Add new stretching transformation to mesh tools

CONTRIBUTORS.md

@@ -32,3 +32,4 @@
 | Adrian-Lock      | Adrian Lock            | Met Office            | 2026-05-21 |
 | ukmo-juan-castillo  | Juan M. Castillo    | Met Office            | 2026-03-24 |
 | mcdalvi          | Mohit Dalvi            | Met Office            | 2026-06-19 |
+| cjohnson-pi      | Christine Johnson      | Met Office            | 2026-06-29 |

applications/lbc_demo/example/mesh_config.nml

@@ -16,15 +16,16 @@
 /
 
 &planar_mesh
-  edge_cells_x    = 30
-  edge_cells_y    = 30
-  domain_size     = 24.0, 22.0
-  domain_centre   = 0.0, 0.0
-  periodic_x      = .false.
-  periodic_y      = .false.
-  create_lbc_mesh = .true.
-  lbc_rim_depth   = 6
-  lbc_parent_mesh = 'primary'
+  edge_cells_x     = 30
+  edge_cells_y     = 30
+  domain_size      = 24.0, 22.0
+  domain_centre    = 0.0, 0.0
+  periodic_x       = .false.
+  periodic_y       = .false.
+  create_lbc_mesh  = .true.
+  lbc_rim_depth    = 6
+  lbc_parent_mesh  = 'primary'
+  stretch_function = 'uniform'
 /
 
 &partitions

mesh_tools/example/planar_mesh_gen.nml

@@ -51,16 +51,16 @@
 !       be a factor of the other.
 
 &planar_mesh
-  edge_cells_x    = 48, 24
-  edge_cells_y    = 24, 12
+  edge_cells_x    = 48,24
+  edge_cells_y    = 24,12
   domain_size     = 48.0, 24.0
   domain_centre   = 0.0, 0.0
   periodic_x      = .false.
   periodic_y      = .false.
   create_lbc_mesh = .true.
   lbc_rim_depth   = 1
   lbc_parent_mesh = 'mesh_A'
-  apply_stretch_transform = .false.
+  stretch_function = 'uniform'
 /
 
 &partitions
@@ -81,8 +81,10 @@
 &stretch_transform
   cell_size_inner = 2.0, 2.0
   cell_size_outer = 3.0, 3.0
-  n_cells_outer   = 2, 2
-  n_cells_stretch = 5, 5
+  n_cells_outer_wsen   = 2, 2, 2, 2
+  n_cells_stretch_wsen = 5, 5, 5, 5
   stretching_on   = 'cell_centres'
+  poly_power      = 3
   transform_mesh  = 'mesh_A'
+  stretching_on   = 'cell_nodes'
 /

mesh_tools/rose-meta/lfric-mesh_tools/HEAD/rose-meta.conf

@@ -319,16 +319,6 @@ ns=namelist/mesh/planar
 sort-key=Section-A03
 title=Planar
 
-[namelist:planar_mesh=apply_stretch_transform]
-compulsory=true
-description=Apply stretched grid transformation.
-help=Enables using a larger cell spacing in the outer region of the domain,
-    =compared to the inner region.
-!kind=default
-sort-key=Panel-D01
-trigger=namelist:stretch_transform: .true.;
-type=logical
-
 [namelist:planar_mesh=create_lbc_mesh]
 compulsory=true
 description=Create LBC mesh
@@ -470,6 +460,36 @@ help=Specifies whether the topology of the mesh should be periodic in the y dire
 sort-key=Panel-A10
 type=logical
 
+[namelist:planar_mesh=stretch_function]
+compulsory=true
+description=Type of stretch transformation applied to the unit mesh
+!enumeration=true
+help=The function applied is:
+    = * "Uniform"
+    = Uniform resolution mesh
+    =   y = ax
+    =
+    = * "Inflation"
+    = Low resolution outer and a high resolution interior
+    = with a stretch region in between.
+    =   y_n = y_n-1 + dy * inflation ** stretch
+    =
+    = * "Polynomial"
+    = Low resolution outer and a high resolution interior
+    = with a stretch region in between.
+    =    y = bx + ax^n
+    = where n is the power
+!kind=default
+trigger=namelist:stretch_transform: this != "'uniform'";
+       =namelist:stretch_transform=stretching_on: this == "'inflation'";
+       =namelist:stretch_transform=n_cells_outer: this == "'inflation'";
+       =namelist:stretch_transform=n_cells_stretch: this == "'inflation'";
+       =namelist:stretch_transform=poly_power: this == "'polynomial'";
+       =namelist:stretch_transform=n_cells_outer_wsen: this == "'polynomial'";
+       =namelist:stretch_transform=n_cells_stretch_wsen: this == "'polynomial'";
+value-titles=uniform, inflation, polynomial
+values='uniform','inflation','polynomial'
+
 [namelist:rotation]
 compulsory=true
 ns=namelist/mesh/Rotation
@@ -578,20 +598,34 @@ type=real
 
 [namelist:stretch_transform=n_cells_outer]
 compulsory=true
-description=Depth (in cells) of outer region
+description=Depth of outer region [cells]
 help=[n_outer_x, n_outer_y]
-    =This is the number of cells in an outer region
-    =near to a domain edge. i.e. There are
+    =Number of cells in an outer region
+    =adjacent to domain boundary. i.e. There are
     =2*n_cells_outer along the whole axis
 !kind=default
 length=2
-range=2:
+range=1:
+sort-key=Panel-A04
+type=integer
+
+[namelist:stretch_transform=n_cells_outer_wsen]
+compulsory=true
+description=Depth of outer region [cells]
+help=[n_outer_w, n_outer_s, n_outer_e, n_outer_n]
+    =Number of cells in an outer region
+    =adjacent to domain boundary. Values are ordered
+    =for the domain boundaries as West, South,
+    =East and North.
+!kind=default
+length=4
+range=1:
 sort-key=Panel-A04
 type=integer
 
 [namelist:stretch_transform=n_cells_stretch]
 compulsory=true
-description=Depth (in cells) of stretch region
+description=Depth of stretch region [cells]
 help=[n_stretch_x, n_stretch_y]
     =A stretch region is defined to be the
     =cells in between the inner region and an
@@ -601,7 +635,39 @@ help=[n_stretch_x, n_stretch_y]
     =and 2*n_cells_stretch along the whole axis
 !kind=default
 length=2
-range=0:
+range=1:
+sort-key=Panel-A04
+type=integer
+
+[namelist:stretch_transform=n_cells_stretch_wsen]
+compulsory=true
+description=Depth of stretch region [cells]
+help=[n_stretch_w, n_stretch_s, n_stretch_e, n_stretch_n]
+    =
+    =Number of cells in stretch region toward a give domain
+    =boundary. These are ordered for the West, South
+    =East and North boundaries of the domain.
+    =
+    =A stretch region is defined to be the
+    =cells in between the inner region and an
+    =outer region. Cell sizes within the stretch
+    =region will lie between cell sizes within
+    =the inner/outer regions.
+    =i.e. cell_size_outer > size > cell_size_inner
+!kind=default
+length=4
+range=1:
+sort-key=Panel-A04
+type=integer
+
+[namelist:stretch_transform=poly_power]
+compulsory=true
+description=Polynomial Power
+help=Integer polynomial power, n, for
+    =stretch transform function
+    =   y = bx + ax^n
+!kind=default
+range=2:
 sort-key=Panel-A04
 type=integer
 

mesh_tools/rose-meta/lfric-mesh_tools/versions.py

@@ -31,3 +31,33 @@ def upgrade(self, config, meta_config=None):
         # Add settings
         return config, self.reports
 """
+
+class vn31_t270(MacroUpgrade):
+    # Upgrade macro for #270 by cjohnson-pi
+
+    BEFORE_TAG = "vn3.1"
+    AFTER_TAG = "vn3.1_t270"
+
+    def upgrade(self, config, meta_config=None):
+        # Add settings
+        nml = "namelist:planar_mesh"
+        apply_stretch_transform = self.get_setting_value(
+            config, [nml, "apply_stretch_transform"])
+
+        if apply_stretch_transform == ".true.":
+            self.add_setting(
+                config,
+                [nml, "stretch_function"],
+                "'inflation'",
+            )
+        else:
+            self.add_setting(
+                config,
+                [nml, "stretch_function"],
+                "'uniform'",
+            )
+        self.remove_setting(
+            config, [nml, "apply_stretch_transform"]
+        )
+
+        return config, self.reports

mesh_tools/source/planar_mesh_generator.f90

@@ -70,6 +70,9 @@ program planar_mesh_generator
                                  key_from_geometry
   use rotation_config_mod, only: ROTATION_TARGET_NULL_ISLAND, &
                                  ROTATION_TARGET_NORTH_POLE
+  use planar_mesh_config_mod, &
+                           only: STRETCH_FUNCTION_INFLATION, &
+                                 STRETCH_FUNCTION_POLYNOMIAL
 
   implicit none
 
@@ -169,11 +172,9 @@ program planar_mesh_generator
   logical :: periodic_y
   logical :: create_lbc_mesh
   integer(i_def) :: lbc_rim_depth
+  integer(i_def) :: stretch_function
 
   character(str_def) :: lbc_parent_mesh
-
-  logical :: apply_stretch_transform
-
   character(str_def) :: transform_mesh
 
   type(local_mesh_collection_type) :: local_mesh_collection
@@ -226,17 +227,16 @@ program planar_mesh_generator
   topology       = config%mesh%topology()
   geometry       = config%mesh%geometry()
 
-  edge_cells_x    = config%planar_mesh%edge_cells_x()
-  edge_cells_y    = config%planar_mesh%edge_cells_y()
-  periodic_x      = config%planar_mesh%periodic_x()
-  periodic_y      = config%planar_mesh%periodic_y()
-  domain_size     = config%planar_mesh%domain_size()
-  domain_centre   = config%planar_mesh%domain_centre()
-  create_lbc_mesh = config%planar_mesh%create_lbc_mesh()
-  lbc_rim_depth   = config%planar_mesh%lbc_rim_depth()
-  lbc_parent_mesh = config%planar_mesh%lbc_parent_mesh()
-
-  apply_stretch_transform = config%planar_mesh%apply_stretch_transform()
+  edge_cells_x     = config%planar_mesh%edge_cells_x()
+  edge_cells_y     = config%planar_mesh%edge_cells_y()
+  periodic_x       = config%planar_mesh%periodic_x()
+  periodic_y       = config%planar_mesh%periodic_y()
+  domain_size      = config%planar_mesh%domain_size()
+  domain_centre    = config%planar_mesh%domain_centre()
+  create_lbc_mesh  = config%planar_mesh%create_lbc_mesh()
+  lbc_rim_depth    = config%planar_mesh%lbc_rim_depth()
+  lbc_parent_mesh  = config%planar_mesh%lbc_parent_mesh()
+  stretch_function = config%planar_mesh%stretch_function()
 
   if (partition_mesh) then
     max_stencil_depth    = config%partitions%max_stencil_depth()
@@ -251,7 +251,8 @@ program planar_mesh_generator
     target_null_island = config%rotation%target_null_island()
   end if
 
-  if (apply_stretch_transform) then
+  if (stretch_function == STRETCH_FUNCTION_INFLATION .or. &
+      stretch_function == STRETCH_FUNCTION_POLYNOMIAL) then
     transform_mesh = config%stretch_transform%transform_mesh()
   end if
 
@@ -408,9 +409,9 @@ program planar_mesh_generator
     call log_event( log_scratch_space, LOG_LEVEL_ERROR )
   end if
 
-  if (apply_stretch_transform) then
-    ! This enables support meshes to be created with a variable
-    ! resolution stretching function.
+  ! This enables support meshes to be created with a variable
+  ! resolution stretching function.
+  if (config%namelist_exists('stretch_transform')) then
     do j=1, n_meshes
       if (trim(mesh_names(j)) == trim(transform_mesh)) then
         fine_mesh_edge_cells_x = edge_cells_x(j)
@@ -743,6 +744,7 @@ program planar_mesh_generator
                         periodic_y         = periodic_y,             &
                         domain_size        = domain_size,            &
                         domain_centre      = domain_centre,          &
+                        stretch_function   = stretch_function,       &
                         rotate_mesh        = rotate_mesh,            &
                         target_north_pole  = set_north_pole,         &
                         target_null_island = set_null_island )
@@ -779,6 +781,7 @@ program planar_mesh_generator
                         fine_mesh_edge_cells_y,                    &
                         periodic_x, periodic_y,                    &
                         domain_size, domain_centre,                &
+                        stretch_function    = stretch_function,    &
                         target_mesh_names   = target_mesh_names,   &
                         target_edge_cells_x = target_edge_cells_x, &
                         target_edge_cells_y = target_edge_cells_y, &