Tunneling ``zero-bias'' anomaly in the quasi-ballistic regime

For the first time, we study the tunneling density of states (DOS) of the interacting electron gas beyond the diffusive limit. A strong correction to the DOS persists even at electron energies exceeding the inverse transport relaxation time, which could not be expected from the well-known Altshuler-Aronov-Lee (AAL) theory. This correction originates from the interference between the electron waves scattered by an impurity and by the Friedel oscillation this impurity creates. Account for such processes also revises the AAL formula for the DOS in the diffusive limit.Comment: 4 pages, 2 .eps figures, submitted to Phys. Rev. Let

A vortex-based model of velocity and shear stress in a partially vegetated shallow channel

This paper presents a method for predicting the distributions of velocity and shear stress in shallow channels with a boundary of emergent vegetation. Experiments in a laboratory channel with model vegetation show that the velocity profile exhibits a distinct two-layer structure, consisting of a rapidly varying shear layer across the vegetation interface and a more gradual boundary layer in the main channel. In addition, coherent vortices are observed which span both layers, and are the dominant contributors to lateral momentum fluxes. From these observations, we propose a model for the vortex-induced exchange and find expressions for the width of momentum penetration into the vegetation, the velocity and shear stress at the vegetation edge, and the width of the boundary layer in the main channel. These variables, along with a momentum balance in the main channel, comprise a modeling framework which accurately reproduces the observed velocity and shear stress distributions. The predictions for the velocity and shear stress can provide a basis for modeling flood conveyance, overbank sediment transport, and scalar residence time in the vegetated layer

A model for thin layer formation by delayed particle settling at sharp density gradients

Thin layers – regions where plankton or particles accumulate vertically on scales of a few meters or less – are common in coastal waters, and have important implications for both trophic dynamics and carbon cycling. These features can form by a variety of biological and physical mechanisms, including localized growth, shear-thinning, and directed swimming. An additional mechanism may result in the formation of thin layers of marine aggregates, which have been shown to decrease their settling velocity when passing through sharp density gradients, a behavior termed delayed settling. Here, we apply a simple vertical advection-diffusion model to predict the properties of aggregate thin layers formed by this process. We assume a constant vertical flux of particles from the surface, which is parameterized by observations from laboratory experiments with marine aggregates. The formation, maintenance, and shape of the layers are described in relation to non-dimensional numbers that depend on environmental conditions and particle settling properties. In particular, model results demonstrate layer intensity and sharpness both increase with higher Péclet number (Pe), that is, under conditions with weaker mixing relative to layer formation. Similarly, more intense and sharper layers are found when the delayed settling behavior of aggregates is characterized by a lower velocity minimum. The model also predicts layers that are vertically asymmetric and highly “peaky” when compared with a Gaussian distribution, features often seen in thin layers in natural environments. Lastly, by comparing model predictions with observations of thin layers in the field, we are able to gain some insight into the applicability of delayed settling as a thin layer formation mechanism in different environmental conditions

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

The efficiency of the conversion of mechanical to potential energy, often expressed as the flux Richardson number, Rif, is an important determinant of vertical mixing in the ocean. To examine the dependence of Rif on the buoyancy Reynolds number, ReB, we analyze three sets of data: microstructure profiler data for which mixing is inferred from rates of dissipation of turbulent kinetic energy (ε) and temperature variance (χ) measured in the open ocean, time series of spectrally fit values of ε and covariance-derived buoyancy fluxes measured in nearshore internal waves, and time series of spectrally fit values of ε and χ measured in an energetic estuarine flow. While profiler data are well represented by Rif ≈ 0.2 for 1 < ReB < 1,000, the covariance data have much larger values of ReB and, consistent with direct numerical simulation results, show that Rif ~ ReB −0.5. The estuarine data have values of ReB that fall between those of the other two data sets but also shows Rif ≈ 0.2 for ReB < 5000. Overall, these data suggest that Rif is in general not constant and may be substantially less than 0.2 when ReB is large, although the value at which the transition from constant to ReB-dependent mixing may depend on additional parameters that are yet to be determined. Nonetheless, for much of the ocean, ReB < 100 and so Rif is constant there

Available potential energy and the general circulation: Partitioning wind, buoyancy forcing, and diapycnal mixing

The ocean energy cycle is calculated using a new available potential energy (APE) decomposition, which partitions adiabatic buoyancy fluxes from diapycnal mixing, applied to results from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), eddy-permitting ocean state estimate and observed surface buoyancy fluxes from theWHOIOAFlux project. Compared with the traditional Lorenz energy cycle, this framework provides a more accurate estimate of the background potential energy (PE) of the global oceans and the surface generation and interior fluxes of APE. Calculations of the global energy budget using 16 yr of ECCO2 output suggest that the adiabatic portion of the general circulation is maintained by a balance between the mean wind-driven upwelling that increases APE (+0.27 TW) and time-fluctuating processes, including mesoscale eddies, which release APE (-0.27 TW). The APE generated by surface buoyancy fluxes (0.46 TW) is comparable to the generation by the mean winds. The global rate of irreversible mixing (0.46 TW), which balances surface APE generation, is consistent with previous estimates of the diapycnal fluxes associated with maintaining deep stratification (see Munk and Wunsch) and a global diapycnal diffusivity of O(1 × 10-4)m2 s-1. However, the net contribution of diapycnal mixing to the total potential energy is negligible, which suggests that mixing, contrary to one current paradigm, does not place a global demand on kinetic energy dissipation. However, there are regions where mixing is significant, for example, between 3000 and 5000m (in ECCO2), where mixing increases PE by 0.1 TW. The work provides a new framework for separating adiabatic-diabatic fluxes and for monitoring the global rate of diapycnal mixing rate using measurable surface properties such as SST and heat flux

Optimal transient growth in thin-interface internal solitary waves

The dynamics of perturbations to large-amplitude internal solitary waves (ISWs) in two-layered flows with thin interfaces is analysed by means of linear optimal transient growth methods. Optimal perturbations are computed through direct-adjoint iterations of the Navier-Stokes equations linearized around inviscid, steady ISWs obtained from the Dubreil-Jacotin-Long (DJL) equation. Optimal perturbations are found as a function of the ISW phase velocity (alternatively amplitude) for one representative stratification. These disturbances are found to be localized wave-like packets that originate just upstream of the ISW self-induced zone (for large enough ) of potentially unstable Richardson number, <[CDATA Ri. They propagate through the base wave as coherent packets whose total energy gain increases rapidly with . The optimal disturbances are also shown to be relevant to DJL solitary waves that have been modified by viscosity representative of laboratory experiments. The optimal disturbances are compared to the local Wentzel-Kramers-Brillouin (WKB) approximation for spatially growing Kelvin-Helmholtz (K-H) waves through the <[CDATA Ri zone. The WKB approach is able to capture properties (e.g. carrier frequency, wavenumber and energy gain) of the optimal disturbances except for an initial phase of non-normal growth due to the Orr mechanism. The non-normal growth can be a substantial portion of the total gain, especially for ISWs that are weakly unstable to K-H waves. The linear evolution of Gaussian packets of linear free waves with the same carrier frequency as the optimal disturbances is shown to result in less energy gain than found for either the optimal perturbations or the WKB approximation due to non-normal effects that cause absorption of disturbance energy into the leading face of the wave. Two-dimensional numerical calculations of the nonlinear evolution of optimal disturbance packets leads to the generation of large-amplitude K-H billows that can emerge on the leading face of the wave and that break down into turbulence in the lee of the wave. The nonlinear calculations are used to derive a slowly varying model of ISW decay due to repeated encounters with optimal or free wave packets. Field observations of unstable ISW by Moum et al. (J. Phys. Oceanogr., vol. 33 (10), 2003, pp. 2093-2112) are consistent with excitation by optimal disturbances

Particle radial velocity and concentration kernels estimation in isotropic grid turbulence experiments of inertialess particles at small separation distances

We report experimental measurements of kinematic and dynamic particle concentration kernels conditioned by the separation distances of solid inertialess particles in isotropic turbulence by three-dimensional particle tracking velocimetry with particle diameters smaller than the Kolmogorov length scale. Particle radial relative velocity statistics are measured from the dissipation to the integral length-scale range. The radial scaling of particle and fluid relative velocity variance in the inertial subrange, consistent with Kolmogorov's theory, is reported, while a new scaling is found for small distances due to finite-size effects between particles. The measured concentration kernels at small separation distances therefore deviate from those in the theory of Saffman & Turner (J. Fluid Mech., vol. 1, 1956, pp. 16-30) at small inter-particle distances due to hydrodynamic interactions. A real kernel taking into account the history of the particle tracks and excluding multiple events is also calculated, while the normalised particle concentration kernels are found to be essentially insensitive to the flow Reynolds number

Detection of undiagnosed HIV among state prison entrants

substantial proportion of individuals infected with the human immunodeficiency virus (HIV) in the United States enter a correctional facility annually.1,2 Therefore, incarceration presents an opportunity for HIV detection. Even thoughmany states have adopted policies of mass HIV screening of inmates,2-4 the extent to which HIV testing on prison entry detects new infections is unclear

“Inside these fences is our own little world”: Prison-based hiv testing and hiv-related stigma among incarcerated men and women

Correctional facilities offer opportunities to provide comprehensive HIV services including education, testing, treatment, and coordination of post- release care. However, these services may be undermined by unaddressed HIV stigma. As part of a prison-based HIV testing study, we interviewed 76 incarcerated men and women from the North Carolina State prison system. The sample was 72% men, median age 31.5 years (range: 19 to 60). Thematic analysis revealed high levels of HIV-related fear and stigma, homophobia, incomplete HIV transmission knowledge, beliefs that HIV is highly contagious within prisons (“HIV miasma”), and the View of HIV testing as protective. Interviewees described social distancing behaviors and coping mechanisms they perceived to be protective, including knowing their HIV status and avoiding contact with others and shared objects. Interviewees endorsed universal testing, public HIV status disclosure, and segregation of HIV-positive inmates. Intensified education and counseling efforts are needed to ameliorate entrenched HIV-transmission fears and stigmatizing beliefs

What makes me screen for HIV? Perceived barriers and facilitators to conducting recommended routine HIV testing among primary care physicians in the southeastern United States

The Centers for Disease Control and Prevention have recommended routinely testing patients (aged 13-64) for HIV since 2006. However, many physicians do not routinely test. From January 2011 to March 2012, we conducted 18 in-depth individual interviews and explored primary care physicians' perceptions of barriers and facilitators to implementing routine HIV testing in North Carolina. Physicians' comments were categorized thematically and fell into 5 groups: policy, community, practice, physician, and patient. Lack of universal reimbursement was identified as the major policy barrier. Participants believed endorsement from the United States Preventive Services Tasks Force would facilitate adoption of routine HIV testing policies. Physicians reported HIV/AIDS stigma, socially conservative communities, lack of confidentiality, and rural geography as community barriers. Physicians believed public HIV testing campaigns would legitimize testing and decrease stigma in communities. Physicians cited time constraints and competing clinical priorities as physician barriers that could be overcome by delegating testing to nursing staff. HIV test refusal, low HIV risk perception, and stigma emerged as patient barriers. Physicians recommended adoption of routine HIV testing for all patients to facilitate and destigmatize testing. Physicians continue to experience a variety of barriers when implementing routine HIV testing in primary care settings. Our findings support multilevel approaches to enhance physician routine HIV testing in primary care settings. © The Author(s) 2014