2) Check cutoff critical neutral slope #711
Description
Redi parameterizes the diffusion along isopycnals using the Redi diffusion tensor ...
| 1+S_x^2 S_x*S_y S_x |
| S_x*S_y 1+S_y^2 S_y | *K_redi
| S_x S_y S_x^2+S_y^2 |
where K_redi is the isoneutral diffusivity, S_x= [drho/dx]/[-drho/dz] and S_y=[drho/dy]/[-drho/dz] are the isoneutral slopes, with S_x*S_y <<1
| 1 0 S_x |
| 0 1 S_y | *K_redi
| S_x S_y S_x^2+S_y^2 |
If the isopycnals become very steep (outcropping) with this tensor we would basically produce vertical diffusion which we dont want. Therefor we taper the isoneutral slope to zero in areas where the slopes a very strong.
The neutral slope is tapered with a tanh(...) function ...
! in FESOM1.4 ODM95=True hyperbolic tangent slope tapering tapering
! Danabasoglu and McWilliams, 1995, sensitivity of the global ocean circulation
! to parameterizations of mesoscale tracer transport,
c1 = 1.0_WP
if (scaling_ODM95) then
do nz = ul1, nl1
c1(nz)=0.5_WP*(1.0_WP + tanh((ODM95_Scr - neutral_slope(3,nz,n))/ODM95_Sd))
if ((bvfreq(nz,n) <= 0.0_WP) .or. (bvfreq(nz+1,n) <= 0.0_WP)) c1(nz)=0.0_WP
enddo
end if
...where Scr controls the cutoff for the neutral slope and Sd controls the width of smoothing region where tapering is switched on/off. Larger Scr means less tapering, smaller Scr means stronger tapering. Stronger tapering means that the areas where the neutral slope i set to zero and thus Redi diffusivity along isopycnal slope are switched off are larger.
So far we used in FESOM2.0 S_cr =1e-2, values in literature are about 0.4e-2. The value in FESOM1.4 was S_cr=0.2e-2. We look now what effect has S_cr. All other parameters are kept the same, here already area scaling is used!
&oce_dyn
C_d=0.0025 ! Bottom drag, nondimensional
A_ver= 1.e-4 ! Vertical viscosity, m^2/s
scale_area=5.8e9 ! Visc. and diffus. are for an element with scale_area
SPP=.false. ! Salt Plume Parameterization
Fer_GM=.true. ! to swith on/off GM after Ferrari et al. 2010
K_GM_max = 2000.0 ! max. GM thickness diffusivity (m2/s)
K_GM_min = 2.0 ! max. GM thickness diffusivity (m2/s)
K_GM_bvref = 2 ! def of bvref in ferreira scaling 0=srf,1=bot mld,2=mean over
mld,3=weighted mean over mld
K_GM_rampmax = -1.0 ! Resol >K_GM_rampmax[km] GM on
K_GM_rampmin = -1.0 ! Resol <K_GM_rampmin[km] GM off, in between linear scaled down
K_GM_resscalorder = 2 ! <--- here area scaling
scaling_Ferreira =.false. ! GM vertical scaling after Ferreira et al.(2005) (as also imp
lemented by Qiang in FESOM 1.4)
scaling_Rossby =.false. ! GM is smoothly switched off according to Rossby radius (from
1. in coarse areas to 0. where resolution reaches 2 points/Rossby radius)
scaling_resolution =.true. ! GM is spatially scaled with resolution; A value of K_GM corre
sponds then to a resolution of 100km
scaling_FESOM14 =.false. ! special treatment of GM in the NH (as also implemented by Qi
ang in FESOM 1.4; it is zero within the boundary layer)
scaling_ODM95 =.true. ! tapering based on critical slope
--> ODM95_Scr = 1.0e-2 <-- ! Critical slope for tapering
-------------------------------------------
ODM95_Sd = 1.0e-3 ! slope width of tapering smoothing zone
scaling_LDD97 =.false. ! tapering in surface
LDD97_c = 2.0 ! [m/s] is the first baroclinic wave speed
LDD97_rmin = 15.0e3 ! rossby radius min cutoff [m]
LDD97_rmax = 100.0e3 ! rossby radius max cutoff [m]
Redi =.true.
visc_sh_limit=5.0e-3 ! for KPP, max visc due to shear instability
mix_scheme='KPP' ! vertical mixing scheme: KPP, PP
Ricr = 0.3 ! critical bulk Richardson Number
concv = 1.6 ! constant for pure convection (eqn. 23) (Large 1.5-1.6; MOM def
ault 1.8)
/
--> March NH MLD for Scr =1.0e.-2, 0.2e-2 and 0.1e-2, with and without activated Monin-Obukov mixing
--> September SH MLD for Scr =1.0e.-2, 0.2e-2 and 0.1e-2, with and without activated Monin-Obukov mixing
While smaller Scr value seems to benefit the MLD in the Labrador Sea, this is not the case in the Weddel Sea. There it looks like that Scr=0.2e-2 creates a better stratification in the weddel sea than the original value of Scr=1.0e-2. But Scr values smaller than 0.2e-2 are not beneficial, there strange things start to happen around the coast of the Antarctis.