The sponge layer

in a nutshell

The sponge layer is necessary to damp vertically propagating waves, which would otherwise non-physically be reflected downward from the model top. In practice its application is thus limited to a few of the topmost layers (usually 4), with decreasing impact with decreasing altitudes. The sponge layer is a relaxation of main variables (winds and/or potential temperature) towards a given value or their zonal mean along a given time scale.

Sponge layer parameters

The parameters controlling the sponge layer are in input file gcm.def (or sometimes in vert_L**.def as they are strongly linked to the vertical grid)

• iflag_top_bound (integer): 0 for no sponge, 1 for a sponge over the last 4 topmost layers, 2 to have a sponge layer extending over topmost layers down to a pressure which is 100 times that of the topmost layer's.
• mode_top_bound (integer): 0 for no relaxation, 1 to relax winds (u,v) to zero, 2 to relax winds (u,v) towards their zonal mean, 3 to relax winds (u,v) and potential temperature towards their zonal mean.
• tau_top_bound (real, Hz): inverse of the relaxation characteristic time scale at the topmost layer; which is then halved at each successive descending layer.

Application of the sponge

The sponge is applied right after dissipation at each dissipation step.

01/12/2021