SET_DTBT using wrong face area
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Background
Subroutine set_dtbt
() is used to estimate barotropic time step size. The subroutine is called in
- barotropic solver initialization
MOM6/src/core/MOM_barotropic.F90
Line 4907 in 7305528
- After a
dtbt_reset_period
MOM6/src/core/MOM_dynamics_split_RK2.F90
Line 646 in 7305528
In the subroutine, there are currently three options in calculating/estimating face area
- With optional input argument
BT_cont
- With optional input argument
eta
, passing to subroutinefind_face_areas
() - With optional input argument
add_SSH
, passing to subroutinefind_face_areas
()
And all of them seem to have an issue.
Issues
1. BT_cont
For reasons unknown, BT_cont
option is never used in this subroutine.
2. eta
Instead, eta
is used in recalculating dtbt. The problem is that eta
is almost never used because of this buggy block (NONLINEAR_BT_CONTINUITY
is by default False and not used with USE_BT_CONT_TYPE
):
MOM6/src/core/MOM_barotropic.F90
Lines 2869 to 2875 in 7305528
3. add_SSH
The bug in the if-block above effectively let the model use zero sea surface height (add_SSH is zero by default) to calculate face area, which may underestimate gravity wave speed.
Moreover, without eta
, subroutine find_face_areas
() will use (Z_ref
-bathymetry) to estimate thickness. This does not apply to cases like the Great Lakes, where 1) Within the domain, the reference height is spatially varying 2) portions of the water bodies are above the sea level reference height . (and runtime parameter SSH_EXTRA
, which is currently only used to estimate an upper bound of eta during initialization would not help.)
Solutions
- Use
BT_cont
when possible - Fix the if-block bug related to
eta
- *Make
Z_ref
a 2D field, rather than a scalar. I put an asterisk because the scalar should be fine for most cases, even with spatially varying reference heights. But I wonder if something weird may come up with OBCs...
The fixes may make the model slower with shorter dtbt.