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Gent-McWilliams isopycnic tracer diffusion

Gent-McWilliams (gent) mixing operates only on tracer species (potential temperature, salinity and other tracers), so it should be used in conjunction with a different option for hmix_momentum_choice, typically either del2 or aniso. No bi-harmonic form of gent has been developed and accepted yet, so it is appropriate to use the del2 values of ah. For vertical dependence of the mixing, a profile with the form κ1 + κ2 exp(-z/D) can be chosen, where D is a depth scale, z is model depth and κ1 and κ2 parameters specifiy factors multiplying the diffusivity. Note that this function is multiplied by the diffusivity ah; for a constant κ the first parameter should be set to 1 and the second to 0. Two diffusivities can be specified for the Redi and bolus parts of the GM parameterization; ah is used for the Redi part, ah_bolus is used for the bolus part. Two different maximum slopes can also be specified to allow different taperings of the Redi and bolus terms. A backgroud horizontal diffusivity ah_bkg can be used for bottom cells. If the gm_bolus flag is set, the bolus velocity is explicitly calculated and used as part of the velocity field, as opposed to the incorporating this process as part of the horizontal mixing. This last option does not currently work with partial bottom cells.


Table: Gent-McWilliams horizontal mixing namelist (LANL)
&hmix_gm_nml Valid values
[LANL default]
Gent-McWilliams isopycnic diffusion
kappa_choice ['constant'], 'variable' constant or (horizontally) variable kappa
slope_control_choice ['notanh'], 'tanh', 'clip', 'gerdes' control slope using tanh, algebraic approximation to tanh (notanh), clipping or method of Gerdes
kappa_depth_1 [1.0] the first term in the function for variation of kappa with depth
kappa_depth_2 [0.0] the coefficient of the exponential in the function for variation of kappa with depth
kappa_depth_scale [150000.0] the depth scale for the exponential in the function for variation of kappa with depth
ah [0.8 x 107] diffusion coeff for Redi part (cm2/s)
ah_bolus [0.8 x 107] diffusion coeff for bolus part (cm2/s)
[ah_bkg] [0.0] diffusion coeff for bottom cells (cm2/s)
slm_r 0.01 max slope for Redi terms
slm_b 0.01 max slope for bolus terms
[gm_bolus] .true., [.false.] option for explicit calculation of bolus velocity
/    


Table: Gent-McWilliams horizontal mixing namelist (CESM1)
&hmix_gm_nml CESM1 gx3 dipole-grid default CESM1 gx1 dipole-grid default CESM1 tripole-grid default Valid values Gent-McWilliams isopycnic diffusion
kappa_isop_choice 'bfre' 'bfre' 'bfre' 'bfre', 'edgr', 'cons', 'dept', 'vmhs', 'hdgr', 'drad', 'bfvm', 'bfhd', 'bfdr' choice for KAPPA_ISOP
kappa_thic_choice 'bfre' 'bfre' 'bfre' 'bfre', 'edgr', 'cons', 'dept', 'vmhs', 'hdgr', 'drad', 'bfvm', 'bfhd', 'bfdr' choice for KAPPA_THIC
kappa_freq_choice 'once_a_day' 'once_a_day' 'once_a_day' 'once_a_day', 'never', 'every' frequency choice for kappa computation (once a day, never, every timestep)
slope_control_choice 'notanh' 'notanh' 'notanh' 'notanh', 'tanh', 'clip', 'gerdes' control slope using tanh, algebraic approximation to tanh (notanh), clipping or method of Gerdes
kappa_depth_1 1.0 1.0 1.0   the first term in the function for variation of kappa with depth
kappa_depth_2 0.0 0.0 0.0   the coefficient of the exponential in the function for variation of kappa with depth
kappa_depth_scale 150000.0 150000.0 150000.0 CESM1 scripts determine value based upon resolution and other option settings the depth scale for the exponential in the function for variation of kappa with depth
ah 4.0e7 3.0e7 3.0e7 CESM1 scripts determine value based upon resolution and other option settings diffusion coeff for Redi part (cm2/s)
ah_bolus 4.0e7 3.0e7 3.0e7 CESM1 scripts determine value based upon resolution and other option settings diffusion coeff for bolus part (cm2/s)
use_const_ah_bkg_srfbl .true. .true. .true. .true., .false. if .true., then the specified constant value of ah_bkg_srfbl is used as the "maximum" background horizontal diffusivity within the surface boundary layer. Otherwise, KAPPA_ISOP is utilized as this "maximum".
ah_bkg_srfbl 4.0e7 3.0e7 3.0e7 automatically set by CESM1 scripts, depending on resolution and options selected backgroud horizontal diffusivity within the surface boundary layer
ah_bkg_bottom 0.0 0.0 0.0   backgroud horizontal diffusivity within the surface boundary layer
slm_r 0.03 0.03 0.03   max slope for Redi terms
slm_b 0.03 0.03 0.03   max slope for bolus terms
diag_gm_bolus .true. .true. .false. .true.,.false. if .true., compute diagnostic bolus velocity
transition_layer_on .true. .true. .true. .true., .false. if .true., use transition layer parameterization
read_n2_data .false. .false. .false. .true., .false. if .true., use climatological N2 data instead of model dependent N2
buoyancy_freq_filename 'bouyancy_freq' 'bouyancy_freq' 'bouyancy_freq' character string <= 256 chars file name for the time-independent buoyancy frequency squared
buoyancy_freq_fmt 'nc' 'nc' 'nc' 'nc', 'bin' file format suffix for the buoyancy_freq_filename file
const_eg 1.2 1.2 1.2   tuning parameter (unitless)
gamma_eg 500.0 500.0 500.0   effective upper limit for inverse eddy time scale (unitless)
kappa_min_eg 0.35 x 107 0.35 x 107 0.35 x 107   minimum KAPPA (cm2/s)
kappa_max_eg 2.0 x 107 2.0 x 107 2.0 x 107   maximum KAPPA (cm2/s)
/          


Table: Variable GM diffusivity for submesoscale eddies namelist (CESM1)
&mix_submeso_nml CESM1 default Valid values submesoscale eddies
efficiency_factor 0.07 (0.6,0.8) efficiency factor
time_scale_constant 8.64e4 (8.64e4,7*2.64e4) time-scale constant (s)
luse_const_horiz_len_scale .false. .true., .false. if .true., use constant horizontal length scale. if .false., hor_length_scale is used as the lower limit
hor_length_scale 5.0e5   horizontal length scale (cm)
/      

The user is referred to the The Parallel Ocean Program (POP) Reference Manual for details on the various options listed in the CESM1 POP2 hmix_gm_nml and mix_submeso_nml namelists.



Next:
Anisotropic viscosity options Up: Horizontal mixing parameterizations Previous: Biharmonic horizontal mixing.   Contents

2010-04-01