Andes Simulation Resolution Study

This page describes a set of numerical simulations at progressively coarser resolution to compare with the Andes07 results, which use 500 m isotropic mesh spacing. Four new simulations were conducted, having horizontal resolutions of 1000, 2000, 4000 and 8000 m.

This series utilizes an identical vertical mesh as the original Andes07 run.

Mesh Comparison

Horizontal Spacings

Each computational grid uses the same strategy as in Andes07 with uniform high resolution used over the Andes and then progressively coarser mesh in the horizontal directions when moving towards the lateral boundaries. The white lines in the image below show the region of uniform spacing and this is identical to what was used in the Andes07 simulation. The only difference is that the horizontal spacings in the high-resolution zone is increased from 500 m in the Andes07 simulation to 1000, 2000, 4000, and 8000 m here. The dotted white line in the image below indicates the location of the x-z views shown below.

Region of uniform mesh (solid lines) and x-z plane location (dotted line). (Δ 4000 m case).

Vertical Spacing

Identical to Andes07.

Each computational domain extends to an altitude of 200 km and uses 500 m resolution at the surface to an altitude of 144 km. Very gentle stretching is used to the top of the domain.

Stretching

The the rate at which the mesh is stretched in all three directions is approximately equal ~1.6%, which is nearly identical to what was used in the Andes07 run.

Wind and Thermodynamic Profiles

Forcing

Forcing terms gradually introduce winds near the surface with the objective of achieving the wind profile within a twelve hour period. A hyperbolic tangent function is used in order to produce gentle acceleration of the wind near the beginning and end of the forcing period. The maximum forcing rate is equivalent to that of a linear ramp with a duration of four hours.

The wind profile and forcing strategy are idetical to what was done in the Andes07 run.

Results in xz Plane over the South Peak

Results in yz Plane at x = 50km

Results in xy Planes at Altitudes 50km, 70km, 100km, 150km, 180km

50 km

North Peak 2km Anomaly over the Interval 8.0-9.0 hr

Upon futher investigation and analysis it was determined that the 2km case produces a large-scale coherent vortex that extends down to 50 km as turbulence starts to develop during the 8.0-9.0 hr. time period. The vortex is most evidient at a time of 8.5 hr., and the solutions for the 0.5 km and 2 km resolution cases are shown in the images below. As expected, the 0.5 km resolution case produces much smaller-scale turbulent motions with no coherent large-scale vortices present. The 1 km resolution case (not shown) is similar, but with less small-scale detail (but still no large-scale vortex).

Vorticity Magnitude for an xz plane over the North Peak (line at 50km)

The choherent vortex in the 2 km case gives rise to large momentum fluxes and these superimpose with the GW momentum fluxes to create the anomoly in the momentum flux seen in the plot for the 8-9 hr. averaging window above. The figures below showing traces of u', w', and u'w' over the North Peak at an altitude of 50 km demonstrate the enhanced momentum flux due to the vortex in the 2 km case. The abrupt sign change in w' at x~50 km is an indication of the vortex in the 2km case. Meanwhile u' for the 2 km case is roughly constant at a minimum value around x=50 km, with the net result that u'w' also undergoes a sign change close to x=50 km and achieves large peak values on either side of the zero crossing.