Configuration

The atmosphere and the solve are described by a single configuration object — ADConfig in C++ and JAX. The workflow is always the same:

1. Set the dimensions (num_layers, num_quadrature) and any flags that change array sizes (use_delta_m, use_thermal_emission).

2. Call allocate() to size the per-layer / per-level arrays.

3. Fill the arrays and set the boundary/source parameters.

4. Call solve(cfg).

allocate() fills the zeroth Legendre moment of every layer with \(1\) (an isotropic default) and, in thermal mode, sizes the temperature array to num_layers + 1. solve() calls validate() internally, but you may call it yourself to fail early.

Tip

Set use_delta_m and use_thermal_emission before allocate(): delta-M provisions one extra Legendre moment per layer (\(2M+1\) instead of \(2M\)), and thermal mode allocates the temperature array.

Dimensions and flags

Field	Default	Meaning
num_layers	0	Number of atmospheric layers (must be > 0).
num_quadrature	8	Gauss–Legendre streams per hemisphere \(M\) (must be >= 2). Fast paths exist for 2, 4, 8, 16, 32.
use_thermal_emission	false	Compute Planck sources internally from temperatures and the wavenumber band.
use_delta_m	false	Enable delta-M scaling (Using delta-M scaling).
use_diffusion_lower_bc	false	Use the diffusion-approximation lower boundary instead of a surface (Boundary conditions).
index_from_bottom	false	If true, user arrays (and outputs) are ordered bottom-of-atmosphere first.
compute_temperature_jacobian	false	Compute analytic temperature Jacobians (thermal only; C++; Temperature Jacobians).
compute_flux_components	false	Split the net flux into additive thermal and stellar parts (Outputs).

Boundary and source parameters

Field	Default	Meaning
surface_albedo	0.0	Lambertian surface albedo in \([0, 1]\).
surface_emission	0.0	Raw surface thermal emission (only when !use_thermal_emission).
surface_temperature	-1.0	Surface skin temperature [K]. If >= 0 (thermal mode), the surface emits at this temperature instead of temperature[num_layers].
top_emission	0.0	Raw isotropic downwelling diffuse radiation at TOA (only when !use_thermal_emission).
top_temperature	-1.0	Top-boundary temperature [K]. If >= 0 (thermal mode), TOA downwelling is \(B(T_\text{top})\) instead of \(B(T_0)\); set 0 for cold space (DisORT default).
solar_flux	0.0	Collimated solar/stellar flux at TOA (set > 0 to enable the beam).
solar_mu	1.0	Cosine of the solar zenith angle; must be in \((0, 1]\) when solar_flux > 0.
wavenumber_low / wavenumber_high	0.0	Band limits [cm⁻¹] over which the Planck function is integrated (thermal mode).

Layer and level arrays

After allocate() these have the sizes shown. “Layer” arrays have one entry per layer; “level” arrays have one entry per interface (num_layers + 1).

Array	Size	Meaning
delta_tau	num_layers	Optical depth of each layer (\(\geq 0\)).
single_scat_albedo	num_layers	Single-scattering albedo of each layer, in \([0, 1]\).
phase_function_moments	num_layers × nmom	Reduced Legendre moments per layer. nmom is \(2M\) (or \(2M+1\) with delta-M).
temperature	num_layers + 1	Level temperatures [K] (thermal mode).
planck_levels	num_layers + 1	Pre-computed Planck values at levels (an alternative to temperature; skips the internal Planck evaluation).

Validation rules

validate() throws (C++ std::invalid_argument / Python ValueError) when, among other checks:

• num_layers <= 0 or num_quadrature < 2;

• any array size does not match the declared dimensions;

• a delta_tau is negative or a single_scat_albedo leaves \([0, 1]\);

• surface_albedo leaves \([0, 1]\);

• solar_flux > 0 but solar_mu is not in \((0, 1]\);

• surface_temperature or top_temperature is set without use_thermal_emission (use the raw *_emission fields instead);

• use_diffusion_lower_bc is set without Planck data.

The RTOutput object

solve() returns an RTOutput whose arrays are all indexed by interface (0 = TOA … num_layers = surface). See Outputs for the full list, including the mean-intensity convention and the optional Jacobian and flux-component arrays.