MPAS-JEDI | config_do_DAcycling vs config_do_restart

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For simulation using the MPAS-JEDI analysis file (an.nc) as the initial condition, the recommended settings are:

In namelist.atmosphere file,

• config_do_DAcycling = true
• config_do_restart = false

Why These Settings?

• config_do_DAcycling = true: This option tells MPAS to recalculate the model's coupled prognostic fields (like theta_m, rho_zz, and ru) from the uncoupled analysis variables in the file . Because JEDI's analysis output (whether a restart or mpasout file) contains uncoupled fields, setting this to true is necessary to ensure the model initializes correctly from the updated analysis .
  • When JEDI updates state variables like temperature (\(T\)), surface pressure (\(p_{s}\)), or moisture (\(q_{v}\)), coupled and diagnostic state variables (such as modify-coupled potential temperature \(\theta_m\), dry air density \(\rho_{zz}\), and pressure \(p\)) must be recalculated to restore thermodynamic consistency across the grid.
    • Setting config_do_DAcycling = .true. instructs MPAS-A upon initialization to recompute these coupled atmospheric variables directly from the newly analyzed state fields rather than assuming they were inherited unchanged from a prior hydrostatic/prognostic step.
• config_do_restart = false: While the an.nc file is technically a restart-style file, the JEDI documentation specifies that config_do_restart should be set to false for the MPAS-A forecast when using the recommended 2-stream input approach (where the analysis is stored in mpasout files) . If you are instead using the restart stream for your JEDI input, you would set it to true . Given the common use of an.nc as a JEDI analysis output, false is the correct choice for the forecast step.

In short, setting DO_DACYCLING = true is the key step that ensures your forecast starts correctly from the JEDI analysis field.

The Key Difference: Initialization vs. Direct Read

There is a significant difference. Setting config_do_DAcycling = true fundamentally changes how MPAS initializes the model state from the input file, compared to a standard restart run.

The core distinction lies in what happens to the coupled variables (theta_m, rho_zz, etc.) when the model starts.

• Standard Restart (config_do_DAcycling = false): The model trusts the file completely. It reads the prognostic variables directly from the restart file and uses them "as-is" to continue the simulation. There is no recalculation of the coupled state.
• DA Cycling Setup (config_do_DAcycling = true): The model distrusts the file's coupled state. Even if the file contains theta_m and rho_zz, the model ignores those values and forces a call to the atm_init_coupled_diagnostics routine . This routine recalculates the coupled fields (theta_m, rho_zz, ru, etc.) from the uncoupled analysis variables (like theta, rho, qv, u).

Core Differences

Using config_do_DAcycling = .true. with config_do_restart = .false. is fundamentally different from a standard Restart Run (config_do_restart = .true.).

Feature	Data Assimilation Run (DAcycling = .true., restart = .false.)	Standard Restart Run (restart = .true.)
Input Source	Reads state variables from an updated analysis file (e.g., an.nc supplied as init.nc).	Reads state variables from a binary restart file generated by a prior MPAS run.
Coupled Variables (\(\theta_m, \rho_{zz}\))	Recomputed dynamically from updated basic state variables (\(T, p_s, q_v\)).	Loaded directly as exact values saved at the restart time step.
Integrity of Physics/Dynamics	Recalculates missing hydrostatic and diagnostic fields to ensure physical balance before stepping forward.	Resumes exact mathematical state continuously from the last model time step.
Accumulated Diagnostics	Reset to zero (e.g., accumulated rainfall, diagnostic radiation flux totals).	Preserved continuously from the start of the simulation sequence.

What Happens Behind the Scenes

1. Variable Coupling & Recalculation

In MPAS-A, the prognostic variables are coupled mass/thermodynamic state variables like modified coupled potential temperature (\(\theta_m\)) and dry air mass density (\(\rho_{zz}\)).

• DA Cycling Mode: JEDI (or DART) updates standard meteorological variables (\(T\), \(u\), \(v\), \(q\), \(p_s\)). When MPAS boots up with config_do_DAcycling = .true., it reads those updated analysis fields and recalculates internal coupled parameters (\(\theta_m, \rho_{zz}\)) to keep the model hydrostatically consistent.
• Restart Mode: The model expects that no fields have been changed externally. It bypasses diagnostic recalculation and assumes \(\theta_m\) and \(\rho_{zz}\) are perfectly matched to the exact millisecond the prior run stopped.

2. Diagnostic Accumulators

• DA Cycling Mode: Resets time-integrated diagnostics (like rainc or rainnc precipitation accumulation).
• Restart Mode: Retains time-integrated diagnostics so total rainfall maps reflect the entire run duration.

When to Use Which?

• Use config_do_DAcycling = .true. & config_do_restart = .false.: Whenever you insert a new analysis state into MPAS from JEDI, WRFDA, or DART.
• Use config_do_restart = .true.: Whenever you are simply extending a forecast that paused or stopped (e.g., running hours 0–24, stopping, then resuming hours 24–48 with no external data modification).

Why This Matters for MPAS-JEDI

This behavior is specifically designed for data assimilation workflows:

1. JEDI Output Format: The MPAS-JEDI analysis file (an.nc) primarily updates uncoupled variables (potential temperature theta, dry density rho, water vapor qv, and edge-normal wind u) .
2. Necessary Coupling: To run the forecast model, these uncoupled variables must be mathematically coupled into the model's prognostic variables (theta_m is moist potential temperature, rho_zz is the dry-air density in the terrain-following coordinate, etc.).
3. The Solution: By setting config_do_DAcycling = true, you tell MPAS, "Do not read the coupled variables from this file. Instead, take the uncoupled analysis variables and run the initialization routine to generate a consistent, coupled model state from them" .

The "Bit-Level" Difference

A crucial technical detail: even if you take a standard restart file and run it with config_do_DAcycling = true, you will get different results than if you ran it with config_do_DAcycling = false .

This is because the atm_init_coupled_diagnostics routine uses floating-point calculations that introduce bit-level differences. The recalculated theta_m will be mathematically equivalent to the stored value in the restart file, but not bitwise identical. This is expected and correct behavior for a DA run—the model is starting from a fresh, internally consistent initialization derived from the analysis variables.

References

1. config_do_DAcycling in MPAS | JiaWang Start dateSep 24, 2026
   1. For a restart run, setting config_do_DAcycling = true will force a call to the atm_init_coupled_diagnostics routine, which is otherwise only called at the beginning of a cold-start simulation; see the logic in atm_mpas_init_block: MPAS-Model/src/core_atmosphere/mpas_atm_core.F at v8.4.2 · MPAS-Dev/MPAS-Model . The re-computation of theta_m, rho_zz, etc. in the atm_init_coupled_diagnostics routine will generally lead to bit-level differences in these fields compared with their values in the restart file.
2. Some questions about the parameter cong_do_DAcycling | ateNov 21, 2022
   1. The 'config_do_DAcycling' option essentially controls whether the atm_init_coupled_diagnostics routine is called at the beginning of a restart simulation. The logic for this can be found around L.389 of mpas_atm_core.F (in MPAS v7.3).
   2. Within the atm_init_coupled_diagnostics routine -- among other fields -- we compute theta_m, rho_zz, ru, and rw from the uncoupled theta, rho, u, w, and qv fields in the restart file.