14,644 research outputs found

    Cauldron subsidence and subglacial floods

    Get PDF
    Ice cauldrons are depressions which form at the surface of ice sheets when an underlying subglacial lake empties. Notable examples of such cauldrons occur on the surface of the Vatnajökull ice cap in Iceland, and in particular are formed when subglacial volcanic eruptions occur. More generally, cauldrons will form when a subglacial lake empties during a jökulhlaup. The rate of subsidence of the ice surface is related to the rate at which the subglacial water empties from the lake. We use a viscous version of classical beam theory applied to the ice sheet to determine the relation between the subsidence rate and flood discharge, and we use the results to make inferences concerning ring fracture spacings in cauldrons, the consequent effect on flood discharge dynamics, and the likely nature of subsidence events in the Antarctic Ice Sheet

    Periodic breathing at high altitude.

    Get PDF
    Periodic breathing is often associated with heart disease or stroke, and commonly Cheyne-Stokes breathing has a period of about a minute. Periodic breathing also commonly occurs in healthy subjects at high altitude, and here the periods may be much shorter, of the order of 15-20 s. In this paper we study such periodic breathing using the classical model of Grodins et al. (1967, J. Appl. Physiol. 22, 260-276), together with a prescription for the dependence of ventilation on the blood CO2 concentration, modulated by the reduced oxygen pressure (the 'Oxford fan'). The model focusses on the fast dynamics of the arterial blood CO2, and differs in this respect from our previous work which emphasised the brain CO2 concentration; in this sense our model is in fact a generalization of the conceptually simpler Mackey-Glass model

    The role of the central chemoreceptor in causing periodic breathing.

    Get PDF
    In a previous publication (Fowler et aL, 1993), we reduced the classical cardiorespiratory control model of (Grodins et aL, 1967) to a much simpler form, which we then used to study the phenomenon of periodic breathing. In particular, cardiac output was assumed constant, and a single (constant) delay representing arterial blood transport time between lung and brain was included in the model. In this paper we extend this earlier work, both by allowing for the variability in transport delays, due to the dependence of cardiac output on blood gas concentrations, and also by including further delays in the system. In addition, we extensively discuss the physiological implications of parameter variations in the model; several novel mechanisms for periodic breathing in clinical situations are proposed. The results are discussed in the light of recent observational studies. Keywords: Periodic breathing; Cheyne-Stokes respiration; heart-rate variability*, differential-delay equations. 1

    High resolution radiometric measurements of convective storms during the GATE experiment

    Get PDF
    Using passive microwave data from the NASA CV-990 aircraft and radar data collected during the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE), an empirical model was developed relating brightness temperatures sensed at 19.35 GHz to surface rainfall rates. This model agreed well with theoretical computations of the relationship between microwave radiation and precipitation in the tropics. The GATE aircraft microwave data was then used to determine the detailed structure of convective systems. The high spatial resolution of the data permitted identification of individual cells which retained unique identities throughout their lifetimes in larger cloud masses and allowed analysis of the effects of cloud merger

    Lithospheric failure on Venus

    Get PDF
    We develop a predictive model which has the ability to explain a postulated style of episodic plate tectonics on Venus, through the periodic occurrence of lithospheric subduction events. Present-day incipient subduction zones are associated with the existence of arcuate trenches on the Venusian lithosphere. These trenches resemble terrestrial subduction zones, and occur at the rim of coronae, uplift features thought to be due to deep-mantle convective plumes. The model we adopt represents the lithosphere as the thermal boundary layer which lies above a convective plume. We assume a temperature-dependent nonlinear viscoelastic rheology, and we assume a stress-based criterion for plastic yield. In developing this latter criterion, we are led to a re-interpretation of the strength envelope which is commonly used in analysing lithospheric stress, and we propose that the plastic yield strength has meaning (and is finite) below the lithosphere, using behaviour in the Earth as our 'laboratory' justification for this view. An inferred yield stress on the Earth is ca. 300 bar (30 MPa). Our model then shows that a thickening lithosphere becomes progressively more fluid as the stresses induced by the buoyant convective plume become large. Failure occurs when the effective lithosphere viscosity becomes equal to that of the underlying mantle. We show that reasonable expected values of yield stress in the range 100-200 bar (10-20 MPa) for Venusian mantle rocks are consistent within the framework of the model with radii of coronal trenches in the range 100-1200 km, and with the approximate time (200-800 Myr) which they may take to develop

    Topological code Autotune

    Full text link
    Many quantum systems are being investigated in the hope of building a large-scale quantum computer. All of these systems suffer from decoherence, resulting in errors during the execution of quantum gates. Quantum error correction enables reliable quantum computation given unreliable hardware. Unoptimized topological quantum error correction (TQEC), while still effective, performs very suboptimally, especially at low error rates. Hand optimizing the classical processing associated with a TQEC scheme for a specific system to achieve better error tolerance can be extremely laborious. We describe a tool Autotune capable of performing this optimization automatically, and give two highly distinct examples of its use and extreme outperformance of unoptimized TQEC. Autotune is designed to facilitate the precise study of real hardware running TQEC with every quantum gate having a realistic, physics-based error model.Comment: 13 pages, 17 figures, version accepted for publicatio

    Quantum computing with nearest neighbor interactions and error rates over 1%

    Full text link
    Large-scale quantum computation will only be achieved if experimentally implementable quantum error correction procedures are devised that can tolerate experimentally achievable error rates. We describe a quantum error correction procedure that requires only a 2-D square lattice of qubits that can interact with their nearest neighbors, yet can tolerate quantum gate error rates over 1%. The precise maximum tolerable error rate depends on the error model, and we calculate values in the range 1.1--1.4% for various physically reasonable models. Even the lowest value represents the highest threshold error rate calculated to date in a geometrically constrained setting, and a 50% improvement over the previous record.Comment: 4 pages, 8 figure

    The use of LANDSAT data to study mesoscale cloud features

    Get PDF
    The author has identified the following significant results. Analysis of a complex cloud banding case over the Adirondacks on 20 July 1974 gave evidence that processes other than those recognized by the Rayleigh-Kuettner theory are capable of giving rise to cloud bands. Other situations studied verified that elementary wave theory is useful under proper conditions
    corecore