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Field-over-dt diagnostic

Divides any input field by the model timestep dt = t_current − t_start_of_step. This is useful for converting accumulated differences into per-second rates.

Configuration

Append _over_dt to any field name X:

field_names:
  - X_over_dt

The output field has the same layout and grid as X, with units [X_units / s].

Behavior

  • At the start of each timestep (init_timestep), the start-of-step timestamp is saved internally.
  • At evaluation time (compute_diagnostic), dt is computed as the difference between the field's current timestamp and the saved start timestamp (in seconds).
  • The output is X / dt element-wise.
  • Before init_timestep has ever been called, the output is filled with the standard EAMxx fill value (constants::fill_value<Real>). Note this does NOT cover the case where init_timestep HAS been called but no time has elapsed since: there dt is zero, which is an error rather than a fill value (see the caveats below).

Example: atmospheric backward tendency

field_names:
  - T_mid_minus_T_mid_prev_over_dt

This composes three diagnostics:

  1. T_mid_prev — FieldPrevDiag: value of T_mid at the start of the step
  2. T_mid_minus_T_mid_prev — BinaryOpsDiag: difference
  3. T_mid_minus_T_mid_prev_over_dt — FieldOverDtDiag: rate

The shorthand T_mid_atm_backtend expands to the same expression via the built-in alias system (see Built-in aliases).

Caveats

  • dt must be strictly positive; a zero or negative timestep triggers a runtime error.
  • This diagnostic cannot be written to an instant output stream. An instant stream takes a snapshot at t0, before the first step has run, so dt is zero and the run aborts with "dt must be positive". Use an averaged stream (averaging_type: average, max, min) instead. The same applies to everything built on top of it, including X_atm_backtend.
  • Only fields with Real data type are supported.
  • The suffix _over_dt is matched before binary-op patterns, so A_minus_B_over_dt parses as FieldOverDtDiag(A_minus_B), not BinaryOpsDiag(A_minus_B, over, dt). See Parsing precedence for details.