A Deep Water Dispersion Experiment in the Gulf of Mexico

<p>The Deep Water Horizon oil spill has dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment (DWDE), which consisted in the release of surface drifters and RAFOS floats drifting at 300 and 1500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated, and decreases with depth. The separation dependence of relative diffusivity follows a Richardson law at all depths. Time dependence of dispersion suggests a Richardson regime near the surface and a mixed Richardson/ballistic regime in depth at scales of [10-100 km]. Finite Scale Lyapunov Exponents and pair separation Kurtosis suggest the existence of a Lundgren regime at scales smaller than the Rossby radius near the surface, and at smaller scales in depth.</p>

Relative Dispersion in the Antarctic Circumpolar Current

Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or non-local, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit non-local dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~ 10days or ~ 20–30km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis and relative diffusivity suggest non-local dispersion as they are consistent with the model within error, while finite size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, 2nd order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave-like flows that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be non-local at scales of 5-100km.

Relative dispersion in the South Western Mediterranean as derived from satellite-tracked surface drifting buoys

Relative dispersion (D2) in the South Western Mediterranean is analyzed using surface drifter pairs deployed during the period from 1986 to 2016. The results show the existence of four well-known regimes. The first regime, characterized by an exponential increment of the relative dispersion (Lundgren or exponential regime), corresponds to the chaotic advection at small scales and small separation distances, lasts for a few days. In the second regime, extending from 1.5 to roughly 7 days, for scales between 25 and 57 km and 1–3 km of initial distance, D2 increases as time cubed (Richardson regime). The third regime occurs for initial distances of 5–10 km and times of 1.5–13 days; D2 increases quadratically with time (Ballistic regime). The forth regime corresponds to time scales larger than 34 days for initial distances of 1–3 km and to 23 days for 35–40 km with a linear increase in time of D2 (Rayleigh or diffusive regime). The relative diffusivity and characteristic dispersion time exhibit three different phases based on the initial pair separations and corresponding with Lundgren, Richardson and Rayleigh regimes, respectively. In the first phase (enstrophy cascade range) the diffusivity is ~ D2 for distances smaller than 15 km and initial separation distances between 5 km and 10 km, and also for distances smaller than 40 km for initial separation distances between 35 km and 40 km; characteristic dispersion time is constant. In the second phase (inverse energy cascade), the diffusivity and characteristic dispersion time increase with growing distances following the 4/3 and 2/3 power laws, respectively, for scale ranging between 3 and 15 km and for initial distances smaller than 3 km. The third phase occurs for distance larger than 55 km, all pair velocities are uncorrelated and both relative diffusivity and characteristic dispersion time are approximately constants.

A DIFFUSIVITY MODEL FOR GAS DIFFUSION IN DRY POROUS MEDIA COMPOSED OF CONVERGING–DIVERGING CAPILLARIES

Gas diffusion in dry porous media has been a hot topic in several areas of technology for many years. In this paper, a diffusivity model for gas diffusion in dry porous media is developed based on fractal theory and Fick’s law, which incorporates the effects of converging–diverging pores and tortuous characteristics of capillaries as well as Knudsen diffusion. The effective gas diffusivity model is expressed as a function of the fluctuation amplitude of the capillary cross-section size variations, the porosity, the pore area fractal dimension and the tortuosity fractal dimension. The results show that the relative diffusivity decreases with the increase of the fluctuation amplitude and increases with the increase of pore area fractal dimension. To verify the validity of the present model, the relative diffusivity from the proposed fractal model is compared with the existing experimental data as well as two available models of Bruggeman and Shou. Our proposed diffusivity model with pore converging–diverging effect included is in good agreement with reported experimental data.

Turbulent dispersion properties from a model simulation of the western Mediterranean Sea

Using a high-resolution primitive equation model of the western Mediterranean Sea, we analyzed the dispersion properties of a set of homogeneously distributed, passive particle pairs. These particles were initially separated by different distances D0 (D0 = 5.55, 11.1 and 16.65 km), and were seeded in the model at initial depths of 44 and 500 m. This realistic ocean model, which reproduces the main features of the regional circulation, puts into evidence the three well-known regimes of relative dispersion. The first regime due to the chaotic advection at small scales lasts only a few days (3 days at 44 m depth, a duration comparable with the integral timescale), and the relative dispersion is then exponential. In the second regime, extending from 3 to 20 days, the relative dispersion has a power law tα where α tends to 3 as D0 becomes small. In the third regime, a linear growth of the relative dispersion is observed starting from the twentieth day. For the relative diffusivity, the D2 growth is followed by the Richardson regime D4/3. At large scales, where particle velocities are decorrelated, the relative diffusivity is constant. At 500 m depth, the integral timescale increases (> 4 days) and the intermediate regime becomes narrower than that at 44 m depth due to the weaker effect of vortices (this effect decreases with depth). The turbulent properties become less intermittent and more homogeneous and the Richardson law takes place.

Simulations of Nearshore Particle-Pair Dispersion in Southern California

Knowledge of horizontal relative dispersion in nearshore oceans is important for many applications including the transport and fate of pollutants and the dynamics of nearshore ecosystems. Two-particle dispersion statistics are calculated from millions of synthetic particle trajectories from high-resolution numerical simulations of the Southern California Bight. The model horizontal resolution of 250 m allows the investigation of the two-particle dispersion, with an initial pair separation of 500 m. The relative dispersion is characterized with respect to the coastal geometry, bathymetry, eddy kinetic energy, and the relative magnitudes of strain and vorticity. Dispersion is dominated by the submesoscale, not by tides. In general, headlands are more energetic and dispersive than bays. Relative diffusivity estimates are smaller and more anisotropic close to shore. Farther from shore, the relative diffusivity increases and becomes less anisotropic, approaching isotropy ~10 km from the coast. The degree of anisotropy of the relative diffusivity is qualitatively consistent with that for eddy kinetic energy. The total relative diffusivity as a function of pair separation distance R is on average proportional to R5/4. Additional Lagrangian experiments at higher horizontal numerical resolution confirmed the robustness of these results. Structures of large vorticity are preferably elongated and aligned with the coastline nearshore, which may limit cross-shelf dispersion. The results provide useful information for the design of subgrid-scale mixing parameterizations as well as quantifying the transport and dispersal of dissolved pollutants and biological propagules.

Turbulent dispersion properties from a model simulation of the western Mediterranean

Using a high resolution primitive equation model of the western Mediterranean Sea, we analyzed the dispersion properties of a set of homogeneously distributed, passive particle pairs. These particles were initially separated by different distances D0 (D0 = 5.55, 11.1 and 16.5 km), and were seeded in the model at initial depths of 44 and 500 m. This realistic ocean model, which reproduces the main features of the regional circulation, puts in evidence the three well-known regimes of relative dispersion. The first regime due to the chaotic advection at small scales, lasts only a few days (3 days at 44 m depth, a duration comparable with the integral time scale) and the relative dispersion is then exponential. In the second regime, extending from 3 to 20 days, the relative dispersion has a power law tα where α tends to 3 as D0 becomes small. In the third regime, a linear growth of the relative dispersion is observed starting from the twentieth day. For the relative diffusivity, the D2 growth is followed by the Richardson regime D4/3. At large scales, where particle velocities are decorrelated, the relative diffusivity is constant. At 500 m depth, the integral time scale increases (> 4 days) and the intermediate regime becomes narrower than that at 44 m depth due to weaker effect of vortices (this effect decreases with depth). The turbulent properties become less intermittent and more homogeneous and the Richardson law takes place.