18,879 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
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
Probing tiny convective cores with the acoustic modes of lowest degree
Solar-like oscillations are expected to be excited in stars of up to about
1.6 solar masses. Most of these stars will have convective cores during their
Main-sequence evolution. At the edges of these convective cores there is a
rapid variation in the sound speed which influences the frequencies of acoustic
oscillations. In this paper we build on earlier work by Cunha and Metcalfe, to
investigate further the impact that these rapid structural variations have on
different p-mode frequency combinations, involving modes of low degree. In
particular, we adopt a different expression to describe the sound speed
variation at the edge of the core, which we show to reproduce more closely the
profiles derived from the equilibrium models. We analyse the impact of this
change on the frequency perturbation derived for radial modes. Moreover, we
consider three different small frequency separations involving, respectively,
modes of degree l = 0, 1, 2, 3; l = 0, 1; and l = 0, 2, and show that they are
all significantly affected by the sharp sound speed variation at the edge of
the core. In particular, we confirm that the frequency derivative of the
diagnostic tool that combines modes of degree up to 3 can potentially be used
to infer directly the amplitude of the relative sound speed variation at the
edge of the core. Concerning the other two diagnostic tools, we show that at
high frequencies they can be up to a few microhertzs smaller than what would be
expected in the absence of the rapid structural variation at the edge of the
core. Also, we show that the absolute values of their frequency derivatives are
significantly increased, in a manner that is strongly dependent on stellar age.Comment: 7 pages. submitted to A&
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,
The expectations hypothesis of the term structure: some empirical evidence for Portugal
The purpose of this paper is to test the (rational) expectations hypothesis of the term structure of interest rates using Portuguese data for the interbank money market. The results obtained support only a very weak, long-run or "asymptotic" version of the hypothesis, and broadly agree with previous evidence for other countries.
The empirical evidence supports the cointegration of Portuguese rates and the "puzzle" well known in the literature: although its forecasts of future short-term rates are in the correct direction, the spread between longer and shorter rates fails to forecast future longer rates. In the single equation framework, the implications of the hypothesis in terms of the predictive ability of the spread are also clearly rejected
- …