soil_moisture diagnostic mislabelled units kg kg-1 (should be kg m-2 / SMCL)

Start with the named STASH and JULES interface files, especially applications/lfricinputs/source/common/lfricinp_stashmaster_mod.f90 and the soil-moisture kernel files, then locate the field or diagnostic definition that writes the units attribute. Change only the soil_moisture metadata from kg kg-1 to kg m-2 and verify that generated LFRic output reports the corrected units without changing stored values.

Summary

The soil_moisture field is written to LFRic output/checkpoint files with a units attribute of kg kg-1, but the field is actually SMCL — soil moisture content per layer in kg m-2 (= θ · ρ_water · Δz_layer). The units metadata should be corrected to kg m-2.

Evidence that the field is kg m-2 (SMCL), not kg kg-1

soil_moisture is JULES' smcl_soilt prognostic (UM STASH 9), which is a per-layer moisture content in kg m-2, not a mass fraction:

• interfaces/jules_interface/source/kernel/process_soil_kernel_mod.F90 — the interface documents the argument as kg m-2, and the moisture-bounding logic builds its limits as θ · dzsoil · rho_water and compares them directly against soil_moisture:

  !> @param[in,out] soil_moisture   Soil moisture content (kg m-2)
  ...
  smc_min = 0.1_r_def * soil_moist_wilt(map_2d(1)) * dzsoil(n) * rho_water
  smc_max =             soil_moist_sat(map_2d(1))  * dzsoil(n) * rho_water
  

  This is only dimensionally consistent if soil_moisture is kg m-2.
• interfaces/jules_interface/source/kernel/jules_extra_kernel_mod.F90 maps it one-to-one onto smcl_soilt (annotated kg m-2), and jules_exp_kernel_mod.F90 copies it into soil_layer_moisture (kg m-2).
• STASH identity: applications/lfricinputs/source/common/lfricinp_stashmaster_mod.f90 — stashcode_soil_moist = 9 (UM STASH 9 = SMCL, kg m-2 per layer).

So the physics unambiguously treats soil_moisture as kg m-2; the kg kg-1 label on the written field is incorrect.

Where this was observed

The mislabelled units = "kg kg-1" attribute appears on soil_moisture in LFRic UGRID output — e.g. UK-domain restartLFRicAtm_* checkpoints and GAL9 global dumps.

Suggested fix

Correct the units metadata for soil_moisture (STASH 9 / smcl_soilt) in the field/diagnostic definition that drives the written attribute, from kg kg-1 to kg m-2.

Impact

Purely a metadata/label correction — the stored values are already SMCL (kg m-2) and are used correctly by the physics; only the written units string is wrong. Downstream tools that trust the units attribute (rather than the STASH/field identity) would mis-scale the data.