On the stability of the inertial circulation in the 11/2-layer, quasi-geostrophic model

The effect of the time-dependent interface, separating an inertial quasi-geostrophic upper fluid layer from the quiescent abyss, on the non-linear stability of a steady circulation that takes place in this layer is explored. The analysis resorts to the method of Arnol'd's invariant resulting in a conditional stability criterion, which proves the stabilizing effect of the interface with respect to the single-layer case. The uniqueness of the stable basic flow field follows. Finally, non-linear and linear analyses are compared in the special case of a channeled flow with a fluctuating interface, the latter leading to an unconditional stability statement, whose meaning is clarified by resorting to the previously obtained nonlinear criterion. Received 8 June 2000 and Received in final form 18 September 2000     

A note on equilibrium boundary conditions in lattice Boltzmann fluid dynamic simulations

It is shown that Lattice Boltzmann (LB) simulations using simple equilibrium boundary conditions at solid walls, provide quantitatively accurate results for backward-facing step flows at moderate Reynolds numbers. The basic reason for such favorable behavior is that well-resolved LB simulations operate in the so-called weak non-equilibrium regime, in which shear effects at the scale of a single lattice spacing are weak, meaning by this that the cell-shear time scale is much longer than the molecular time scales, so that the LB collisional relaxation takes place in a quasi-homogeneous velocity field. Due to their simplicity, it is suggested that equilibrium boundary conditions may represent a viable option for the LB simulation of complex flows with solid boundaries at moderate Reynolds numbers.     

Turbulent structures dependent on tidal currents in the bottom boundary layer of the Venice lagoon

Summary The time series of horizontal and vertical turbulent velocity fluctuationsu′, w′ have been recorded by means of an electromagnetic currentmeter in proximity of the bottom of a channel feeding the Venetian Lagoon. Simultaneous surface gradients have been recorded at two tide gauge stations, one upstream and the other downstream of the chosen test site. The time series ofu′, w′ andu′ w′ values have been analysed using standard digital methods and, for each record, spectra, cross-spectra, cospectra, quadrature spectra, phase and coherence ofu′ andw′ have been computed. This analysis allows us to determine temporal and spatial dimensions of turbulent structures that give the greatest contribution to Reynolds stress ( , where ρ is the water density). These structures that seem to be dependent on longitudinal surface gradients are primarily responsible for vertical momentum transport and, consequently, for the lift-up and transport of sediments. Statistic distributions ofu′, w′ andu′ w′ values indicate that the greatest turbulent structures are those withu′ w′<0 and withu′<0 predominating.

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Riassunto Registrazioni delle componenti fluttuanti, orizzontali e verticali,u′ andw′ della velocità di corrente di marea sono state eseguite con un correntometro eletrromagnetico in prossimità del fondo di un canale alimentatore della laguna di Venezia. Contemporaneamente sono stati rilevati i gradienti di livello tra due stazioni mareografiche a monte e a valle del luogo prescelto. Le serie temporali diu′, w′ eu′ w′ sono state analizzate con metodi digitali standard e per ogni record sono stati ricavati spettri, spettri incrociati, spettri di coerenza e quadratura, funzioni di fase e di coerenza. L'analisi ha consentito di determinare le dimensioni temporali e spaziali delle strutture turoolente che danno il maggiore contributo allo stress di Reynolds e che, essendo responsabili del trasferimento verticale della quantità di moto, hanno primaria importanza nei meccanismi di sollevamento dei sedimenti. Si è cercata inoltre una relazione tra l'intensità delle strutture turbolente e i valori del gradiente di livello. Le curve di distribuzione statistica diu′, w′ eu′ w′ con i relativi valori di asimmetria e curtosi hanno confermato che esistono strutture turbolente di grandi dimensioni e che sono caratterizzate dau′ w′<0, prevalentemente conu′<0.

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Резюме Регистрируются временные последовательности флуктуаций горизонтальных и вертикальных турбулентнех скоростейu′, w′ с помощью электромагнитной гидрометрической трубки вблизи дна канала, питающего Венецианскую Лагуну. Также регистрируются одновременные поверхностные градиенты надвух станцнях для калибровки прилива, одна вверх по течению и другая вниз по течению для заданного местоположения. Анализируются временные последовательностиu′, w′ и значенияu′, w′, используя стандартные численные методы. Вычисляются фаза и когерентностьu′ иw′ для всех зарегистрированных спектров. Этот аналиэ позволяет определить временные и пространственные размеры турбулентных структур, которые дают наибольший вклад в напряжение Рейнольдса ( , где р есть плотность воды). Эти структуры, которые, по-видимому, зависят от продольных поверхностных градиентов, ответственны, главным образом, за перенос вертикального импульса и, следовательно, за подъем и перенос осадков. Статистипеские распределенияu′, w′ изначенияu′, w′ показывают, что наибольшие турбулентные структуры имеютu′ w′<0 и преобладают структуры сu′<0.

     

A bio-optical model suitable for use in forward and inverse coupled atmosphere-ocean radiative transfer models

A simple, yet complete bio-optical model for the inherent optical properties (IOPs) of oceanic waters is developed. This bio-optical model is specifically designed for use in comprehensive, multiple scattering radiative transfer models for the coupled atmosphere-ocean system. Such models can be used to construct next-generation algorithms for simultaneous retrieval of aerosol and marine parameters. The computed remote sensing reflectance R_rs(λ) is validated against field measurements of R_rs(λ) compiled in the SeaBASS data base together with simultaneous chlorophyll concentrations (C) ranging from 0.03 to . This connection between R_rs and C is used to construct a chlorophyll concentration retrieval algorithm that yields reliable results for a large range of chlorophyll concentrations. The overall performance of a MODIS/VIIRS chlorophyll concentration retrieval algorithm is found to be less satisfactory.     

The effect of free surface on the stability of geophysical flows: a note for a quasi-geostrophic ocean

Summary The role of the free surface on the stability of a wind driven, quasi-geostrophic ocean having an Ekman bottom layer is studied analytically using a perturbative procedure. It is shown that the growth rate range of the developing disturbances is reduced by an amount depending on the energetics of the free surface. In other words, the main free surface effect is to increase the stability of the whole system if this is compared with the rigid-lid ocean model.

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Riassunto Viene analizzato il ruolo della superficie libera nella stabilità di un oceano nell'approssimazione quasi-geostrofica, forzato dal vento ed interessato dallo strato di Ekman sul fondo. Nell'ambito di un approccio lineare, si dimostra che la presenza di detta superficie aumenta la stabilità del sistema quando la si confronta con quella di un oceano nell'approssimazione di ?rigid lid?.

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Резюме Используя процедуру теории возмущений, аналитически исследуется роль свободной поверхности на устойчивость океана в квази-геострофическом приближении. Показывается, что главный эффект свободной поверхности сводится к увеличению устойчивости всей системы по сравнению с моделью океана в приближении ?жесткой крышки?.

     

Ratchets: the problems with boundary conditions in insulating fluids

We have constructed microfabricated devices, designed to have asymmetry in their physical structures, in order to fractionate objects under continuous-flow conditions. The fractionation of particles with the inclusion of diffusion and asymmetric structures requires a knowledge of not only statistical mechanics but also the external forces acting on the particles, since thermal Brownian fluctuations alone cannot serve to separate particles. We explicitly examine this problem in a device designed to fractionate biomolecules dissolved in water, and show that boundary conditions that influence the direction of the applied force are quite important in determining the efficiency of the device. Received: 11 December 2001 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Enrichment and transport of surfactants in marine aerosol formed during particular weather conditions

Summary Surfactants appear responsible for the presence and the enrichment of microfloatable components in marine aerosol. According to the first set of both surface thermodynamic and chemical data, a qualitative model of enrichment and mass transfer is proposed for marine aerosol, near the coast in particular weather conditions. The present data also suggest a quality test for coastal sea-water, integrated over large areas, in those weather conditions in which a direct surface water sampling is very difficult. Paper presented at the 1° Congresso del Gruppo Nazionale per la Fisica dell'atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

TASEP related models with traffic light boundary

Cellular automata models play an important role in traffic modeling. For some variants of the Nagel-Schreckenberg model, the effects of traffic light boundary conditions are considered. Based on previous results, the exact density profiles can be derived easily for deterministic dynamics. Additionally, the exact average outflow per traffic light cycle is presented not only in the deterministic case, but also for an important semi-stochastic variant with slow-to-start behaviour. Thereby, the models are strongly related to the well-known totally asymmetric simple exclusion process (TASEP) which can be regarded as a generic model for many driven particle systems.     

A theoretical study on various models for the domain boundaries in epitaxial GaN films

Various domain boundaries that are found in epitaxial Wurtzite GaN films were studied by molecular dynamics simulation. The Ewald summation algorithm and Keating potential model are adopted to calculate the long-range Coulomb interaction and the short-range bonding force in the semiconductor system, respectively. The research results show that the domain formation energies of (100) and (110) boundaries are significantly different. The latter ones have general quite higher formation energies than the formers. The like-atom (i.e. atoms of the same kind) bonding domain boundaries (LABDB) have higher formation energies than their counterparts of unlike-atom (i.e. atoms of different kinds) bonding domain boundaries (UABDB) in all GaN (100) and (110) interfaces. The UABDB structures are all stable while most of the LABDB are unstable. The advantage and the limitation of Keating potential model in Molecular Dynamics simulation for covalent crystal are discussed. Received: 7 April 1999 / Accepted: 1 November 1999 / Published online: 8 March 2000     

A process of thermocline erosion on the Sicilian continental shelf

Summary We study a process of thermocline erosion measured in the Channel of Sicily in the period 16 November–December 1985. Evidence is presented that the transition from Summer to Winter stratification has been an impulsive process. We show that the erosion of the seasonal thermocline and the deepening of the surface mixed layer were stimulated by the action of two strong atmospheric perturbations. Possible relations between mixing events and internal wave breaking are discussed. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

Fringe waves radiated by a half-plane for the boundary conditions of Neumann

The fringe waves of the physical theory of diffraction are obtained in terms of Fresnel integrals for a half-plane satisfying the Neumann boundary condition. The approximate expressions of the radiated waves are also evaluated for sufficiently large wavenumbers. The fields are plotted and compared numerically.     

Dependence of T<Subscript>c</Subscript> on hydrostatic pressure in a 123 superconductor

Hydrostatic pressure studies on the tetragonal 123 superconductor (La_1-xCa_x) (Ba_1.75-xLa_0.25+x)Cu₃O_y for x =0.1 and variable oxygen content y show that increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in is observed only for the most underdoped sample. Received 26 August 1999     

Effect of toroidal moment on a macroscopic self-organization of electrons in the quantum Hall regime

We have studied CR lineshape of terahertz-light-induced current in InAs quantum wells in tilted quantizing magnetic fields. We have observed dramatic modification of the lineshape with increasing of in-plane component of magnetic field as well as with increasing of transverse built-in electric field in the well. Scenario of the modification shows that the energy spectrum asymmetry is determined by so-called toroidal moment of the system and is a function of Landau quantum number. Macroscopic self-organization of electrons under the conditions of quantum Hall effect has also been directly demonstrated in both linear and saturation regimes of the light absorption.     

Topological properties of phylogenetic trees in evolutionary models

The extent to which evolutionary processes affect the shape of phylogenetic trees is an important open question. Analyses of small trees seem to detect non-trivial asymmetries which are usually ascribed to the presence of correlations in speciation rates. Many models used to construct phylogenetic trees have an algorithmic nature and are rarely biologically grounded. In this article, we analyze the topological properties of phylogenetic trees generated by different evolutionary models (populations of RNA sequences and a simple model with inheritance and mutation) and compare them with the trees produced by known uncorrelated models as the backward coalescent, paying special attention to large trees. Our results demonstrate that evolutionary parameters as mutation rate or selection pressure have a weak influence on the scaling behavior of the trees, while the size of phylogenies strongly affects measured scaling exponents. Within statistical errors, the topological properties of phylogenies generated by evolutionary models are compatible with those measured in balanced, uncorrelated trees.     

Variations in the boundary conditions of coupled dynamic systems

The paper is a sequel to reference [1]. In that paper, isolation of the basic dynamic systems was achieved by imposing uncoupled boundary conditions in which the coupling impedances were removed. It is illustrated that alternate isolations, involving different boundary conditions, can be imposed. In particular, the modification introduced in the formalism when blocked boundary conditions are substituted for the uncoupled boundary conditions is discussed in some detail.     

A stability criterion for time-dependent basic zonal flows in the homogeneous model of the ocean

Summary The homogeneous model of the oceanic circulation gives the simplest understanding of the effect of the wind stress on the dynamics of the bulk interior. The annual and interannual variability of the wind stress suggests to consider the problem of the linear stability of a time-dependent flow driven by the mechanism ?wind stress forcing-bottom dissipation?. In the context of the homogeneous model a stability criterion based on the energy method has been obtained for unsteady zonal flows. An example of the application of the criterion has clarified the role of an oscillating wind stress in the maintenance of the stability of the basic flow.

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Riassunto Il modello omogeneo della circolazione oceanica fornisce la comprensione piú semplice dell'effetto dello stress del vento sulla dinamica del nucleo centrale della massa d'acqua. La variabilità annuale e interannule dello stress del vento pone il problema della stabilità lineare di una corrente di base controllata dal meccanismo ?forzatura del vento—dissipazione del fondo?. Nel contesto del modello omogeneo è stato ottenuto un criterio per la stabilità lineare di un flusso di base zonale, non stazionario, basato sul metodo dell'energia. Un esempio di applicazione del criterio serve a chiarire il ruolo di un windstress periodico nel mantenimento della stabilità del fluosso base.

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Резюме Одюродная модель океанической циркуляции позволяет понять влияние воздействия ветра на динамику внутренней области. Непостоянство воздействия ветра предполагает, что необходимо рассмотреть проблему линеиной устойчивости зависяшего от времени течения, которое приводится в действие за сч?eт механизма ?воздействие ветра-придонная диссипация?. Для нестцционарных зональных течений получается критерий устойчивости на основе знергетического метода в рамках однородной модели. В качестве примера применения этого критерия рассматривается роль осциллируюэего воздействия ветра в поддержании устойчивости основного течения.

     

Langmuir probe measurements in the sheath of a highly charged body

Summary We perform a model study of the electrical characteristics of a Langmuir probe moving in the field of a highly charged central body. The study is mainly motivated by a recent proposal, in the context of the Tethered Satellite System project, where the use of probes to study the space charge region surrounding a highly charged satellite is foreseen.

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Riassunto Si presenta un’analisi delle caratteristiche elettriche di una sonda di Langmuir nel campo elettrico di un corpo centrale ad alto stato di carica. Lo studio è soprattutto motivato da una recente proposta di esperimento per il progetto Tethered Satellite System dove si prevede l’uso di sonde per studiare la regione di carica spaziale attorno da un satellite a elevati potenziali.

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Измерения с помошью Ленгмюровского зонда в оболочке сильно заряженного тела

Резюме Предлагается анализ Электрических характеристик Ленгмюровского зонда, движушегося в поле центрального сильно заряженного тела. Йсследование, в основном, было обусловлено недавно предложенным Экспериментом в связи с проектом ? привязанной спутниковой системы ? в котором предполагается использование зондов для исследования области пространственного заряда, окружаюшей сильно заряженный спутник.

     

Influence of external tidal and meteorological forces on the bradyseismic phenomenon in the Phlegraean Fields

Summary Large tidal and meteorological influences on the present behaviour of the bradyseismic phenomenon in the Phlegraean Fields have been significantly determined. The two recent reinstatements of the uplift in 1970 and 1982 were found to be preceded by anomalous large increase of the mean oceanic loading. A significant covariance of the annual cycle of oceanic loading (and rainfall) with the number of shocks was also ascertained. Similarly it has been shown that the amplitudes and the significance level of the tidal terms in the number of shocks, related to extensional tidal stresses activing faulting, increase with the volcanic activity. Both these results indicate that oceanic loading (and rainfall) and tidal terms may contribute in warning the phenomenon. The tidal triggering is better explained as the effect of the product of each concurrent tidal component. Moreover, it is shown that the geophysical system in the area is getting more sensitive to the examined weak external forces and therefore becoming closer to unstable conditions. On the other hand, since the sea-level will certainly decrease in the near, future, a significant reduction of the volcanic activity is expected in the next months.

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Riassunto Nel presente lavoro si quantizza e si spiega l'influenza delle forze esterne meteomarine e mareali sulla evoluzione del fenomeno bradisismico in atto a Pozzuoli. Le recrudescenze del fenomeno nel 1970 e nel 1982 sono state precedute ad anomalo innalzamento, a periodo dell'ordine degli anni, nel livello medio del mare; cosí come sono state rilevate cospiranti variazioni annuali nel livello del mare, nella sismicità e nel sollevamento del suolo. Poiché pioggia e livello del mare sono strettamente legati non è stato possibile in questa fase dell'indagine una distinzione tra gli effetti delle due possibili concause sul fenomeno bradisismico, che comunque contribuiscono all'incremento del contenuto di acqua nelle rocce. è stato mostrato che l'ampiezza e la significatività delle onde mareali nel numero dei sismi crescono con l'aumentare dell'attività vulcanica. è stato anche mostrato che la probabilità di un evento sismico di piú elevata energia in una particolare ora connessa a date posizioni del sole e della luna è circa 3 volte maggiore rispetto ad altra ora pure predeterminabile. Si fornisce anche una possibile spiegazione dell'effetto degli stress mareali sull'attività sismica. Livello del mare e stress mareali possono perciò contribuire allo studio, della previsione dell'evoluzione del rischio sismico e vulcanico. Mentre da una parte l'accertata amplificazione delle onde mareali nei terremori del 1983 rispetto al 1970 lascia desumere che il sistema flegreo si approssimi a condizioni critiche, dall'altra il sicuro prossimo abbassamento del livello del mare lascia prevedere un'assai prossima e cospicua attenuazione del fenomeno in atto.

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Резюме Определяется большое влияние приливных и метеорологических сил на поведение орадисейсмического явления в Флегрчнных Лолях. Обнаружено, что двум недавним восстановлениям подъема в 1970 г. и в 1982 г. предшествовало аномально большое увеличение средней океанической нагрузки. Также установлена заметная ковариантность годового цикла океанической нагрузки (и осадков) с числом толчков. Показывается, что амилитуды и уровень значимости приливных членов увеличиваются с вулканической деятельностью. Оба этих результата указывают, что океаническая нагрузка (и осадки) и приливные члены могут давать вклад в предупреждение явления. Приливный спусковой механизм может быть объяснен, как эффект умножения для каждой приливной компоненты. Кроме того показывается, что геофизическая система в свободном пространстве является более чувст-вительной к исследованным слабым внешним силам и, следовательно, становится ближе к неустойчивым условиям. С другой стороны, т. к. уровень моря будет понижаться в ближайшем будущем, то ожидается заметное уменьшение вулканической деятельности в ближайшие месяцы.

     

Optimizing conflicts in the formation of strategic alliances

Coalition setting among a set of actors (countries, firms, individuals) is studied using concepts from the theory of spin glasses. Given the distribution of respective bilateral propensities to either cooperation or conflict, the phenomenon of local aggregation is modeled. In particular the number of coalitions is determined according to a minimum conflict principle. It is found not to be always two. Along these lines, previous studies are revisited and are found not to be consistent with their own principles. The model is then used to describe the fragmentation of former Yugoslavia. Results are compared to the actual situation. Received 23 February 2000     

Nonlinear voter models: the transition from invasion to coexistence

In nonlinear voter models the transitions between two states depend in a nonlinear manner on the frequencies of these states in the neighborhood. We investigate the role of these nonlinearities on the global outcome of the dynamics for a homogeneous network where each node is connected to m = 4 neighbors. The paper unfolds in two directions. We first develop a general stochastic framework for frequency dependent processes from which we derive the macroscopic dynamics for key variables, such as global frequencies and correlations. Explicit expressions for both the mean-field limit and the pair approximation are obtained. We then apply these equations to determine a phase diagram in the parameter space that distinguishes between different dynamic regimes. The pair approximation allows us to identify three regimes for nonlinear voter models: (i) complete invasion; (ii) random coexistence; and – most interestingly – (iii) correlated coexistence. These findings are contrasted with predictions from the mean-field phase diagram and are confirmed by extensive computer simulations of the microscopic dynamics.