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radiative_transfer block ​

Top-level configuration for the RT solver — quadrature, polarization, truncation, and the projection-cap knobs introduced in v0.7. The full file at src/CoreRT/DefaultParameters.yaml shows a representative config; use this page to look up what each field does, what the defaults are, and which legacy aliases still work.

Required fields ​

  • spec_bands — Vector{String}. Each string is a Julia range expression in cm⁻¹, e.g. "(1e7/777):0.015:(1e7/757)". Multiple bands run as a band-batched solve.

  • surface — Vector{String}. One BRDF constructor per spectral band. See Schema/surface.md for the surface vocabulary (LambertianSurfaceScalar, CoxMunkSurface, rpvSurfaceScalar, etc.).

  • polarization_type — String. One of "Stokes_I()", "Stokes_IQ()", "Stokes_IQU()", "Stokes_IQUV()". Sets the Stokes vector dimension (1/2/3/4 channels respectively).

  • Δ_angle — Real, optional (default 0.0). Forward-peak exclusion angle in degrees, consumed by δBGE truncation. Ignored when truncation: NoTruncation() or when truncation: auto resolves to NoTruncation(). Legacy YAMLs commonly set 2.0 to reproduce VLIDORT's DO_DELTAM_SCALING default — set explicitly when matching a benchmark that pins this value.

  • depol — Real. Rayleigh depolarization factor (0 = no depolarization, the Natraj-2009 idealization).

  • float_type — String. "Float64" or "Float32". Float32 is fine for most retrievals and gives a ~2× speedup on large spectral windows; Float64 is required for reproducing published benchmarks at full precision.

  • architecture — String. "Architectures.CPU()", "Architectures.GPU()", or "Architectures.MetalGPU()". The CUDA GPU path is loaded via the vSmartMOMCUDAExt weak-dependency extension (using CUDA activates it).

Resolution knobs (Phase D — v0.7) ​

Pick one of the schemas below. The new nstreams knob is the recommended path; legacy max_m + l_trunc configs keep working.

  • nstreams — optional, default 8. Weighted streams per hemisphere, the primary user-facing resolution knob. Public contract: stream_l_cap = 2·nstreams - 1 regardless of quadrature_type. For VLIDORT users: this is the Julia equivalent of VLIDORT's NSTREAMS.

  • m_max — optional, default null. Explicit Fourier loop bound (order). When set, clamps the trait-aggregator output below stream_l_cap. Useful when you want to truncate the loop more aggressively than the per-component traits would.

  • truncation — optional, default auto. Controls how the aerosol phase function is treated relative to the projection cap:

    • "auto" (default): VLIDORT-DO_DELTAM_SCALING-style. With no aerosols, resolves to NoTruncation(). With aerosols, picks δBGE(stream_l_cap, Δ_angle) and the per-band Mie loop applies it only when length(greek.β) > stream_l_cap — short phase functions automatically fall back to NoTruncation() so the truncation step can never crash on a fits-the-cap series. Logs the choice via @info.

    • "NoTruncation()" or null: exactly no transform. Coefficients above stream_l_cap are silently dropped from the Fourier projection (per Phase C trait clamps).

    • "δBGE(L, Δ)": explicit δ-BGE-fit truncation (Sanghavi & Stephens 2015). Used for benchmark reproducibility when you want exact parity with a published numerical convention.

Legacy schema (still supported, deprecated v0.7) ​

  • max_m — Integer. Number of Fourier moments (count, loop runs m = 0:max_m-1). Triggers legacy parsing — nstreams will be left unset and the old min(ceil((l_max+1)/2), max_m) aggregator semantics apply.

  • l_trunc — Integer. Legendre cutoff. Plays the role of stream_l_cap under the legacy schema.

Setting both nstreams and l_trunc produces a @warn — the legacy l_trunc is ignored and nstreams wins.

Other optional fields ​

  • quadrature_type — optional, default "GaussLegQuad()". The recommended default. "RadauQuad()" is supported but documented as expert/legacy: it includes the SZA as a quadrature node (handy for DNI-on-node configurations) at the cost of being 5–50× less accurate per stream than Gauss-Legendre on Rayleigh-only setups (see the Natraj-2009 commit f9403eb and the v0.7 release notes). Sanghavi's guidance: keep Radau as an option, but reach for Gauss by default.

  • numerics — optional. Sub-block with rarely-touched solver knobs:

    • dτ_max_threshold (default 0.001) — doubling-step cap. Sets dτ_max = threshold · μ_min where μ_min is the smallest TRUE quadrature stream. Smaller → finer initial layers / more doublings; raising to ~0.01 reduces doublings.

    • dτ_min_floor (default 1024·eps(FT)) — absolute floor on dτ_max. ~1.2e-4 for Float32 / 2.3e-13 for Float64. Prevents grazing-VZA configs from collapsing dτ below FT precision.

    • blas_threads (default null) — per-model BLAS thread cap. null leaves BLAS.get_num_threads() alone; an integer pins it for the duration of rt_run. See commit 37390d2.

    • verbose (default false) — when true, rt_run prints the TimerOutputs timing tree at the end of each call (useful for profiling, noisy in batched loops).

Example — minimal new schema ​

A new-schema config can omit nstreams (default 8), quadrature_type (default GaussLegQuad), and truncation (default auto). The leanest valid radiative_transfer block is:

yaml
radiative_transfer:
  spec_bands:
    - "[18867.92 18868.92]"
  surface:
    - LambertianSurfaceScalar(0.0)
  polarization_type: Stokes_IQU()
  Δ_angle: 2.0
  depol: 0.0
  float_type: Float64
  architecture: Architectures.CPU()

This expands to: nstreams = 8 (so stream_l_cap = 15), quadrature_type = GaussLegQuad(), truncation = auto. For a Rayleigh-only Lambertian scene this resolves to NoTruncation() and the Fourier loop runs m = 0:2 (Phase C trait dispatch).

Example — Cox-Munk ocean retrieval ​

yaml
radiative_transfer:
  spec_bands: ["[18867 18870]"]
  surface: ["CoxMunkSurface(wind_speed=5.0)"]
  polarization_type: Stokes_IQU()
  nstreams: 16          # 16 weighted streams ⇒ stream_l_cap = 31
  truncation: auto      # auto → δBGE(31, 2.0) once Mie aerosol is added
  Δ_angle: 2.0
  depol: 0.0
  float_type: Float64
  architecture: Architectures.CPU()

See also ​