Testing the assumptions underlying ocean mixing methodologies using direct numerical simulations

AbstractDirect numerical simulations of stratified turbulence are used to test several fundamental assumptions involved in the Osborn, Osborn–Cox, and Thorpe methods commonly used to estimate the turbulent diffusivity from field measurements. The forced simulations in an idealized triply periodic computational domain exhibit characteristic features of stratified turbulence including intermittency and layer formation. When calculated using the volume-averaged dissipation rates from the simulations, the vertical diffusivities inferred from the Osborn and Osborn–Cox methods are within 40% of the value diagnosed using the volume-averaged buoyancy flux for all cases, while the Thorpe-scale method performs similarly well in the simulation with a relatively large buoyancy Reynolds number (Reb ≃ 240) but significantly overestimates the vertical diffusivity in simulations with Reb &lt; 60. The methods are also tested using a limited number of vertical profiles randomly selected from the computational volume. The Osborn, Osborn–Cox, and Thorpe-scale methods converge to their respective estimates based on volume-averaged statistics faster than the vertical diffusivity calculated directly from the buoyancy flux, which is contaminated with reversible contributions from internal waves. When applied to a small number of vertical profiles, several assumptions underlying the Osborn and Osborn–Cox methods are not well supported by the simulation data. However, the vertical diffusivity inferred from these methods compares reasonably well to the exact value from the simulations and outperforms the assumptions underlying these methods in terms of the relative error. Motivated by a recent theoretical development, it is speculated that the Osborn method might provide a reasonable approximation to the diffusivity associated with the irreversible buoyancy flux.</jats:p

Public Beach Assessment Report Aqua-PO Stafford county, Virginia

The Aqua-Po Public Beach report is the first in a series of reports on the public beaches in the Commonwealth of Virginia. There are 14 localities with public beaches totaling about 23 miles of shoreline. The public beach reports are an assessment of the history of each public beach and their current status in terms of loss of beach fill and maintenance needed. This is measured by the beach monitoring program sponsored by the Virginia Board on the Conservation and Development of Public Beaches

Clostridium perfringens Epsilon Toxin Compromises the Blood-Brain Barrier in a Humanized Zebrafish Model

This is the final version. Available on open access from Elsevier vis the DOI in this recordClostridium perfringens epsilon toxin (ETX) is hypothesized to mediate blood-brain barrier (BBB) permeability by binding to the myelin and lymphocyte protein (MAL) on the luminal surface of endothelial cells (ECs). However, the kinetics of this interaction and a general understanding of ETX's behavior in a live organism have yet to be appreciated. Here we investigate ETX binding and BBB breakdown in living Danio rerio (zebrafish). Wild-type zebrafish ECs do not bind ETX. When zebrafish ECs are engineered to express human MAL (hMAL), proETX binding occurs in a time-dependent manner. Injection of activated toxin in hMAL zebrafish initiates BBB leakage, hMAL downregulation, blood vessel stenosis, perivascular edema, and blood stasis. We propose a kinetic model of MAL-dependent ETX binding and neurovascular pathology. By generating a humanized zebrafish BBB model, this study contributes to our understanding of ETX-induced BBB permeability and strengthens the proposal that MAL is the ETX receptor.National Institute of Neurological Disorders and StrokeWilma S. and Laurence A. Tisch Foundatio

Churchman\u27s Inquiring Systems: Kernel Theories for Knowledge Management

Churchman [1971] defines inquiry as an activity that produces knowledge. He examines the epistemologies of five schools of philosophy from the perspective of general systems theory, asking the question as to whether each is suitable as the basis for the design of computer-based inquiring systems. He considers systems design and design theory in some detail. We believe that Churchman\u27s inquiring systems can form the basis for the design of knowledge management systems and that the IS research community has hardly tapped the potential of inquiring systems in that regard. Mason and Mitroff [1973] brought inquiring systems into the IS literature early on, essentially making the work endogenous to the field. We argue that building on inquiring systems can contribute to developing IS as a discipline by maintaining continuity in research and developing a theory that IS can call its own. We believe that the lack of use of Churchman\u27s work may be due to its lack of visibility in recent years and attempt to remedy that by summarizing the basics of the inquirers in some detail, trying not to interpret, but to remain faithful to the original. The paper encourages readers to study the original and develop their own notion of how the inquirers might be used in knowledgemanagement work. There are probably as many different perspectives on how inquiring systems could support KMS as there are IS researchers willing to study them. We would like to encourage a proliferation of such perspectives

The Star Formation in Radio Survey: Jansky Very Large Array 33 GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-Forming Regions

We present 33 GHz imaging for 112 pointings towards galaxy nuclei and extranuclear star-forming regions at $\approx$2" resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation in Radio Survey. A comparison with 33 GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover $78\pm4$ of the total flux density over 25" regions ($\approx$ kpc-scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the $\approx30-300$ pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete HII regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii $r_{\rm G} \geq 250$ pc) and detected at $>3\sigma$ significance at 33 GHz and in H$\alpha$. Assuming a typical 33 GHz thermal fraction of 90 %, the ratio of optically-thin 33 GHz-to-uncorrected H$\alpha$ star formation rates indicate a median extinction value on $\approx30-300$ pc scales of $A_{\rm H\alpha} \approx 1.26\pm0.09$ mag with an associated median absolute deviation of 0.87 mag. We find that 10 % of these sources are "highly embedded" (i.e., $A_{\rm H\alpha}\gtrsim3.3$ mag), suggesting that on average HII regions remain embedded for $\lesssim1$ Myr. Finally, we find the median 33 GHz continuum-to-H$\alpha$ line flux ratio to be statistically larger within $r_{\rm G}<250$ pc relative the outer-disk regions by a factor of $1.82\pm0.39$, while the ratio of 33 GHz-to-24 $\mu$m flux densities are lower by a factor of $0.45\pm0.08$, which may suggest increased extinction in the central regions.E.J.M. acknowledges the hospitality of the Aspen Center for Physics, which is supported by National Science Foundation grant No. PHY-1066293. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com

Diapycnal mixing in layered stratified plane Couette flow quantified in a tracer-based coordinate

The mixing properties of statically stable density interfaces subject to imposed vertical shear are studied using direct numerical simulations of stratified plane Couette flow. The simulations are designed to investigate possible self-maintaining mechanisms of sharp density interfaces motivated by Phillips’ argument (Deep-Sea Res., vol. 19, 1972, pp. 79–81) by which layers and interfaces can spontaneously form due to vertical variations of diapycnal flux. At the start of each simulation, a sharp density interface with the same initial thickness is introduced at the midplane between two flat, horizontal walls counter-moving at velocities ±$U_w$. Particular attention is paid to the effects of varying Prandtl number $Pr$ ≡ ν/κ, where ν and κ are the molecular kinematic viscosity and diffusivity respectively, over two orders of magnitude from 0.7, 7 and 70. Varying $Pr$ enables the system to access a considerable range of characteristic turbulent Péclet numbers $Pe_∗$ ≡ $U_∗$$L_∗$/κ, where $U_∗$ and $L_∗$ are characteristic velocity and length scales, respectively, of the motion which acts to ‘scour’ the density interface. The dynamics of the interface varies with the stability of the interface which is characterised by a bulk Richardson number $Ri$ ≡ $b_0$$h$/$U^2_w$, where $b_0$ is half the initial buoyancy difference across the interface and $h$ is the half-height of the channel. Shear-induced turbulence occurs at small $Ri$, whereas internal waves propagating on the interface dominate at large $Ri$. For a highly stable (i.e. large $Ri$) interface at sufficiently large $Pe_∗$, the complex interfacial dynamics allows the interface to remain sharp. This ‘self-sharpening’ is due to the combined effects of the ‘scouring’ induced by the turbulence external to the interface and comparatively weak molecular diffusion across the core region of the interface. The effective diapycnal diffusivity and irreversible buoyancy flux are quantified in the tracer-based reference coordinate proposed by Winters & D’Asaro (J. Fluid Mech., vol. 317, 1996, pp. 179–193) and Nakamura (J. Atmos. Sci., vol. 53, 1996, pp. 1524–1537), which enables a detailed investigation of the self-sharpening process by analysing the local budget of buoyancy gradient in the reference coordinate. We further discuss the dependence of the effective diffusivity and overall mixing efficiency on the characteristic parameters of the flow, such as the buoyancy Reynolds number and the local gradient Richardson number, and highlight the possible role of the molecular properties of fluids on diapycnal mixing

Anticyclonic precession of a plume in a rotating environment

Motivated by potential effects of the Earth's rotation on the Deepwater Horizon oil plume, we conducted laboratory experiments on salt-water point plumes in a homogeneous rotating environment across a wide range of Rossby numbers $0.02<Ro<1.3$. We report a striking physical instability in the plume dynamics near the source: after approximately one rotation period, the plume tilts laterally and starts to precess anticyclonically. The mean precession frequency $\bar\omega$ scales linearly with the rotation rate $\Omega$ as $\bar\omega\approx0.4\Omega$. We find no evidence of a critical Rossby number above which precession ceases. We infer that a conventionally defined Rossby number is not an appropriate parameter when the plume is maintained over a long time: provided $\Omega \neq 0$, rotation is always important to the dynamics. This indicates that precession may occur in persistent oceanic or atmospheric plumes even at low latitudes