Large Eddy Simulation of Particle Settling in the Ocean Mixed Layer

The estimation of the particle flux from the upper ocean plays a critical role in the global carbon cycle modeling. However, no direct information is yet available with regard to the particle settling process in the Ocean Mixed Layer (OML) and mean concentrations of particles are usually calculated with a diffusion equation. Usually it is assumed that the eddy diffusivity is the same as that of the fluid and that the settling velocity remains invariant from that in the still fluid (e.g. Lande and Wood 1987). However, there are many evidences contrary to these assumptions (e.g. Maxey and Riley 1986). Therefore, we investigated the settling process of suspended particles within the OML by means of Large-Eddy Simulation (LES). We used the LES model PALM (PArallel LES Model), which has been used extensively for atmospheric simulations (e.g. Raasch and Harbusch 2001), the ocean deep convection (Noh et al. 2003) and which recently has been modified to allow for the simulation of the OML, including wave breaking and Langmuir circulation.

In all simulations we used neutral thermal stratification, free-slip surface boundary condition and a wind stress in x-direction given by a constant friction velocity. We generated two different types of OML - with and without Langmuir Circulation. The latter was realized by the modification of the equation of motion with an additional vortex force and the Stokes velocity, which is caused by a steady and monochromatic wave field.

After a simulation time of 8 h the turbulence was fully developed and we added a large number of Lagrangian particles to the Eulerian turbulent flow field. The particles were assumed to be solid, spherical and small, so that only Stokes drag acts on the particles. They are characterized by size and a density larger than the fluid density, resulting in an inertial response time and a terminal settling velocity.

The particles were released close to the surface and uniformly in horizontal direction in order to investigate the time dependent settling behavior. We investigated the motions and settling behavior of two types of particles that differ in the terminal settling velocity. In particular, we were interested in how long particles are sustained in the OML before escaping to the deep ocean.

Flow field: Without Langmuir circulation the strength of turbulent elements caused by the surface stress decreases with depth. The lower half of the OML only experiences minor perturbations. However, the structure of the OML is significantly altered by Langmuir circulations. Strong parallel downwelling zones beneath the surface convergence zones are associated with the formation of Langmuir cells. Near the surface the stripes are orientated along the direction of the wind stress, whereas with increasing depth the direction of the stripes spirals clockwise towards a diagonal orientation due to the Coriolis effect. The fully developed Langmuir circulation has a vertical extent comparable to the OML depth.

Particle behavior: In cases without consideration of Langmuir circulation, particles in the lower part of the OML still have a mean downward velocity and finally leave the OML. A drastic change in the pattern of particle settling occurs in the presence of Langmuir circulation whose downward jets quickly carry particles along with them. After a short time, the particles are distributed uniformly throughout the OML. As a consequence, Langmuir circulation accelerates the downward dispersion of particles initially, but ultimately delays the escape from the OML into the deep ocean. The percentage of particles that stay within the OML is remains higher.

In all experiments, the mean particle settling velocity is smaller than the theoretical terminal settling velocity. The magnitude of the decrease is larger in the presence of Langmuir circulation as particles spend more time in the upward parts of the Langmuir cell structure. The variance of the vertical distribution of particles and the probability distribution of the vertical particle velocity component show the significant effects of the Langmuir circulation and the terminal particle velocity.