18,864 research outputs found
Quasi-geostrophic dynamics in the presence of moisture gradients
The derivation of a quasi-geostrophic (QG) system from the rotating shallow
water equations on a midlatitude beta-plane coupled with moisture is presented.
Condensation is prescribed to occur whenever the moisture at a point exceeds a
prescribed saturation value. It is seen that a slow condensation time scale is
required to obtain a consistent set of equations at leading order. Further,
since the advecting wind fields are geostrophic, changes in moisture (and
hence, precipitation) occur only via non-divergent mechanisms. Following
observations, a saturation profile with gradients in the zonal and meridional
directions is prescribed. A purely meridional gradient has the effect of
slowing down the dry Rossby waves, through a reduction in the "equivalent
gradient" of the background potential vorticity. A large scale unstable moist
mode results on the inclusion of a zonal gradient by itself, or in conjunction
with a meridional moisture gradient. For gradients that are are representative
of the atmosphere, the most unstable moist mode propagates zonally in the
direction of increasing moisture, matures over an intraseasonal timescale and
has small phase speed.Comment: 9 pages, 8 figures, Quarterly Journal of the Royal Meteorological
Society, DOI:10.1002/qj.2644, 201
Quantum chaos with spin-chains in pulsed magnetic fields
Recently it was found that the dynamics in a Heisenberg spin-chain subjected
to a sequence of periodic pulses from an external, parabolic, magnetic field
can have a close correspondence with the quantum kicked rotor (QKR). The QKR is
a key paradigm of quantum chaos; it has as its classical limit the well-known
Standard Map. It was found that a single spin excitation could be converted
into a pair of non-dispersive, counter-propagating spin coherent states
equivalent to the accelerator modes of the Standard Map. Here we consider how
other types of quantum chaotic systems such as a double-kicked quantum rotor or
a quantum rotor with a double-well potential might be realized with spin
chains; we discuss the possibilities regarding manipulation of the one-magnon
spin waves.Comment: 10 pages, 4 figures. Submitted to PTP special issue for QMC200
Can an Industry Voluntary Agreement on Food Traceability Minimize the Cost of Food Safety Incidents?
In the recent past the United States has had a number of severe food-safety outbreaks in the produce, vegetable and beef industry that greatly disrupted the food system. In all these outbreaks here were severe disruptions on sales that affected the whole industry, and it took an extended period of time to correctly locate the source of the outbreak. Traceability can be an effective tool to reduce the impact of food safety incidents my expediting the search for the origin of outbreaks. This paper investigates to what extent an industry-led voluntary agreement for providing traceability can reduce the cost of a food-safety outbreak. We find that a voluntary agreement on traceability can successfully reduce the cost of a food-safety outbreak but will unlikely achieve the optimal social level of traceability because of significant free riding.Traceability, voluntary agreements, food safety, Food Consumption/Nutrition/Food Safety,
Optimal alarm systems for count processes
In many phenomena described by stochastic processes, the implementation of an alarm system becomes fundamental to predict the occurrence of future events. In this work we develop an alarm system to predict whether a count process will upcross a certain level and give an alarm whenever the upcrossing level is predicted. We consider count models with parameters being functions of covariates of interest and varying on time. This article presents classical and Bayesian methodology for producing optimal alarm systems. Both methodologies are illustrated and their performance compared through a simulation study. The work finishes with an empirical application to a set of data concerning the number of sunspot on the surface of the sun
Construction of a non-standard quantum field theory through a generalized Heisenberg algebra
We construct a Heisenberg-like algebra for the one dimensional quantum free
Klein-Gordon equation defined on the interval of the real line of length .
Using the realization of the ladder operators of this type Heisenberg algebra
in terms of physical operators we build a 3+1 dimensional free quantum field
theory based on this algebra. We introduce fields written in terms of the
ladder operators of this type Heisenberg algebra and a free quantum Hamiltonian
in terms of these fields. The mass spectrum of the physical excitations of this
quantum field theory are given by , where denotes the level of the particle with mass in an infinite
square-well potential of width .Comment: Latex, 16 page
Periodically-driven cold atoms: the role of the phase
Numerous theoretical and experimental studies have investigated the dynamics
of cold atoms subjected to time periodic fields. Novel effects dependent on the
amplitude and frequency of the driving field, such as Coherent Destruction of
Tunneling have been identified and observed. However, in the last year or so,
three distinct types of experiments have demonstrated for the first time,
interesting behaviour associated with the driving phase: i.e. for systems
experiencing a driving field of general form ,
different types of large scale oscillations and directed motion were observed.
We investigate and explain the phenomenon of Super-Bloch Oscillations (SBOs) in
relation to the other experiments and address the role of initial phase in
general. We analyse and compare the role of in systems with homogeneous
forces (), such as cold atoms in shaken or amplitude-modulated
optical lattices, as well as non-homogeneous forces (), such
as the sloshing of atoms in driven traps, and clarify the physical origin of
the different -dependent effects.Comment: 10 pages, 1 figur
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