implement simultaneous transport with SIQN

gusto/equations/prognostic_equations.py

@@ -305,12 +305,13 @@ def add_tracers_to_prognostics(self, domain, active_tracers):
                 name of the active tracer.
         """
 
-        # Check if there are any conservatively transported tracers.
-        # If so, ensure that the reference density is indexed before this tracer.
+        # If there are any conservatively transported tracers, ensure
+        # that the reference density, if it is also an active tracer,
+        # is indexed earlier.
         for i in range(len(active_tracers) - 1):
             tracer = active_tracers[i]
             if tracer.transport_eqn == TransportEquationType.tracer_conservative:
-                ref_density = next(x for x in active_tracers if x.name == tracer.density_name)
+                ref_density = next((x for x in active_tracers if x.name == tracer.density_name), tracer)
                 j = active_tracers.index(ref_density)
                 if j > i:
                     # Swap the indices of the tracer and the reference density

gusto/time_discretisation/time_discretisation.py

@@ -132,14 +132,22 @@ def setup(self, equation, apply_bcs=True, *active_labels):
         self.residual = equation.residual
 
         if self.field_name is not None and hasattr(equation, "field_names"):
-            self.idx = equation.field_names.index(self.field_name)
-            self.fs = equation.spaces[self.idx]
-            self.residual = self.residual.label_map(
-                lambda t: t.get(prognostic) == self.field_name,
-                lambda t: Term(
-                    split_form(t.form)[self.idx].form,
-                    t.labels),
-                drop)
+            if isinstance(self.field_name, list):
+                # Multiple fields are being solved for simultaneously.
+                # This enables conservative transport to be implemented with SIQN.
+                # Use the full mixed space for self.fs, with the
+                # field_name, residual, and BCs being set up later.
+                self.fs = equation.function_space
+                self.idx = None
+            else:
+                self.idx = equation.field_names.index(self.field_name)
+                self.fs = equation.spaces[self.idx]
+                self.residual = self.residual.label_map(
+                    lambda t: t.get(prognostic) == self.field_name,
+                    lambda t: Term(
+                        split_form(t.form)[self.idx].form,
+                        t.labels),
+                    drop)
 
         else:
             self.field_name = equation.field_name
@@ -152,6 +160,34 @@ def setup(self, equation, apply_bcs=True, *active_labels):
             self.residual = self.residual.label_map(
                 lambda t: any(t.has_label(time_derivative, *active_labels)),
                 map_if_false=drop)
+            if isinstance(self.field_name, list):
+                # Multiple fields are being solved for simultaneously.
+                # Keep all time derivative terms:
+                residual = self.residual.label_map(
+                    lambda t: t.has_label(time_derivative),
+                    map_if_false=drop)
+
+                # Only keep active labels for prognostics in the list:
+                for subname in self.field_name:
+                    field_residual = self.residual.label_map(
+                        lambda t: t.get(prognostic) == subname,
+                        map_if_false=drop)
+
+                    residual += field_residual.label_map(
+                        lambda t: t.has_label(*active_labels),
+                        map_if_false=drop)
+
+                self.residual = residual
+            else:
+                self.residual = self.residual.label_map(
+                    lambda t: any(t.has_label(time_derivative, *active_labels)),
+                    map_if_false=drop)
+
+        # Set the field name if using simultaneous transport.
+        if isinstance(self.field_name, list):
+            self.field_name = equation.field_name
+
+        bcs = equation.bcs[self.field_name]
 
         self.evaluate_source = []
         self.physics_names = []
@@ -244,9 +280,19 @@ def setup(self, equation, apply_bcs=True, *active_labels):
         if not apply_bcs:
             self.bcs = None
         elif self.wrapper is not None:
-            # Transfer boundary conditions onto test function space
-            self.bcs = [DirichletBC(self.fs, bc.function_arg, bc.sub_domain)
-                        for bc in bcs]
+            if self.wrapper_name == 'mixed_options':
+                # Define new Dirichlet BCs on the wrapper-modified
+                # mixed function space.
+                self.bcs = []
+                for idx, field_name in enumerate(self.equation.field_names):
+                    for bc in equation.bcs[field_name]:
+                        self.bcs.append(DirichletBC(self.fs.sub(idx),
+                                                    bc.function_arg,
+                                                    bc.sub_domain))
+            else:
+                # Transfer boundary conditions onto test function space
+                self.bcs = [DirichletBC(self.fs, bc.function_arg, bc.sub_domain)
+                            for bc in bcs]
         else:
             self.bcs = bcs
 

gusto/timestepping/semi_implicit_quasi_newton.py

@@ -115,6 +115,9 @@ def __init__(self, equation_set, io, transport_schemes, spatial_methods,
         self.reference_update_freq = reference_update_freq
         self.to_update_ref_profile = False
 
+        # Flag for if we have simultaneous transport
+        self.simult = False
+
         # default is to not offcentre transporting velocity but if it
         # is offcentred then use the same value as alpha
         self.alpha_u = Constant(alpha) if off_centred_u else Constant(0.5)
@@ -148,15 +151,30 @@ def __init__(self, equation_set, io, transport_schemes, spatial_methods,
         self.transported_fields = []
         for scheme in transport_schemes:
             assert scheme.nlevels == 1, "multilevel schemes not supported as part of this timestepping loop"
-            assert scheme.field_name in equation_set.field_names
-            self.active_transport.append((scheme.field_name, scheme))
-            self.transported_fields.append(scheme.field_name)
-            # Check that there is a corresponding transport method
-            method_found = False
-            for method in spatial_methods:
-                if scheme.field_name == method.variable and method.term_label == transport:
-                    method_found = True
-            assert method_found, f'No transport method found for variable {scheme.field_name}'
+            if isinstance(scheme.field_name, list):
+                # This means that multiple fields are being transported simultaneously
+                self.simult = True
+                for subfield in scheme.field_name:
+                    assert subfield in equation_set.field_names
+
+                    # Check that there is a corresponding transport method for
+                    # each field in the list
+                    method_found = False
+                    for method in spatial_methods:
+                        if subfield == method.variable and method.term_label == transport:
+                            method_found = True
+                    assert method_found, f'No transport method found for variable {scheme.field_name}'
+                self.active_transport.append((scheme.field_name, scheme))
+            else:
+                assert scheme.field_name in equation_set.field_names
+
+                # Check that there is a corresponding transport method
+                method_found = False
+                for method in spatial_methods:
+                    if scheme.field_name == method.variable and method.term_label == transport:
+                        method_found = True
+                        self.active_transport.append((scheme.field_name, scheme))
+                assert method_found, f'No transport method found for variable {scheme.field_name}'
 
         self.diffusion_schemes = []
         if diffusion_schemes is not None:
@@ -240,7 +258,11 @@ def transporting_velocity(self):
     def setup_fields(self):
         """Sets up time levels n, star, p and np1"""
         self.x = TimeLevelFields(self.equation, 1)
-        self.x.add_fields(self.equation, levels=("star", "p", "after_slow", "after_fast"))
+        if self.simult is True:
+            # If there is any simultaneous transport, add a temporary field:
+            self.x.add_fields(self.equation, levels=("star", "p", "simult", "after_slow", "after_fast"))
+        else:
+            self.x.add_fields(self.equation, levels=("star", "p", "after_slow", "after_fast"))
         for aux_eqn, _ in self.auxiliary_equations_and_schemes:
             self.x.add_fields(aux_eqn)
         # Prescribed fields for auxiliary eqns should come from prognostics of
@@ -339,6 +361,9 @@ def timestep(self):
         xrhs_phys = self.xrhs_phys
         dy = self.dy
 
+        if self.simult:
+            xsimult = self.x.simult
+
         # Update reference profiles --------------------------------------------
         self.update_reference_profiles()
 
@@ -368,9 +393,20 @@ def timestep(self):
                 self.io.log_courant(self.fields, 'transporting_velocity',
                                     message=f'transporting velocity, outer iteration {outer}')
                 for name, scheme in self.active_transport:
-                    logger.info(f'Semi-implicit Quasi Newton: Transport {outer}: {name}')
-                    # transports a field from xstar and puts result in xp
-                    self.transport_field(name, scheme, xstar, xp)
+                    if isinstance(name, list):
+                        # Transport multiple fields from xstar simultaneously.
+                        # We transport the mixed function space from xstar to xsimult, then
+                        # extract the updated fields and pass them to xp; this avoids overwriting
+                        # any previously transported fields.
+                        logger.info(f'Semi-implicit Quasi Newton: Transport {outer}: '
+                                    + f'Simultaneous transport of {name}')
+                        self.transport_field(self.field_name, scheme, xstar, xsimult)
+                        for field_name in name:
+                            xp(field_name).assign(xsimult(field_name))
+                    else:
+                        logger.info(f'Semi-implicit Quasi Newton: Transport {outer}: {name}')
+                        # transports a field from xstar and puts result in xp
+                        self.transport_field(name, scheme, xstar, xp)
 
             # Fast physics -----------------------------------------------------
             x_after_fast(self.field_name).assign(xp(self.field_name))