Building a Model

This guide shows how to assemble representations and processes into a complete MIAM Model and wire it into MICM’s solver.

Basic Setup

#include <miam/miam.hpp>
#include <micm/CPU.hpp>
using namespace micm;
using namespace miam;

A Model is constructed with a name and a list of representations:

auto model = Model{
  .name_ = "AEROSOL",
  .representations_ = { aitken, accumulation, dust }
};

Processes are added separately:

model.AddProcesses({ h2o_dissociation, co2_hydration });

Multi-Model Systems

Multiple Models can represent different particle categories (e.g., cloud droplets vs. background aerosol). Each Model is registered independently with MICM:

auto cloud_model = Model{
  .name_ = "CLOUD",
  .representations_ = { small_drop, large_drop }
};
cloud_model.AddProcesses({ reactions });

auto aerosol_model = Model{
  .name_ = "AEROSOL",
  .representations_ = { aitken, accumulation }
};
aerosol_model.AddProcesses({ reactions });

MICM Integration

Register Models as external model systems when building the solver:

auto system = System(gas_phase, cloud_model, aerosol_model);

auto solver = CpuSolverBuilder<RosenbrockSolverParameters>(
    RosenbrockSolverParameters::ThreeStageRosenbrockParameters())
    .SetSystem(system)
    .AddExternalModel(cloud_model)
    .AddExternalModel(aerosol_model)
    .SetIgnoreUnusedSpecies(true)
    .Build();

SetSystem registers all state variables (gas-phase species from MICM plus MIAM’s species and aerosol parameters). AddExternalModel registers each Model’s forcing and Jacobian functions with the solver.

State Initialization

After building the solver, initialize the state:

State state = solver.GetState();

// Environmental conditions
state.conditions_[0].temperature_ = 287.45;  // K
state.conditions_[0].pressure_ = 101319.9;   // Pa
state.conditions_[0].CalculateIdealAirDensity();

// Gas-phase species (short name)
state[co2] = 0.2;  // mol m-3

// Condensed-phase species (fully qualified name)
state[droplets.Species(aqueous_phase, h2o)] = 300.0;  // mol m-3 (liquid water content)

// Number concentration (TwoMomentMode only)
state[aitken.NumberConcentration()] = 1.0e8;  // m-3

// Representation parameters (GMD, GSD, section bounds)
droplets.SetDefaultParameters(state);
aitken.SetDefaultParameters(state);

Time Integration

for (int i = 0; i < 100; ++i)
{
  solver.CalculateRateConstants(state);
  auto result = solver.Solve(10.0, state);  // 10-second steps
}

CalculateRateConstants calls MIAM’s UpdateStateParametersFunction, which evaluates temperature-dependent constants (HLC, K_eq, k_f, k_r). The Rosenbrock solver then calls MIAM’s forcing and Jacobian functions internally at each stage.