A self-sustaining nonlinear dynamo process in Keplerian shear flows

A three-dimensional nonlinear dynamo process is identified in rotating plane Couette flow in the Keplerian regime. It is analogous to the hydrodynamic self-sustaining process in non-rotating shear flows and relies on the magneto-rotational instability of a toroidal magnetic field. Steady nonlinear solutions are computed numerically for a wide range of magnetic Reynolds numbers but are restricted to low Reynolds numbers. This process may be important to explain the sustenance of coherent fields and turbulent motions in Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review Letter

Chain-wide learning for inclusive agrifood market development : a guide to multi-stakeholder processes for linking small-scale producers to modern markets

This guide provides a set of concepts and analytical tools for finding ways to better link small-scale producers to the modern markets associated with today’s largescale supermarket retail and wholesale operations. It is has been developed through iterative testing with partners in several organisations and countries. The guide is a product of the Regoverning Markets Programme, a multi-agency programme to generate strategic information and anticipatory policy advice on small-scale producers in these fast changing markets

The terminal area simulation system. Volume 1: Theoretical formulation

A three-dimensional numerical cloud model was developed for the general purpose of studying convective phenomena. The model utilizes a time splitting integration procedure in the numerical solution of the compressible nonhydrostatic primitive equations. Turbulence closure is achieved by a conventional first-order diagnostic approximation. Open lateral boundaries are incorporated which minimize wave reflection and which do not induce domain-wide mass trends. Microphysical processes are governed by prognostic equations for potential temperature water vapor, cloud droplets, ice crystals, rain, snow, and hail. Microphysical interactions are computed by numerous Orville-type parameterizations. A diagnostic surface boundary layer is parameterized assuming Monin-Obukhov similarity theory. The governing equation set is approximated on a staggered three-dimensional grid with quadratic-conservative central space differencing. Time differencing is approximated by the second-order Adams-Bashforth method. The vertical grid spacing may be either linear or stretched. The model domain may translate along with a convective cell, even at variable speeds

The terminal area simulation system. Volume 2: Verification cases

The numerical simulation of five case studies are presented and are compared with available data in order to verify the three-dimensional version of the Terminal Area Simulation System (TASS). A spectrum of convective storm types are selected for the case studies. Included are: a High-Plains supercell hailstorm, a small and relatively short-lived High-Plains cumulonimbus, a convective storm which produced the 2 August 1985 DFW microburst, a South Florida convective complex, and a tornadic Oklahoma thunderstorm. For each of the cases the model results compared reasonably well with observed data. In the simulations of the supercell storms many of their characteristic features were modeled, such as the hook echo, BWER, mesocyclone, gust fronts, giant persistent updraft, wall cloud, flanking-line towers, anvil and radar reflectivity overhang, and rightward veering in the storm propagation. In the simulation of the tornadic storm a horseshoe-shaped updraft configuration and cyclic changes in storm intensity and structure were noted. The simulation of the DFW microburst agreed remarkably well with sparse observed data. The simulated outflow rapidly expanded in a nearly symmetrical pattern and was associated with a ringvortex. A South Florida convective complex was simulated and contained updrafts and downdrafts in the form of discrete bubbles. The numerical simulations, in all cases, always remained stable and bounded with no anomalous trends

Visual Processing Speed in Capuchin Monkeys (Cebus apella) and Rhesus Macaques (Macaca mulatta)

Visual acuity is a defining feature of the primates. Humans can process visual stimuli at extremely rapid presentation durations, as short as 14 ms. Evidence suggests that other primates, including chimpanzees and rhesus macaques, can process visual information at similarly rapid rates. What is lacking is information on the abilities of New World monkeys, which is necessary to determine whether rapid processing is present across the primates or is specific to Old World primates. We tested capuchin (Cebus apella) and rhesus (Macaca mulatta) monkeys on a computerized matching-to-sample paradigm to determine the shortest presentation duration at which stimuli could be correctly identified. In Study 1, using clip art images, both species achieved presentation durations as short as 25 ms while maintaining high accuracy rates. In Study 2, we used logographic Asian language characters to see if stimuli that were more similar in appearance would reveal species differences. Neither species was as accurate, or achieved as short of presentation durations, as with clip-art images. In particular, capuchins were initially less accurate than rhesus in Study 2, but with experience, achieved similar accuracy rates and presentation durations. These data indicate that rapid visual processing abilities are widespread in the primate lineage, and that the form of the stimuli presented can have an effect on processing across species

Oscillations and secondary bifurcations in nonlinear magnetoconvection

Complicated bifurcation structures that appear in nonlinear systems governed by partial differential equations (PDEs) can be explained by studying appropriate low-order amplitude equations. We demonstrate the power of this approach by considering compressible magnetoconvection. Numerical experiments reveal a transition from a regime with a subcritical Hopf bifurcation from the static solution, to one where finite-amplitude oscillations persist although there is no Hopf bifurcation from the static solution. This transition is associated with a codimension-two bifurcation with a pair of zero eigenvalues. We show that the bifurcation pattern found for the PDEs is indeed predicted by the second-order normal form equation (with cubic nonlinearities) for a Takens-Bogdanov bifurcation with Z2 symmetry. We then extend this equation by adding quintic nonlinearities and analyse the resulting system. Its predictions provide a qualitatively accurate description of solutions of the full PDEs over a wider range of parameter values. Replacing the reflecting (Z2) lateral boundary conditions with periodic [O(2)] boundaries allows stable travelling wave and modulated wave solutions to appear; they could be described by a third-order system