Add halo exchanges to ForwardBackward Timestepper

[andrewdnolan]

This PR add missing halo exchanges between sub steps for the ForwardBackward time stepper. Without these PEM test using ForwardBackward time stepping failed, even after rebasing onto #488 .

Fixes #489.

Checklist

• Linting

  • Pre-commit run locally

• Building

  • CMake build does not produce any new warnings from changes in this PR

• Testing

  aurora, oneapi-ifx, mpich

  • CTests Pass
  • Polaris omega_pr Pass

  chrysalis, oneapi-ifx, openmpi

  • CTests Pass
  • Polaris omega_pr Pass

  frontier, craygnu, mpich

  • CTests Pass
  • Polaris omega_pr Pass

  frontier, craygnu-mphipcc, mpich

  • CTests Pass
  • Polaris omega_pr Pass

  pm-cpu, gnu, mpich

  • CTests Pass
  • Polaris omega_pr Pass

  pm-gpu, gnugpu, mpich

  • CTests Pass
  • Polaris omega_pr Pass

• Provide relevant details in a comment to the PR titled Testing with the following:

  • Which machines CTest unit tests
    have been run on and indicate that are all passing.
  • The Polaris omega_pr test suite
    has passed, using the Polaris e3sm_submodules/Omega baseline
  • Document machine(s), compiler(s), and the build path(s) used for -p for both the baseline (Polaris e3sm_submodules/Omega) and the PR build
  • Indicate "All tests passed" or document failing tests
  • Document testing used to verify the changes including any tests that are added/modified/impacted.

[andrewdnolan]

Frontier CTest unit tests:

• Machine: fontier
• Compiler: craygnu / craygnu-mphipcc
• Build type: Release
• Result: All tests passed
• Logs:
  • craygnu: /lustre/orion/cli115/proj-shared/anolan/omega_PR490_halo/craygnu/build/ctest.log
  • craygnu-mphipcc: /lustre/orion/cli115/proj-shared/anolan/omega_PR490_halo/craygnu-mphipcc/build/ctest.log

[xylar]

@hyungyukang, please give this a look when you can.

[andrewdnolan]

Testing:

• PASS: ERS_Vmct.T62_oQU240.COMEGA-IAF.frontier_craygnu.20260812_111903_jba01b
• PASS: PEM_Vmct.T62_oQU240.COMEGA-IAF.frontier_craygnu.20260812_120844_9qcobw

Where I needed to apply this diff:

diff --git a/components/omega/cime_config/buildnml b/components/omega/cime_config/buildnml
index 9f86189bdb..30817523bc 100755
--- a/components/omega/cime_config/buildnml
+++ b/components/omega/cime_config/buildnml
@@ -275,12 +275,8 @@ def _build_time_integration_config(default_time_integration, case):
     # RunDuration is not used in coupled runs
     time_integration["RunDuration"] = "none"
 
-    # All meshes use RK4 timestepping (for now)
-    time_integration["TimeStepper"] = "RungeKutta4"
-
-    if ocn_grid == "oQU240":
-        time_integration["TimeStep"] = "0000_00:05:00"
-    elif ocn_grid == "EC30to60E2r2":
+    if ocn_grid == "EC30to60E2r2":
+        time_integration["TimeStepper"] = "RungeKutta4"
         time_integration["TimeStep"] = "0000_00:01:00"
 
     return time_integration

in order to use Forward-Backward timestepping. As an aside, #494 will make changes to the omega.yml file much easier, but until that's merged this manual diff is required.

[hyungyukang]

CTests passed on both Frontier GPU and CPU.
The Polaris decomp and restart tests for the baroclinic channel also passed using the Forward-Backward time stepper.

Based on @andrewdnolan 's testing, my own testing, and visual inspection, I am approving this PR. Thanks, @andrewdnolan !