mom_offline_main module reference

The routines here implement the offline tracer algorithm used in MOM6. These are called from step_offline Some routines called here can be found in the MOM_offline_aux module.

More…

Data Types

offline_transport_cs

The control structure for the offline transport module.

Functions/Subroutines

`offline_advection_ale()`	3D advection is done by doing flux-limited nonlinear horizontal advection interspersed with an ALE regridding/remapping step. The loop in this routine is exited if remaining residual transports are below a runtime-specified value or a maximum number of iterations is reached.
`offline_redistribute_residual()`	In the case where the main advection routine did not converge, something needs to be done with the remaining transport.
`remaining_transport_sum()`	Returns the sums of any non-negligible remaining transport [H L2 ~> m3 or kg] to check for advection convergence.
`offline_diabatic_ale()`	The vertical/diabatic driver for offline tracers.
`offline_fw_fluxes_into_ocean()`	Apply positive freshwater fluxes (into the ocean) and update netMassOut with only the negative (out of the ocean) fluxes.
`offline_fw_fluxes_out_ocean()`	Apply negative freshwater fluxes (out of the ocean)
`offline_advection_layer()`	When in layer mode, 3D horizontal advection using stored mass fluxes must be used.
`update_offline_fields()`	Update fields used in this round of offline transport.
`register_diags_offline_transport()`	Initialize additional diagnostics required for offline tracer transport.
`post_offline_convergence_diags()`	Posts diagnostics related to offline convergence diagnostics.
`extract_offline_main()`	Extracts members of the offline main control structure.
`insert_offline_main()`	Inserts (assigns values to) members of the offline main control structure.
`offline_transport_init()`	Initializes the control structure for offline transport and reads in some of the.
`read_all_input()`	Coordinates the allocation and reading in all time levels of uh, vh, hend, temp, and salt from files.
`offline_transport_end()`	Deallocates (if necessary) arrays within the offline control structure.

Detailed Description

Offline Tracer Transport in MOM6

‘Offline tracer modeling’ uses physical fields (e.g. mass transports and layer thicknesses) saved from a previous integration of the physical model to transport passive tracers. These fields are accumulated or averaged over a period of time (in this test case, 1 day) and used to integrate portions of the MOM6 code base that handle the 3d advection and diffusion of passive tracers.

The distribution of tracers in the ocean modeled offline should not be expected to match an online simulation. Accumulating transports over more than one online model timestep implicitly assumes homogeneity over that time period and essentially aliases over processes that occur with higher frequency. For example, consider the case of a surface boundary layer with a strong diurnal cycle. An offline simulation with a 1 day timestep, captures the net transport into or out of that layer, but not the exact cycling. This effective aliasing may also complicate online model configurations which strongly-eddying regions. In this case, the offline model timestep must be limited to some fraction of the eddy correlation timescale. Lastly, the nonlinear advection scheme which applies limited mass-transports over a sequence of iterations means that tracers are not transported along exactly the same path as they are in the online model.

This capability has currently targeted the Baltic_ALE_z test case, though some work has also been done with the OM4 1/2 degree configuration. Work is ongoing to develop recommendations and best practices for investigators seeking to use MOM6 for offline tracer modeling.

Implementation of offline routine in MOM6

The subroutine step_tracers that coordinates this can be found in MOM.F90 and is only called using the solo ocean driver. This is to avoid issues with coupling to other climate components that may be relying on fluxes from the ocean to be coupled more often than the offline time step. Other routines related to offline tracer modeling can be found in tracers/MOM_offline_control.F90

As can also be seen in the comments for the step_tracers subroutine, an offline time step comprises the following steps: #. Using the layer thicknesses and tracer concentrations from the previous timestep, half of the accumulated vertical mixing (eatr and ebtr) is applied in the call to tracer_column_fns. For tracers whose source/sink terms need dt, this value is set to 1/2 dt_offline

Half of the accumulated surface freshwater fluxes are applied START ITERATION
Accumulated mass fluxes are used to do horizontal transport. The number of iterations used in advect_tracer is limited to 2 (e.g x->y->x->y). The remaining mass fluxes are stored for later use and resulting layer thicknesses fed into the next step
Tracers and the h-grid are regridded and remapped in a call to ALE. This allows for layers which might ‘vanish’ because of horizontal mass transport to be ‘reinflated’ and essentially allows for the vertical transport of tracers
Check that transport is done if the remaining mass fluxes equals 0 or if the max number of iterations has been reached END ITERATION
Repeat steps 1 and 2
Redistribute any residual mass fluxes that remain after the advection iterations in a barotropic manner, progressively upward through the water column.
Force a remapping to the stored layer thicknesses that correspond to the snapshot of the online model at the end of an accumulation interval
Reset T/S and h to their stored snapshotted values to prevent model drift

Evaluating the utility of an offline tracer model

How well an offline tracer model can be used as an alternative to integrating tracers online with the prognostic model must be evaluated for each application. This efficacy may be related to the native coordinate of the online model, to the length of the offline timestep, and to the behavior of the tracer itself.

A framework for formally regression testing the offline capability still needs to be developed. However, as a simple way of testing whether the offline model is nominally behaving as expected, the total inventory of the advection test tracers (tr1, tr2, etc.) should be conserved between time steps except for the last 4 decimal places. As a general guideline, an offline timestep of 5 days or less.

Runtime parameters for offline tracers

OFFLINEDIR: Input directory where the offline fields can be found
OFF_SUM_FILE: Filename where the accumulated fields can be found (e.g. horizontal mass transports)
OFF_SNAP_FILE: Filename where snapshot fields can be found (e.g. end of timestep layer thickness)
START_INDEX: Which timelevel of the input files to read first
NUMTIME: How many timelevels to read before ‘looping’ back to 1
FIELDS_ARE_OFFSET: True if the time-averaged fields and snapshot fields are offset by one time level, probably not needed -NUM_OFF_ITER: Maximum number of iterations to do for the nonlinear advection scheme -REDISTRIBUTE_METHOD: Redistributes any remaining horizontal fluxes throughout the rest of water column. Options are ‘barotropic’ which “evenly distributes flux throughout the entire water column,’upwards’ which adds the maximum of the remaining flux in each layer above, and ‘none’ which does no redistribution”

Type Documentation

type mom_offline_main/offline_transport_cs