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Salort J.,CNRS Neel Institute | Baudet C.,Laboratoire des Ecoulements Geophysiques et Industriels | Castaing B.,Ecole Normale Superieure de Lyon | Chabaud B.,CNRS Neel Institute | And 11 more authors.
Physics of Fluids | Year: 2010

We present velocity spectra measured in three cryogenic liquid H4e steady flows: grid and wake flows in a pressurized wind tunnel capable of achieving mean velocities up to 5 m/s at temperatures above and below the superfluid transition, down to 1.7 K, and a "chunk" turbulence flow at 1.55 K, capable of sustaining mean superfluid velocities up to 1.3 m/s. Depending on the flows, the stagnation pressure probes used for anemometry are resolving from one to two decades of the inertial regime of the turbulent cascade. We do not find any evidence that the second-order statistics of turbulence below the superfluid transition differ from the ones of classical turbulence, above the transition. © 2010 American Institute of Physics.

Gliere A.,CEA Grenoble | Roux J.-M.,CEA Grenoble | Achard J.-L.,Laboratoire des Ecoulements Geophysiques et Industriels
Microfluidics and Nanofluidics | Year: 2013

A conducting drop in partial wetting regime, placed on the lower electrode of a parallel-plate capacitor and surrounded by a dielectric fluid, is considered. The drop, initially flattened by gravity, is elongated by the electrostatic force and possibly lifts-off when a uniform DC electric field is applied. The electrostatic force and the lift-off condition were calculated in two previous articles, respectively, for undeformable and for slightly deformable drops in the absence of gravity (zero Eötvos number). In this paper, numerical models are put to work to study accurately the complex lift-off process resulting from the competition between gravitational, electrical and capillary forces. Large deformations of the drop surface at any value of the Eötvos number may be addressed by such a numerical procedure. Computational results allow assessing the accuracy and limits of previous analytical and asymptotic relations. © 2013 Springer-Verlag Berlin Heidelberg.

Chemel C.,University of Hertfordshire | Staquet C.,Laboratoire des Ecoulements Geophysiques et Industriels | Chollet J.-P.,Laboratoire des Ecoulements Geophysiques et Industriels
Nonlinear Processes in Geophysics | Year: 2010

Entrainment at the top of the convectively-driven boundary layer (CBL) is revisited using data from a highresolution large-eddy simulation (LES). In the range of values of the bulk Richardson number RiB studied here (about 15-25), the entrainment process is mainly driven by the scouring of the interfacial layer (IL) by convective cells. We estimate the length and time scales associated with these convective cells by computing one-dimensional wavenumber and frequency kinetic energy spectra. Using a Taylor assumption, based upon transport by the convective cells, we show that the frequency and wavenumber spectra follow the Kolmogorov law in the inertial range, with the multiplicative constant being in good agreement with previous measurements in the atmosphere. We next focus on the heat flux at the top of the CBL, Fi , which is parameterized in classical closure models for the entrainment rate we at the interface. We show that Fi can be computed exactly using the method proposed by Winters et al. (1995), from which the values of a turbulent diffusivity K across the IL can be inferred. These values are recovered by tracking particles within the IL using a Lagrangian stochastic model coupled with the LES. The relative difference between the Eulerian and Lagrangian values of K is found to be lower than 10%. A simple expression of we as a function of K is also proposed. Our results are finally used to assess the validity of the classical firstorder model for we. We find that, when RiB is varied, the values for we derived from the first-order model with the exact computation of Fi agree to better than 10% with those computed directly from the LES (using its definition). The simple expression we propose appears to provide a reliable estimate of we for the largest values of RiB only. © 2010 Author(s).

Jullien S.,French National Center for Space Studies | Menkes C.E.,Institute Of Recherche Pour Le Developpement | Menkes C.E.,University Pierre and Marie Curie | Marchesiello P.,French National Center for Space Studies | And 8 more authors.
Journal of Physical Oceanography | Year: 2012

The present study investigates the integrated ocean response to tropical cyclones (TCs) in the South Pacific convergence zone through a complete ocean heat budget. The TC impact analysis is based on the comparison between two long-term (1979-2003) oceanic simulations forced by a mesoscale atmospheric model solution in which extreme winds associated with cyclones are either maintained or filtered. The simulations provide a statistically robust experiment that fills a gap in the current modeling literature between coarse-resolution and short-term studies. The authors' results show a significant thermal response of the ocean to at least 500-m depth, driven by competing mixing and upwelling mechanisms. As suggested in previous studies, vertical mixing largely explains surface cooling induced by TCs. However, TC-induced upwelling of deeper waters plays an unexpected role as it partly balances the warming of subsurface waters induced by vertical mixing. Below 100 m, vertical advection results in cooling that persists long after the storm passes and has a signature in the ocean climatology. The heat lost through TC-induced vertical advection is exported outside the cyclogenesis area with strong interannual variability. In addition, 60% of the heat input below the surface during the cyclone season is released back to the oceanic mixed layer through winter entrainment and then to the atmosphere. Therefore, seasonal modulation reduces the mean surface heat flux due to TCs to about 331023 PW in this region exposed to 10%-15% of the world's cyclones. The resulting climatological anomaly is a warming of about 0.18°C in the subsurface layer and cooling below the thermocline (less than 0.18°C). © 2012 American Meteorological Society.

Krysta M.,Laboratoire des Ecoulements Geophysiques et Industriels | Krysta M.,Laboratoire Jean Kuntzmann | Blayo E.,Laboratoire Jean Kuntzmann | Cosme E.,Laboratoire des Ecoulements Geophysiques et Industriels | Verron J.,Laboratoire des Ecoulements Geophysiques et Industriels
Monthly Weather Review | Year: 2011

In the standard four-dimensional variational data assimilation (4D-Var) algorithm the background error covariance matrix B remains static over time. It may therefore be unable to correctly take into account the information accumulated by a system into which data are gradually being assimilated. A possible method for remedying this flaw is presented and tested in this paper. A hybrid variationalsmoothing algorithm is based on a reduced-rank incremental 4D-Var. Its consistent coupling to a singular evolutive extended Kalman (SEEK) smoother ensures the evolution of the B matrix. In the analysis step, a low-dimensional error covariance matrix is updated so as to take into account the increased confidence level in the state vector it describes, once the observations have been introduced into the system. In the forecast step, the basis spanning the corresponding control subspace is propagated via the tangent linear model. The hybrid method is implemented and tested in twin experiments employing a shallow-water model. The background error covariance matrix is initialized using an EOF decomposition of a sample of model states. The quality of the analyses and the information content in the bases spanning control subspaces are also assessed. Several numerical experiments are conducted that differ with regard to the initialization of the B matrix. The feasibility of the method is illustrated. Since improvement due to the hybrid method is not universal, configurations that benefit from employing it instead of the standard 4D-Var are described and an explanation of the possible reasons for this is proposed. © 2011 American Meteorological Society.

Flor J.B.,Laboratoire des Ecoulements Geophysiques et Industriels | Hopfinger E.J.,Laboratoire des Ecoulements Geophysiques et Industriels | Guyez E.,Laboratoire des Ecoulements Geophysiques et Industriels
Journal of Geophysical Research: Oceans | Year: 2010

The wind blowing over the water surface causes, even for low wind speeds, Langmuir circulation in addition to shear-generated turbulence. Both of these mechanisms mix the upper layer of oceans and lakes, but since the mixing efficiency of Langmuir cells is unknown, their relevance to mixed layer deepening is still an open question. In order to estimate the contribution to mixing by Langmuir vortex cells relative to shear-induced mixing, we employ results on entrainment rate obtained from laboratory experiments with turbulent Taylor vortex cells. These are coherent horizontal vortices analogous to Langmuir cells. To relate the two, we define a surface friction velocity u * that would be necessary to drive cells of strength equivalent to the Taylor vortices. It is then shown that up to a friction Richardson number of Ri* ≈ 50, layer deepening is predominantly caused by shear-generated turbulence, whereas for Ri* > 50, the contribution by turbulent coherent vortices (Langmuir cells) dominates the mixing process. For Richardson numbers Ri* > 120, the entrainment rate decreases, but there is no criterion for the arrest of mixing by turbulent Langmuir cells as was previously suggested. The present results confirm observations that shear-generated turbulence dominates during initial layer deepening under relatively weak buoyancy effects and that subsequently Langmuir cell mixing dominates the mixed layer deepening. The entrainment rates obtained from laboratory experiments are discussed and predict values for the mixed layer deepening that are in good agreement with in situ observations. Copyright 2010 by the American Geophysical Union.

Verhille G.,Ecole Normale Superieure de Lyon | Plihon N.,Ecole Normale Superieure de Lyon | Bourgoin M.,Laboratoire des Ecoulements Geophysiques et Industriels | Odier P.,Ecole Normale Superieure de Lyon | Pinton J.-F.,Ecole Normale Superieure de Lyon
New Journal of Physics | Year: 2010

We study magnetohydrodynamics in a von Kármán flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impellers' electric and magnetic characteristics differ from that of the fluid. Implications with regard to the VKS dynamo are discussed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Dufour C.O.,Laboratoire des Ecoulements Geophysiques et Industriels | Le Sommer J.,Laboratoire des Ecoulements Geophysiques et Industriels | Penduff T.,Laboratoire des Ecoulements Geophysiques et Industriels | Penduff T.,Florida State University | And 2 more authors.
Journal of Physical Oceanography | Year: 2011

The subsurface variability of potential temperature and salinity south of Australia along 130°E is studied over a 25-yr period (1980-2004). The study is done with fields provided by a global eddy-permitting model of the DRAKKARproject forced by atmospheric reanalysis. The analysis performed by C. Sun and D. R. Watts with in situ hydrographic data is repeated. Sun and Watts have investigated the EOF modes in streamfunction space along the World Ocean Circulation Experiment (WOCE) SR3 section. In particular, they found that an EOF mode, which they called the "pulsation mode," strongly dominates subsurface thermohaline variations. Here, it is found that, in the model, an EOF mode with spatial structure similar to the Sun and Watts pulsation mode dominates subsurface thermohaline variations in streamfunction space. The mode displays a maximum of variability at the Subantarctic Front (SAF) between Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The associated time series exhibits an intermittent interseasonal frequency (3-6 months), especially during three periods (1983-84, 1990, and 1994-96). Some energy is also found with a 4-yr period. Further analyses reveal that the pulsation mode can also be observed in physical space. The pulsation mode is found to be related to movements of the SAF constrained by the bathymetry of the Southeast Indian Ridge. The pulsation mode displays many similarities with cold-core eddy events rather than being related to variations of the westerly wind stress, as previously proposed. The impact of those events on SAMW properties remains unclear. © 2011 American Meteorological Society.

Scott R.B.,University of Texas at Austin | Scott R.B.,French National Center for Scientific Research | Ferry N.,Mercator Ocean | Drevillon M.,Mercator Ocean | And 6 more authors.
Ocean Dynamics | Year: 2012

We compared the estimates of surface drifter trajectories from 1 to 7 days in the equatorial Atlantic over an 18-month period with five eddying ocean general circulation model (OGCM) reanalyses and oneobservational product. The cumulative distribution of trajectory error was estimated using over 7,000 days of drifter trajectories. The observational product had smaller errors than any of the individual OGCM reanalyses. Three strategies for improving trajectory estimates using the ensemble of five operational ocean analysis and forecasting products were explored: two methods using a multi-model ensemble estimate and also spatial low-pass filtering. The results were insensitive to the method used to create the ensemble estimates, and by most measures, the results were better than the observational product. Comparison of relative skill of the various OGCM reanalyses suggested promising avenues for exploration for further improvements: forcing with higher frequency wind stress and quality control of input data. One of the lowest horizontal resolution OGCMs, with 1/4° longitude horizontal resolution, made the best trajectory estimates. The individual OGCMs were dominated by errors at spatial scales smaller than about 100 to 200 km, i.e., less than the local deformation radius. But buried in those errors were valuable signals that could be retrieved by combining all the OGCM velocity fields to produce a multimodel ensemble-based estimate. This estimate had skill down to spatial scales about 75 km. Results from this study are consistent with previous work showing that ensemble-mean forecast skill is superior to individual forecasts. © Springer-Verlag 2012.

Juza M.,Laboratoire des Ecoulements Geophysiques et Industriels | Penduff T.,Laboratoire des Ecoulements Geophysiques et Industriels | Penduff T.,Florida State University | Brankart J.-M.,Laboratoire des Ecoulements Geophysiques et Industriels | Barnier B.,Laboratoire des Ecoulements Geophysiques et Industriels
Journal of Operational Oceanography | Year: 2012

This global study evaluates how the varying geometry of the Argo array of profiling floats has affected the actual distributions of mixed layer depth (MLD), temperature (MLT) and heat content (MLHC) annual cycles between 2004 and 2009. These quantities' monthly distributions are computed regionally from a global 1/4° simulation with and without Argo-like subsampling, and the subsequent medians are compared. Argo-like subsampling is shown to bias the medians of MLD, MLT and MLHC distributions by about ±10m, ±1°C, ±1GJ/m2, respectively, with maximum values reaching ±100m, ±5°C, ±5GJ/m2 in certain regions and months. MLD distributions are most distorted where and when the array geometry is irregular, and where MLD distributions are far from Gaussian. The differences between medians of subsampled and fully-sampled distributions are also compared to the actual width of fully-sampled MLHC distributions in every monthly regional bin to evaluate the intrinsic accuracy of the array. Comparing results from several periods (2004-2005, 2006-2007 and 2008-2009), it is shown that Argo-based estimates of mixed layer statistics have improved when the array reached its target density at the end of 2007.

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