Swapping trajectories: a new wall-induced cross-streamline particle migration mechanism in a dilute suspension of spheres

Binary encounters between spherical particles in shear flow are studied for a system bounded by a single planar wall or two parallel planar walls under creeping flow conditions. We show that wall proximity gives rise to a new class of binary trajectories resulting in cross-streamline migration of the particles. The spheres on these new trajectories do not pass each other (as they would in free space) but instead they swap their cross-streamline positions. To determine the significance of the wall-induced particle migration, we have evaluated the hydrodynamic self-diffusion coefficient associated with a sequence of uncorrelated particle displacements due to binary particle encounters. The results of our calculations quantitatively agree with the experimental value obtained by \cite{Zarraga-Leighton:2002} for the self-diffusivity in a dilute suspension of spheres undergoing shear flow in a Couette device. We thus show that the wall-induced cross-streamline particle migration is the source of the anomalously large self-diffusivity revealed by their experiments.Comment: submited to JF

Low-frequency suppression of Southern Hemisphere tropospheric eddy heat flux

This paper analyzes the variability of the zonal cospectrum of Southern Hemisphere tropospheric eddy heat flux in reanalysis data. It is shown that the reduced spectral power of low-frequency eddy heat flux variability largely arises from the anticorrelation in the eddy heat transports by different zonal wave numbers. Although the most plausible mechanism for this relation invokes baroclinicity as a mediating agent, this hypothesis does not seem to be supported by the observed variability of baroclinicity. Low-frequency baroclinicity variability is primarily driven by the mean meridional circulation, with only a minor role for the eddy heat flux

The interannual variability of the tropical divergence tilt and its connection with the extratropical circulation

Previous theoretical work has suggested that the strength of the divergent eddy momentum flux in the deep tropics, due to correlations between rotational zonal velocities and divergent meridional velocities, increases with the meridional tilt of the large-scale divergence field. To test that idea, this work investigates the interannual variability of the divergent eddy momentum flux in reanalysis data. Consistent with the theory, it is found that the eddy momentum flux variability is driven by two main parameters: the amplitude of the tropical stationary wave and the tilt of the divergence field. Together, these two parameters account for 80% (90%) of the interannual eddy momentum flux variance during boreal (austral) winter. The interannual variability of these parameters is governed by the internal atmospheric dynamics. During boreal winter, interannual changes in MJO variability explain nearly half of the interannual variance in the stationary wave amplitude, depending on whether on average MJO anomalies interfere constructively or destructively with the stationary wave. The interannual variability of the divergence phase tilt is modulated by tropical–extratropical interactions in the Pacific. The tilt increases during the negative phase of the west Pacific Oscillation associated with a dipole of upper-level divergence (convergence) on the northern (southern) side of the Pacific jet exit region

The role of the divergent circulation for large-scale eddy momentum transport in the tropics. Part II: dynamical determinants of the momentum flux

This paper investigates the coupling between the rotational and divergent circulations aiming to explain the observations that show that the tropical eddy momentum flux is due to correlations between divergent eddy meridional velocities and rotational eddy zonal velocities. A simple linear model in which the observed eddy divergence field is used to force the vorticity equation can reproduce quite well the observed tropical eddy momentum flux. The eddy momentum flux in the model shows little sensitivity to the basic-state winds and is mainly determined by the characteristics of the divergent forcing. Vortex stretching and divergent advection of planetary vorticity produce eddy momentum flux contributions with the same sign but the former forcing dominates. It is shown that the main factor affecting the direction of the eddy momentum flux response to both forcings is the meridional tilt of the divergence phase lines, albeit with an opposite sign to the classical relation between rotational momentum flux and streamfunction phase tilt. How this divergent structure is determined remains an open question

The impact of divergence tilt and meridional flow for cross-equatorial eddy momentum transport in gill-like settings

This work investigates the sensitivity of the cross-equatorial eddy momentum flux and its rotational and divergent components to Hadley cell strength in simple variants of the Gill problem. An expression is derived linking the divergent momentum flux to the mean meridional wavenumber weighted by the spectrum of divergent eddy kinetic energy, supporting the relation between divergence phase tilt and momentum flux suggested by a previous study. Newtonian cooling makes the divergence tilt eastward moving away from the equator as observed, but this tilt is also sensitive to the Hadley cell. As the divergence tilt is enhanced in the downstream direction of the flow, wave propagation increases along that direction when the Hadley cell strengthens. The meridional flow also plays a second, important role for cross-equatorial propagation. With no Hadley cell, inviscid Sverdrup balance requires perfect compensation between the divergent and rotational momentum fluxes at the equator. The model can only produce cross-equatorial propagation when Sverdrup balance is violated, which in the linear, nearly inviscid limit requires vorticity advection by the mean flow. As the Hadley cell attenuates the geopotential tilt imparted by the divergent forcing, the compensation by the rotational momentum flux is reduced. The linear model can reproduce reasonably well previous nonlinear results by Kraucunas and Hartmann when linearized about their zonal-mean climatologies. The sensitivity of the cross-equatorial momentum fluxes to Hadley cell strength in these solutions is dominated by changes in the divergent flux and consistent with diagnosed changes in the divergence tilt

The role of the divergent circulation for large-scale eddy momentum transport in the tropics. Part I: observations

This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

Motion of a rod-like particle between parallel walls with application to suspension rheology

We study the dynamics of elongated axisymmetric particles undergoing shear flow between two parallel planar walls, under creeping-flow conditions. Particles are modeled as linear chains of touching spheres and it is assumed that walls are separated by a distance comparable to particle length. The hydrodynamic interactions of the chains with the walls are evaluated using our Cartesian-representation algorithm Bhattacharya et al., Physica A 356, 294–340 2005b . We find that when particles are far from both walls in a weakly confined system, their trajectories are qualitatively similar to Jeffery orbits in unbounded space. In particular, the periods of the orbits and the evolution of the azimuthal angle in the flow-gradient plane are nearly independent of the initial orientation of the particle. For stronger confinements, however, i.e., when the particle is close to one or both walls a significant dependence of the angular evolution on the initial particle configuration is observed. The phases of particle trajectories in a confined dilute suspension subject to a sudden onset of shear flow are thus slowly randomized due to unequal trajectory periods, even in the absence of interparticle hydrodynamic interactions. Therefore, stress oscillations associated with initially coherent particle motions decay with time. The effect of near contact particle-wall interactions on the suspension behavior is also discussed

Explosive cyclones in the North Atlantic: NAO influence and multidecadal variability

Ponencia presentada en: VIII Congreso de la Asociación Española de Climatología celebrado en Salamanca entre el 25 y el 28 de septiembre de 2012.[EN]In this study we have analyzed the variability of explosive cyclones affecting Europe at different timescales. Cyclones have been identified and tracked through an automatic algorithm that has been applied to the MSLP NCEP reanalysis data. Subsequently, explosive cyclones affecting Europe have been selected from the whole climatology of extratropical North Atlantic cyclones (406 total cases from Oct-Mar 1950-2010).[ES]En este estudio se ha analizado la variabilidad en diferentes escalas temporales de las ciclogénesis explosivas que afectan Europa. La identificación y seguimiento de ciclones se ha realizado a través de la utilización de un algoritmo automático aplicado sobre los datos de MSLP del reanálisis de NCEP. En total, 406 casos de ciclogénesis explosivas que han afectado Europa han sido seleccionados (Oct-Mar 1950-2010).This study has been partially supported by the Spanish National projects DE VIAJE (CGL2009-06944), TRACS (CGL2009-10285) and the UCM-BSCH GR58/08 “Micrometeorology and Climate variability” group

Inhomogeneous potential vorticity homogenization and equilibrium in simple models of baroclinic instability with implications for the extratropical circulation by Pablo Zurita-Gotor.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003.Includes bibliographical references (p. 199-205).Baroclinic eddies are an important component of the General Circulation which regulate the extratropical climate by transporting heat and momentum. An idealization of this feedback is provided by baroclinic adjustment theories (Stone, 1978), which envision a linearly neutralized mean state. Based on the Charney-Stern condition, most baroclinic adjustment formulations propose basic states with homogenized potential vorticity. In this thesis we investigate the degree of potential vorticity homogenization in the extratropical troposphere. We show that homogenization is only observed across a shallow region around 700 mb, and propose an adjusted state with homogenized PV at the steering level alone. We demonstrate that this state can be neutral under certain conditions, and investigate its relevance for the equilibration of an idealized model. Because of the role of the PV flux as an eddy forcing of momentum, it is illuminating to describe the equilibration in terms of the redistribution of momentum. This affects both the PV gradient and the steering level of the waves, but the condition of homogenization at the steering level is very robust. In the 2D problem, a local balance can be written between the dynamical and frictional forcing of momentum. However, in the 3D problem there is an additional redistribution by a remotely forced meridional circulation. To circumvent this difficulty, we have developed a momentum-based formulation that exploits the interchangeability of momentum and temperature for quasi-balanced stratified rotating flow. By rewritting the thermodynamic equation as a momentum equation, we eliminate the forcing by the mean meridional circulation and formulate a local balance between the eddy PV flux and the non-conservative forcing of momentum.(cont.) This introduces a new variable, which we call potential momentum. The circulation can then be described in terms of the conversion between potential and physical momentum. A major simplification of this formulation is that temperature and momentum can be directly compared. For instance, the surface temperature gradient appears as a momentum source, which helps ellucidate the role of the momentum fluxes and the so-called "barotropic governor" (James, 1987) for the baroclinic equilibration. Our results suggest that mechanical friction might prevent thermal homogenization at the surface.Ph.D