly (1975). Partial reflection of wave energy from the tropopause produces strong influence on the value of wave drag for typical middle and upper tropospheric lapse rates, leading to a quasi-periodic dependance of wave drag on a reduced frequency ( is tropopause height) in the troposphere. The flow seems to be statically unstable for k≥2 for sufficiently large obstacles (whose height exceeds 1 km). In this case, vast regions of rotor motions and strong turbulence are predicted from model calculations in the middle troposphere and the lower stratosphere. The model calculations also point to a testify for possible important role of nonlinear effects associated with finite height of the mountain on the conditions of wave drag amplification in the process of overflow of real mountains.

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Bismans, Francis. « L'Euro-dilemme ». La Revue Internationale des Économistes de Langue Française 4, n^o 1 (30 juin 2019) : 54–64. http://dx.doi.org/10.18559/rielf.2019.1.4.

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L'euro est une « monnaie politique ». S'il en est ainsi, c'est parce que sa naissance est directement liée à un compromis franco-allemand. Plus précisément, la France de Mitterrand acceptait la réunification de l'Allemagne moyennant l'accord de cette dernière sur l'introduction de l'euro. Ce que le président français avait en tête, c'était aussi, bien sûr, d'arrimer solidement la nouvelle Allemagne à l'Europe. Mais ce n'était pas tout, car dans le marchandage, celle-ci obtenait de surcroît deux « compensations » significatives : 1.Elle imposait une discipline budgétaire stricte à tous les pays participant à l'euro via les deux fameux critères dits de convergence : d'une part, le déficit public devait être inférieur à 3% du Produit Intérieur Brut (PIB); d'autre part, l'endettement public total ne pouvait excéder 60% de ce même PIB. 2.Une banque centrale était créée (la BCE ou Banque Centrale Européenne), totalement indépendante des gouvernements et avec comme seul objectif la stabilité des prix, définie comme une inflation égale à 2% annuellement. Dans ces conditions, il n'est pas étonnant que pratiquement rien n'ait été retenu de ce que l'économie politique - pour reprendre un terme un peu vieilli - avait produit de mieux en matière d'analyse des processus d'unification monétaire : la théorie des zones monétaires optimales et celle de la politique macroéconomique.

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Candelier, Julien, Stéphane Le Dizès et Christophe Millet. « Shear instability in a stratified fluid when shear and stratification are not aligned ». Journal of Fluid Mechanics 685 (13 septembre 2011) : 191–201. http://dx.doi.org/10.1017/jfm.2011.306.

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AbstractThe effect of an inclination angle of the shear with respect to the stratification on the linear properties of the shear instability is examined in the work. For this purpose, we consider a two-dimensional plane Bickley jet of width $L$ and maximum velocity ${U}_{0} $ in a stably stratified fluid of constant Brunt–Väisälä frequency $N$ in an inviscid and Boussinesq framework. The plane of the jet is assumed to be inclined with an angle $\theta $ with respect to the vertical direction of stratification. The stability analysis is performed using both numerical and theoretical methods for all the values of $\theta $ and Froude number $F= {U}_{0} / (LN)$. We first obtain that the most unstable mode is always a two-dimensional Kelvin–Helmholtz (KH) sinuous mode. The condition of stability based on the Richardson number $Ri\gt 1/ 4$, which reads here $F\lt 3 \sqrt{3} / 2$, is recovered for $\theta = 0$. But when $\theta \not = 0$, that is, when the directions of shear and stratification are not perfectly aligned, the Bickley jet is found to be unstable for all Froude numbers. We show that two modes are involved in the stability properties. We demonstrate that when $F$ is decreased below $3 \sqrt{3} / 2$, there is a ‘jump’ from one two-dimensional sinuous mode to another. For small Froude numbers, we show that the shear instability of the inclined jet is similar to that of a horizontal jet but with a ‘horizontal’ length scale ${L}_{h} = L/ \sin \theta $. In this regime, the characteristics (oscillation frequency, growth rate, wavenumber) of the most unstable mode are found to be proportional to $\sin \theta $. For large Froude numbers, the shear instability of the inclined jet is similar to that of a vertical jet with the same scales but with a different Froude number, ${F}_{v} = F/ \hspace *{-.1pc}\cos \theta $. It is argued that these results could be valid for any type of shear flow.

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Bouvier, Clément, Daan van den Broek, Madeleine Ekblom et Victoria A. Sinclair. « Analytical and adaptable initial conditions for dry and moist baroclinic waves in the global hydrostatic model OpenIFS (CY43R3) ». Geoscientific Model Development 17, n^o 7 (16 avril 2024) : 2961–86. http://dx.doi.org/10.5194/gmd-17-2961-2024.

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Abstract. This article presents a description of an analytical, stable, and flexible initial background state for both dry and moist baroclinic wave simulation on an aquaplanet in order to test the dynamical core of numerical weather prediction models and study the dynamics and evolution of extratropical cyclones. The initial background state is derived from an analytical zonal wind speed field, or jet structure, and the hydrostatic primitive equations for moist adiabatic and frictionless flow in spherical coordinates. A baroclinic wave can develop if a perturbation is added to the zonal wind speed field. This new baroclinic wave configuration has been implemented in the Open Integrated Forecasting System (OpenIFS) CY43R3, a global numerical weather prediction model developed by the European Centre for Medium-Range Weather Forecasts. In total, seven parameters can be used to control the generation of the initial background state and hence the development of the baroclinic waves in the OpenIFS configuration file: the jet's width, the jet's height, the maximum zonal mean wind speed of the jet, the horizontal mean of the surface virtual temperature, the surface relative humidity, the lapse rate, and the surface roughness. Nine dry and nine moist initial background states have been generated to test their stability without perturbations. The meteorological stability of the initial states is investigated by examining the spatial distributions of the equivalent potential temperature, the absolute vorticity, and the Brunt–Väisälä frequency. Moreover, the root mean square error (RMSE) of the zonal wind speed has been computed to assess their numerical stability. Finally, six dry and six moist initial background state have been used with an unbalanced perturbation to ensure that the baroclinic life cycles that develop are physically realistic. The resulting baroclinic wave is shown to be sensitive to the jet's width. This configuration for baroclinic wave simulations will be used to create a large ensemble of baroclinic life cycles to study how extratropical cyclones may evolve in the future.

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BILLANT, PAUL, et JEAN-MARC CHOMAZ. « Three-dimensional stability of a vertical columnar vortex pair in a stratified fluid ». Journal of Fluid Mechanics 419 (25 septembre 2000) : 65–91. http://dx.doi.org/10.1017/s0022112000001178.

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This paper investigates the three-dimensional stability of a Lamb–Chaplygin columnar vertical vortex pair as a function of the vertical wavenumber kz, horizontal Froude number Fh, Reynolds number Re and Schmidt number Sc. The horizontal Froude number Fh (Fh = U/NR, where U is the dipole travelling velocity, R the dipole radius and N the Brunt–Väisälä frequency) is varied in the range [0.033, ∞[ and three set of Reynolds-Schmidt numbers are investigated: {Re = 10 000, Sc = 1}, Re = 1000, Sc = 1}, {Re = 200, Sc = 637}. In the whole range of Fh and Re, the dominant mode is always antisymmetric with respect to the middle plane between the vortices but its physical nature and properties change when Fh is varied. An elliptic instability prevails for Fh > 0.25, independently of the Reynolds number. It manifests itself by the bending of each vortex core in the opposite direction to the vortex periphery. The growth rate of the elliptic instability is reduced by stratification effects but its spatial structure is almost unaffected. In the range 0.2 < Fh < 0.25, a continuous transition occurs from the elliptic instability to a different instability called zigzag instability. The transitional range Fhc = 0.2–0.25 is in good agreement with the value Fh = 0.22 at which the elliptic instability of an infinite uniform vortex is suppressed by the stratification. The zigzag instability dominates for Fh [les ] 0.2 and corresponds to a vertically modulated bending and twisting of the whole vortex pair. The experimental evidence for this zigzag instability in a strongly stratified fluid reported in the first part of this study (Billant & Chomaz 2000a) are therefore confirmed and extended. The numerically calculated wavelength and growth rate for low Reynolds number compare well with experimental measurements.The present numerical stability analysis fully agrees with the inviscid asymptotic analysis carried out in the second part of this investigation (Billant & Chomaz 2000b) for small Froude number Fh and long wavelength. This confirms that the zigzag instability is related to the breaking of translational and rotational invariances. As predicted, the growth rate of the zigzag instability is observed to be self-similar with respect to the variable Fhkz, implying that the maximum growth rate is independent of Fh while the most amplified dimensional wavenumber varies with N/U. The numerically computed eigenmode and dispersion relation are in striking agreement with the analytical results.

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Chen, Ze, Yufang Tian et Daren Lü. « Turbulence Parameters in the Troposphere—Lower Stratosphere Observed by Beijing MST Radar ». Remote Sensing 14, n^o 4 (15 février 2022) : 947. http://dx.doi.org/10.3390/rs14040947.

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Based on three years of data observed by Beijing MST radar between 2012 and 2014, this study gives the distribution characteristics of the turbulence energy dissipation rate ε and the vertical turbulence diffusion coefficient Kz in the troposphere–lower stratosphere over Beijing based on the spectral width method. Additionally, from the perspective of atmospheric stability expressed by the Brunt–Väisälä frequency N and the gradient Richardson number Ri, the seasonal variations of turbulence parameters at different heights are analyzed. The results show that ε (Kz) is distributed in the range of 10−4 to 10−1.5 m2 s−3 (100 to 101 m2 s−1), with median values of 10−3 to 10−2.5 m2 s−3 (100.4 to 100.75 m2 s−1). In the height range of −1 km to about +1 km from the tropopause, the profiles of ε have no noticeable seasonal difference from October to next May, and ε increases sharply with the slope of linear regression ∇log(ε)/∇h = 0.25 (m2 s−3) km−1. The seasonal variation of turbulence parameters has noticeable differences at different atmospheric layers. Furthermore, the atmospheric static/dynamic stability and turbulence intensity are the influencing factors of turbulence parameters. From the tropopause to 14 km, the values of ε and N both have a peak from October to next May, and this peak layer disappears from July to August. Within the range of 2–4 km from the ground, Kz and the frequency of Ri < 0.25 have a significant positive correlation, with a Pearson correlation coefficient greater than 0.5, and both have large value layers in March to May and September to October. The turbulence intensity above 15 km is likely the main factor affecting the turbulence dissipation rate and vertical turbulence diffusion coefficient.

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Gong, Shaohua, Guotao Yang, Jiyao Xu, Xiao Liu et Qinzeng Li. « Gravity Wave Propagation from the Stratosphere into the Mesosphere Studied with Lidar, Meteor Radar, and TIMED/SABER ». Atmosphere 10, n^o 2 (16 février 2019) : 81. http://dx.doi.org/10.3390/atmos10020081.

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A low-frequency inertial atmospheric gravity wave (AGW) event was studied with lidar (40.5° N, 116° E), meteor radar (40.3° N, 116.2° E), and TIMED/SABER at Beijing on 30 May 2012. Lidar measurements showed that the atmospheric temperature structure was persistently perturbed by AGWs propagating upward from the stratosphere into the mesosphere (35–86 km). The dominant contribution was from the waves with vertical wavelengths λ z = 8 − 10 km and wave periods T ob = 6.6 ± 0.7 h . Simultaneous observations from a meteor radar illustrated that MLT horizontal winds were perturbed by waves propagating upward with an azimuth angle of θ = 247 ° , and the vertical wavelength ( λ z = 10 km ) and intrinsic period ( T in = 7.4 h ) of the dominant waves were inferred with the hodograph method. TIMED/SABER measurements illustrated that the vertical temperature profiles were also perturbed by waves with dominant vertical wavelength λ z = 6 − 9 km . Observations from three different instruments were compared, and it was found that signatures in the temperature perturbations and horizontal winds were induced by identical AGWs. According to these coordinated observation results, the horizontal wavelength and intrinsic phase speed were inferred to be ~560 km and ~21 m/s, respectively. Analyses of the Brunt-Väisälä frequency and potential energy illustrated that this persistent wave propagation had good static stability.

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Achatz, Ulrich, et Gerhard Schmitz. « Shear and Static Instability of Inertia–Gravity Wave Packets : Short-Term Modal and Nonmodal Growth ». Journal of the Atmospheric Sciences 63, n^o 2 (1 février 2006) : 397–413. http://dx.doi.org/10.1175/jas3636.1.

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Abstract The problem of nonmodal instabilities of inertia–gravity waves (IGW) in the middle atmosphere is addressed, within the framework of a Boussinesq model with realistic molecular viscosity and thermal diffusion, by singular-vector analysis of horizontally homogeneous vertical profiles of wind and buoyancy obtained from IGW packets at their statically least stable or most unstable horizontal location. Nonmodal growth is always found to be significantly stronger than that of normal modes, most notably at wave amplitudes below the static instability limit where normal-mode instability is very weak, whereas the energy gain between the optimal perturbation and singular vector after one Brunt–Väisälä period can be as large as two orders of magnitude. Among a multitude of rapidly growing singular vectors for this optimization time, small-scale (wavelengths of a few 100 m) perturbations propagating in the horizontal parallel to the IGW are most prominent. These parallel optimal perturbations are amplified by a roll mechanism, while transverse perturbations (with horizontal scales of a few kilometers) are to a large part subject to an Orr mechanism, both controlled by the transverse wind shear in the IGW at its statically least stable altitude, but further enhanced by reduced static stability. The elliptic polarization of the IGW leaves its traces in an additional impact of the roll mechanism via the parallel wind shear on the leading transverse optimal perturbation.

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Heinemann, Günther, Lukas Glaw et Sascha Willmes. « A Satellite-Based Climatology of Wind-Induced Surface Temperature Anomalies for the Antarctic ». Remote Sensing 11, n^o 13 (28 juin 2019) : 1539. http://dx.doi.org/10.3390/rs11131539.

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It is well-known that katabatic winds can be detected as warm signatures in the surface temperature over the slopes of the Antarctic ice sheets. For appropriate synoptic forcing and/or topographic channeling, katabatic surges occur, which result in warm signatures also over adjacent ice shelves. Moderate Resolution Imaging Spectroradiometer (MODIS) ice surface temperature (IST) data are used to detect warm signatures over the Antarctic for the winter periods 2002–2017. In addition, high-resolution (5 km) regional climate model data is used for the years of 2002 to 2016. We present a case study and a climatology of wind-induced IST anomalies for the Ross Ice Shelf and the eastern Weddell Sea. The IST anomaly distributions show maxima around 10–15K for the slopes, but values of more than 25K are also found. Katabatic surges represent a strong climatological signal with a mean warm anomaly of more than 5K on more than 120 days per winter for the Byrd Glacier and the Nimrod Glacier on the Ross Ice Shelf. The mean anomaly for the Brunt Ice Shelf is weaker, and exceeds 5K on about 70 days per winter. Model simulations of the IST are compared to the MODIS IST, and show a very good agreement. The model data show that the near-surface stability is a better measure for the response to the wind than the IST itself.

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Teng, Chenyu. « The Catering Industry under the Epidemic : A Case Study on Yum China ». Advances in Economics, Management and Political Sciences 30, n^o 1 (10 novembre 2023) : 110–17. http://dx.doi.org/10.54254/2754-1169/30/20231450.

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Since the outbreak of the epidemic in 2019, the world economy has been in a lot of trouble. Among the various sectors, the catering industry has borne the brunt of this unprecedented crisis, facing unparalleled challenges and setbacks. During this time, Yum China has handled the epidemic differently than other food service businesses and hasn't had any trouble with business because of it. The goal of this paper was to look at why Yum China can still be successful even though there is an epidemic. This paper demonstrated the reasons for Yum Chinas success in the epidemic. This paper finally found that Yum Chinas success in the epidemic had the following three reasons: At first, the high-quality digital platform enabled Yum China to take a leading position in the industry. In the future, Yum China will continue to increase its investment in digitalization and maintain its leading role. Secondly, the strong safety and health measures had ensured the safety and stability of Yum Chinas customer base during the epidemic. However, Yum China should pay more attention to food safety after the epidemic. Thirdly, Yum China's strong supply chain management had ensured the stable import of raw materials and cost control during the epidemic. Yum China should continue to strengthen its investment in supply chain management to consolidate strong supply chain advantages.

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CHATELLIER, Vincent, Jean-Marc CHAUMET et Thierry POUCH. « La pandémie de Covid-19, l’économie agricole internationale et les filières animales : le cas de la Chine, des États-Unis et de l’UE ». INRAE Productions Animales 35, n^o 1 (17 mai 2022) : 1–20. http://dx.doi.org/10.20870/productions-animales.2022.35.1.5376.

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La pandémie de Covid-19 est un évènement mondial majeur. Une réflexion sur les premières implications de cette crise sanitaire pour l’économie agricole mondiale et les filières animales est proposée dans le cadre de cet article. Les secteurs agricole et agroalimentaire ont été fortement concernés par cette crise dont les impacts économiques globaux (baisse de 3,5 % du produit intérieur brut mondial et de 5,3 % des échanges internationaux de marchandises entre 2019 et 2020) affectent le pouvoir d’achat des consommateurs finaux. Ils ont cependant bien résisté à la crise, tant au niveau de l’offre (stabilité ou légère croissance des volumes produits de viandes et de lait à l’échelle mondiale) que du commerce. En 2021, la forte hausse des prix internationaux des produits agricoles et la reprise rapide de la croissance économique, notamment dans les trois zones plus spécifiquement étudiées ici (Chine, États-Unis et Union européenne), suggèrent que l’agriculture devrait rester sous la pression d’une demande mondiale soutenue. Plus que la Covid-19, l’impact de la peste porcine africaine en Chine a eu, depuis plusieurs années, des répercussions majeures sur les courants d’échanges internationaux de viandes. De plus, l’appétit croissant des chinois pour les produits laitiers joue un rôle central dans le développement des marchés laitiers internationaux, au bénéfice des zones exportatrices dont l’Union européenne et les États-Unis.

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Yim, Eunok, et Paul Billant. « On the mechanism of the Gent–McWilliams instability of a columnar vortex in stratified rotating fluids ». Journal of Fluid Mechanics 780 (2 septembre 2015) : 5–44. http://dx.doi.org/10.1017/jfm.2015.426.

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In stably stratified and rotating fluids, an axisymmetric columnar vortex can be unstable to a special instability with an azimuthal wavenumber $m=1$ which bends and slices the vortex into pancake vortices (Gent & McWilliams Geophys. Astrophys. Fluid Dyn., vol. 35 (1–4), 1986, pp. 209–233). This bending instability, called the ‘Gent–McWilliams instability’ herein, is distinct from the shear, centrifugal or radiative instabilities. The goals of the paper are to better understand the origin and properties of this instability and to explain why it operates only in stratified rotating fluids. Both numerical and asymptotic stability analyses of several velocity profiles have been performed for wide ranges of Froude number $\mathit{Fr}_{h}={\it\Omega}_{0}/N$ and Rossby number $\mathit{Ro}=2{\it\Omega}_{0}/f$, where ${\it\Omega}_{0}$ is the angular velocity on the vortex axis, $N$ the Brunt–Väisälä frequency and $f$ the Coriolis parameter. Numerical analyses restricted to the centrifugally stable range show that the maximum growth rate of the Gent–McWilliams instability increases with $\mathit{Ro}$ and is independent of $\mathit{Fr}_{h}$ for $\mathit{Fr}_{h}\leqslant 1$. In contrast, when $\mathit{Fr}_{h}>1$, the maximum growth rate decreases dramatically with $\mathit{Fr}_{h}$. Long axial wavelength asymptotic analyses for isolated vortices prove that the Gent–McWilliams instability is due to the destabilization of the long-wavelength bending mode by a critical layer at the radius $r_{c}$ where the angular velocity ${\it\Omega}$ is equal to the frequency ${\it\omega}$: ${\it\Omega}(r_{c})={\it\omega}$. A necessary and sufficient instability condition valid for long wavelengths, finite Rossby number and $\mathit{Fr}_{h}\leqslant 1$ is that the derivative of the vertical vorticity of the basic vortex is positive at $r_{c}$: ${\it\zeta}^{\prime }(r_{c})>0$. Such a critical layer $r_{c}$ exists for finite Rossby and Froude numbers because the real part of the frequency of the long-wavelength bending mode is positive instead of being negative as in a homogeneous non-rotating fluid ($\mathit{Ro}=\mathit{Fr}_{h}=\infty$). When $\mathit{Fr}_{h}>1$, the instability condition ${\it\zeta}^{\prime }(r_{c})>0$ is necessary but not sufficient because the destabilizing effect of the critical layer $r_{c}$ is strongly reduced by a second stabilizing critical layer $r_{c2}$ existing at the radius where the angular velocity is equal to the Brunt–Väisälä frequency. For non-isolated vortices, numerical results show that only finite axial wavenumbers are unstable to the Gent–McWilliams instability.

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Bucher-Koenen, Tabea, et Caroline Knebel. « Finanzwissen und Finanzbildung in Deutschland – Was wissen wir eigentlich ? » Vierteljahrshefte zur Wirtschaftsforschung 90, n^o 1 (1 janvier 2021) : 11–32. http://dx.doi.org/10.3790/vjh.90.1.11.

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Zusammenfassung: Im internationalen Vergleich verfügen die Deutschen über ein relativ hohes Finanzwissen. Allerdings bedeutet dies nicht, dass Finanzwissen universell verbreitet ist. Der Anteil der Befragten, der drei grundlegende Fragen zu Zins, Inflation und Risikodiversifikation richtig beantworten kann, liegt zwischen 53 % und 62 %. Dieser Anteil liegt bei Frauen, älteren Menschen, Personen mit geringem Einkommen und geringer Bildung deutlich niedriger. Wissenschaftliche Untersuchungen zeigen, dass sich Finanzwissen positiv auf Finanzentscheidungen auswirkt. Zudem können Finanzbildungsprogramme Finanzwissen und Finanzverhalten verbessern. In Deutschland gibt es bisher keine breit angelegte Financial Literacy-Strategie und damit auch keine gezielten Evaluationen und Qualitätssicherungsmaßnahmen für die angebotenen Programme. Eine solche Strategie könnte sowohl dazu beitragen Finanzentscheidungen Einzelner als auch die gesamtwirtschaftliche Stabilität zu verbessern. Summary: In an international comparison, Germans have a relatively high level of financial knowledge. However, this does not imply that financial knowledge is universally distributed. The share of respondents, who are able to answer three basic questions about interest rate, inflation, and risk diversification correctly, lies between 53 % and 62 %. Among women, older individuals, and people with low income or low education, this share is substantially lower. Research has shown that financial knowledge positively affects financial decision-making. Furthermore, financial education programs can enhance financial knowledge and behavior. In Germany, no broad financial literacy strategy exists and, therefore, no targeted evaluations and quality assurance measures for programs are in place. Such a strategy could improve people’s financial decisions as well as the overall financial stability.

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Wilson, Richard, Hiroyuki Hashiguchi et Masanori Yabuki. « Vertical Spectra of Temperature in the Free Troposphere at Meso-and-Small Scales According to the Flow Regime : Observations and Interpretation ». Atmosphere 9, n^o 11 (23 octobre 2018) : 415. http://dx.doi.org/10.3390/atmos9110415.

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This article addresses the properties of stably-stratified and unstable layers in the free troposphere. Thorpe’s method of analysis has been applied to potential temperature (PT) profiles obtained from the raw measurements of operational radiosondes. In principle, this method distinguishes stably stratified and unstable regions. The background static stability, quantified by the square Brunt-Väisälä frequency estimated on the sorted PT profiles (stable everywhere), is observed to be significantly smaller in the unstable regions, likely due to turbulent mixing. The vertical power spectral densities (PSDs) of temperature fluctuations are shown to be proportional to m − p , where p is in the average 2.8 ± 0.2 in the stably stratified regions, and is 1.7 ± 0.3 in the unstable regions, for wavenumbers m in the range [ 10 − 2 , 10 − 1 ] m − 1 . Such findings validate the Thorpe analysis when applied to radiosondes. Also, the distribution of thicknesses h of unstable layers is observed to approximately follow a power law, varying as h − r with r ≈ 2.1 ± 0.1 . PT profiles for the entire troposphere have also been analyzed as the sum of a sorted profile and an anomaly profile. The PSDs of the sorted PT profiles are scaled as m − 3 down to a few meters on the vertical scale. Simple stochastic models based on random walks with increments having the property of flicker noise are shown to reproduce the spectral properties of the sorted PT profiles, i.e., of the vertical stratification of the free atmosphere.

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Le Reun, Thomas, Benjamin Favier et Michael Le Bars. « Parametric instability and wave turbulence driven by tidal excitation of internal waves ». Journal of Fluid Mechanics 840 (14 février 2018) : 498–529. http://dx.doi.org/10.1017/jfm.2018.18.

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We investigate the stability of stratified fluid layers undergoing homogeneous and periodic tidal deformation. We first introduce a local model which allows us to study velocity and buoyancy fluctuations in a Lagrangian domain periodically stretched and sheared by the tidal base flow. While keeping the key physical ingredients only, such a model is efficient in simulating planetary regimes where tidal amplitudes and dissipation are small. With this model, we prove that tidal flows are able to drive parametric subharmonic resonances of internal waves, in a way reminiscent of the elliptical instability in rotating fluids. The growth rates computed via direct numerical simulations (DNSs) are in very good agreement with Wentzel–Kramers–Brillouin analysis and Floquet theory. We also investigate the turbulence driven by this instability mechanism. With spatio-temporal analysis, we show that it is weak internal wave turbulence occurring at small Froude and buoyancy Reynolds numbers. When the gap between the excitation and the Brunt–Väisälä frequencies is increased, the frequency spectrum of this wave turbulence displays a $-2$ power law reminiscent of the high-frequency branch of the Garett and Munk spectrum (Geophys. Fluid Dyn., vol. 3 (1), 1972, pp. 225–264) which has been measured in the oceans. In addition, we find that the mixing efficiency is altered compared to what is computed in the context of DNS of stratified turbulence excited at small Froude and large buoyancy Reynolds numbers and is consistent with a superposition of waves.

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Wei, Lu, Jinxin Li, Jingyi Zhang, Kaili Qu, Mingxing Wang, Tingting Ni, Yuhuan Miao, Ming Luo, Shumin Feng et Dahui Liu. « Response Surface Modelling of Six Organic Acids from Pinellia ternata (Thunb.) Breit by Ultrasound-Assisted Extraction and Its Determination by High-Performance Liquid Chromatography Coupled to Triple Quadrupole Mass Spectrometry ». Chemosensors 12, n^o 3 (19 mars 2024) : 47. http://dx.doi.org/10.3390/chemosensors12030047.

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Organic acids are a key active component of Pinellia ternata (Thunb.) Breit, and their concentration is closely associated with the quality of P. ternata. Developing an efficient and rapid method for detecting organic acids can offer a valuable technology for real-time assessment of P. ternata quality. In this work, a high-performance liquid chromatography coupled to triple quadrupole mass spectrometry (HPLC-QqQ-MS/MS) approach, combining the optimization of extraction conditions using response surface methodology (RSM), was developed for the simultaneous quantitative analysis of six organic acids in P. ternata. The optimal extraction conditions for organic acids in P. ternata were ultrasonic extraction with a solid–liquid ratio of 1:50, ultrasonic time of 60 min, and extraction temperature of 55 °C. Multiple-reaction monitoring (MRM) scanning was employed for quantification using HPLC-QqQ-MS/MS in negative ionization mode through a single run of 10 min, and the limit of detection (LLOD) and limit of quantification (LLOQ) for organic acids were as low as 0.138 ng/mL and 0.614 ng/mL, respectively. The relative standard deviations (RSDs) of reproducibility, precision, and stability were all lower than 5.0%, and recovery rates were 97.75–107.14%, with RSDs < 5.0%. Finally, this method was successfully applied to determine the investigated organic acids in 12 production regions of P. ternata, revealing significant differences between different production areas. This indicates that the optimized method is suitable for further accurate investigations of organic acids of P. ternata.

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Cheynet, Etienne, Ida Marie Solbrekke, Jan Markus Diezel et Joachim Reuder. « A one-year comparison of new wind atlases over the North Sea ». Journal of Physics : Conference Series 2362, n^o 1 (1 novembre 2022) : 012009. http://dx.doi.org/10.1088/1742-6596/2362/1/012009.

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The New European Wind Atlas (NEWA) and the Norwegian hindcast archive (NORA3) database have become publicly available since the end of 2019 and mid-2021, respectively. They aim to model the long-term wind climatology with a spatial resolution of ca. 3 km and a temporal resolution of 1 h (NORA3) or 30 min (NEWA). Both products have a high potential for wind energy applications. Although their geographical coverages partly overlap, an inter-comparison of the NEWA and NORA3 databases in an offshore environment is still lacking. The paper compares the hourly mean wind speed and wind direction recorded in 2009 at the FINO1 platform (North Sea) with hindcast data from the NEWA and the NORA3 database. Both products were found to provide reliable estimates of the mean wind speed at 101 m above sea level. However, NORA3 shows slightly better performances than NEWA for the mean wind speed in terms of root-mean-square error, bias, earth mover’s distance (EMD) and Pearson correlation coefficient. For the mean wind direction, a larger circular EMD than previously documented is found, which could be due to a directional bias in the wind vane data. Finally, the Brunt-Väisälä frequency is computed using sea-surface temperature analyses and the air temperature from NORA3 and NEWA at 101 m above sea level. The encouraging description of the static atmospheric stability by the wind atlases opens the possibility to study in more detail thermally-induced wind events for wind resource assessment or wind turbine design.

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