Initialise with true gravity by mo-cjsmith · Pull Request #511 · MetOffice/lfric_apps

mo-cjsmith · 2026-05-20T16:26:58Z

PR Summary

Sci/Tech Reviewer: Thomas Bendall (@tommbendall)
Code Reviewer:

Although the shallow atmosphere approximation is not used by UM(ENDgame) , it still retains the constant gravity field of earlier models. If LFRic is to run with true height-varying gravity, initialisation from a UM start-dump must take account of the change in gravity between the two models to avoid a significant change to the meteorology of the data.

The existing code to do this has a known defect, which is to be corrected here. An additional variation of this method is added, along with the option to use a method that takes another approach to the problem.

== Code Changes ==

geo_on_pres_kernel_mod.F90 and pres_lev_diags_alg_mod.x90: Diagnostic of geopotential height on a constant pressure level changed to use the model geopotential, rather than height_w3 field, so that it is valid for shallow=.false. as well as shallow=.true..
rose-meta.conf: New setting in the initialization namelist, regravitate, which selects the method to be used for recalculating theta when switching from constant to height-varying gravity.
map_fd_to_prognostics_alg_mod.x90: Place where regravitate is used.

== New Code ==

initialisation/regrav_isotherm_kernel_mod.F90:

Code Quality Checklist

I have performed a self-review of my own code
My code follows the project's style guidelines
Comments have been included that aid understanding and enhance the readability of the code
My changes generate no new warnings
All automated checks in the CI pipeline have completed successfully

Testing

I have tested this change locally, using the LFRic Apps rose-stem suite
If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Test Suite Results - lfric_apps - test_reset_gravity/run2

Suite Information

Item	Value
Suite Name	test_reset_gravity/run2
Suite User	chris.smith
Workflow Start	2026-05-20T13:14:06
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	mo-cjsmith/lfric_apps@test_reset_gravity	False
lfric_core	MetOffice/lfric_core@2026.03.2	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.2	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@2026.03.2	True

Task Information

✅ succeeded tasks - 1164

Security Considerations

I have reviewed my changes for potential security issues
Sensitive data is properly handled (if applicable)
Authentication and authorisation are properly implemented (if applicable)

Performance Impact

Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

AI Assistance and Attribution

Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

PSyclone Approval

If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the TCD Team

Sci/Tech Review

I understand this area of code and the changes being added
The proposed changes correspond to the pull request description
Documentation is sufficient (do documentation papers need updating)
Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

All dependencies have been resolved
Related Issues have been properly linked and addressed
CLA compliance has been confirmed
Code quality standards have been met
Tests are adequate and have passed
Documentation is complete and accurate
Security considerations have been addressed
Performance impact is acceptable

Thomas Bendall (tommbendall)

Thanks Chris, sorry if I've jumped the gun here with a review since the PR is still in "draft" state. I'm really excited about this potentially facilitating the use deep gravity.

Along with some minor in-line comments, I have two major suggestions:

Do we need to still include all of the different isotherm* options? I don't think any of the isotherm* options are tested, and since the geopot option seems to perform best, should we remove all of the isotherm* options? Or possibly keep only one that preserves the old behaviour?
Do we need unit-tests for the new kernels?

Thomas Bendall (tommbendall) · 2026-05-21T10:14:48Z

+    =Used when the shallow switch is false and reading a start dump
+    =that has been created by a constant gravity model, such as the UM.
+    =All methods calculate an Exner field in hydrostatic balance with
+    =height-varying gravity. This is done by upward integr


there seems to be some of the help section missing here!

Thomas Bendall (tommbendall) · 2026-05-21T10:16:03Z

@@ -3247,23 +3221,39 @@ help=Horizontal winds are read in on a W2h space from a start dump.  If
 sort-key=02b
 type=logical

+[namelist:initialization=regravitate]
+compulsory=true
+description=Method for switching gravity


Could you make this slightly clearer?

Suggested change

description=Method for switching gravity

description=Method for setting up initial hydrostatic balance when using deep gravity but a start dump that assumed shallow gravity

Thomas Bendall (tommbendall) · 2026-05-21T10:16:45Z

+    =that has been created by a constant gravity model, such as the UM.
+    =All methods calculate an Exner field in hydrostatic balance with
+    =height-varying gravity. This is done by upward integr
+    =Isothermodynamic: Preserve absolute temperature and pressure on


If we're to keep the different isothermal methods (I wonder if we don't!), should we explain the differences between them here?

Thomas Bendall (tommbendall) · 2026-05-21T12:54:37Z

+! under which the code may be used.
+!-----------------------------------------------------------------------------
+
+!> @brief   Need to say something here


I think you might have forgotten to fill these in!

Thomas Bendall (tommbendall) · 2026-05-21T13:32:04Z

+  end do
+
+  ! Map temperature from newly computed grid back onto current model grid
+  call interp( nlayers+1, ht_wt, th, nlayers+1, height_wth(map_wt(1)), &


I'm a bit confused about this call, based on the interp function, shouldn't this be height_wth(map_wt(1):map_wt(1)+nlayers)?

It looks like we are passing in an element of an array instead of a section of an array

Thomas Bendall (tommbendall) · 2026-05-21T13:32:56Z

+
+  ! Map temperature from newly computed grid back onto current model grid
+  call interp( nlayers+1, ht_wt, th, nlayers+1, height_wth(map_wt(1)), &
+               temperature(map_wt(1)) )


Similarly, I'm a bit confused about this call, based on the interp function, shouldn't this be temperature(map_wt(1):map_wt(1)+nlayers)?

It looks like we are passing in an element of an array instead of a section of an array

Thomas Bendall (tommbendall) · 2026-05-21T13:34:10Z

+
+end subroutine interp
+
+! Does the input UM data, T(z), pi(z), have the correct z information?


Just checking whether you intended to leave this comment in there?

Thomas Bendall (tommbendall) · 2026-05-21T14:04:21Z

+  ! Geopotential height
+  do k = 0, nlayers
+    ht_wt(k) = planet_radius * height_wth(map_wt(1)+k) / &
+              ( planet_radius - height_wth(map_wt(1)+k) )


I'm wondering if we should enforce that this calculation is done at double precision, since planet_radius could be very large and height_wth could be very small

Thomas Bendall (tommbendall) · 2026-05-21T14:05:18Z

+  weight1 = height_w3( map_w3(1) ) - height_wth( map_wt(1) )
+  exner_surf = exner( map_w3(1) ) * ( cp * temp_virt  ) / &
+               ( cp * temp_virt - ( gravity - coriolis_term( map_wt(1) ) ) * weight1 )
+  theta( map_wt(1) ) = temperature( map_wt(1) ) / exner_surf


I'm a bit confused at the surface theta being updated but no other values. Should theta actually be passed into this kernel?

Thomas Bendall (tommbendall) · 2026-05-21T14:13:24Z

-      call invoke( &
+
+      select case( regravitate )
+      case( regravitate_isotherm1 )


I wonder if we need all of these isotherm methods? Or if you want to preserve the current behaviour, we could just keep your isotherm1 method?

mo-cjsmith added 17 commits May 5, 2026 09:34

Rose stem test config file

1390b69

Metadata and upgrade macro

95b08ef

Regravitation code

b2c25b3

Geopotential height diagnostics

eed41cc

Remove spurious merge

dbc822f

Regravitate rose stem test

0413dc4

Simplify argument lists

a4f131a

Properly delete items from arg list

133d1f9

Missing declaration

4f35a7e

KGO and filename correction

ddc7041

Upgrade macro correction

85d473c

Remove unused variables

e9855a3

Descriptions

313680b

Things caught by developer tests

36c21ee

Undo messing up of files

60cf96e

Unit test for pressure diag

cd41d2b

Corrections to geopotential height diagnostic

c7a8227

github-actions Bot assigned mo-cjsmith May 20, 2026

github-actions Bot added the cla-required The CLA has not yet been signed by the author of this PR - added by GA label May 20, 2026

Thomas Bendall (tommbendall) requested changes May 21, 2026

View reviewed changes

mo-cjsmith added 5 commits May 21, 2026 16:26

Improve comments. Remove redundant code.

ebbf4f3

Exner is inout

0e3512d

Integration must be from top

462e1bb

Remove obsolete code

c4e3ec1

Contribute

a263bec

github-actions Bot added cla-signed The CLA has been signed as part of this PR - added by GA and removed cla-required The CLA has not yet been signed by the author of this PR - added by GA labels May 22, 2026

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Initialise with true gravity#511

Initialise with true gravity#511
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mo-cjsmith:reset_gravity

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	description=Method for switching gravity
	description=Method for setting up initial hydrostatic balance when using deep gravity but a start dump that assumed shallow gravity


		end subroutine interp

		! Does the input UM data, T(z), pi(z), have the correct z information?

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PR Summary

Code Quality Checklist

Testing

trac.log

Test Suite Results - lfric_apps - test_reset_gravity/run2

Suite Information

Task Information

Security Considerations

Performance Impact

AI Assistance and Attribution

Documentation

PSyclone Approval

Sci/Tech Review

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