LES preliminary results

Here are the results of two preliminary simulations performed by the Large-Eddy Simulation (LES). They are presented as animations under mpeg (compact but poor quality and no frame rate control) or gif format (larger, but good quality at 6 fps).

Common features to all simulations

All simulations include cabelling and thermobaricity in the equation of state. The resolution is 6.25x6.25x5.93m. The bottom boundary condition is slip, well below the initial mixed layer. The surface boundary is slip, with a rigid lid and an ice cover.
A thermodynamic ice model has been implemented at the surface, based on the model described by McPhee (1990). Heat, salinity and momentum fluxes are calculated from u* deduced from ANZFLUX measurements, along with the evolution of ice thickness due to freezing and melting.

The box has 64x64x64 grid points and time step is 6s. The model is initialized with one of the least thermobarically stable profile measured near Maud Rise at yo-yo cast YUO75 of the 1994 ANZFLUX Weddell Sea project, the object being to reproduce the 1-D model results of McPhee (2000) . This profile shows a 3-layer system, with 2 interfaces at 100m and 200m depth (fig.6 of the article).
Problems with initial stability of the initial profile in McPhee (2000) led us to shift the salinity profile by 1m downwards. This removed the density inversion at 100m, and produced an agreement between the T-S characteristics of this profile and those also measured in this area by Stanton (personal communication).


Simulation 1

This simulation, like four other variants which applied a momentum flux at the surface, grew to be too warm in the upper layer, but still showed interesting patterns. This version is not as bad as the others because the 'Stanton' number c_h used to determine heat flux is adjusted so that delta-T is between the bottom of the ice and the average of the uppermost model grid, rather than the average mixed layer temperature. Simulation 2 below also uses this augmented coefficient (about 1.4x for our dz), which is recalculated by assuming a log profile of the temperature just below the ice.

Potential temperature is shown on 2 vertical sections, and interfaces are characterized by 2 isothermal surfaces at 0°C (above) and 0.18°C (below). The shear stress driven by surface momentum flux feeds the growth of large internal waves at the first interface which in turn leads to excessive vertical mixing. We believe the most likely solution to this divergence between model and observations is either that the non-slip surface where the momentum flux is uniformly applied does not adequately dissipate the internal wave motions, or that the profile from YU075 cannot be used to uniformly initialize the water column. We are currently reworking this upper boundary condition, but in the meantime we offer a less realistic simulation below which sidesteps this problem.
MPEG Movie (256Kb)
GIF Movie (2.13MB)

Simulation 2


In this simulation, no momentum flux is transferred into the mixed layer. This is not realistic, but it was easier than altering the slip surface boundary condition.
Again, potential temperature is shown on 2 vertical sections, and interfaces are characterized by the isothermal surfaces at 0°C (above) and 0.18°C (below). This simulation was numerically stable, and convective plumes hit the bottom of the modelled box after 7.5 simulated days.
MPEG Movie (468Kb)
GIF Movie (3.64Mb)


Maintained by Arlene Guest