RFM Flags: Vary Planck Function within Layer

BFX Flag

15FEB22

Enabled
Calculate Radiance R = ∫ B.dτ assuming B (Planck Function) varies linearly with altitude and Optical Depth χ = ln(τ) varies linearly with path within each layer (effectively integrate B(χ)dτ for each layer assuming B is linear in χ for Zenith/Nadir paths, and quadratic in χ for limb paths).

Disabled
Calculate radiance R = ∫ B.dτ assuming B is constant within each layer, with weighted value determined by the Curtis-Godson temperatures for each absorbing species.

Restrictions
Incompatible with HOM, NTE and TAB Flags.
At least one flag generating a radiance-type output must be enabled: BBT, RAD, or RJT (otherwise this option would have no effect).

Notes
  1. This flag activates the 'linear in tau' algorithm described, for the nadir case, in Clough et al (1992). An equivalent approximation is used for limb-viewing.
  2. This different method of integrating radiance should be more accurate for thick layers since it allows for the fact that the effective temperature 'seen' for a particular layer will vary according to the temperature gradient through the layer and the optical thickness, eg in optically thick regions of the spectrum only the 'near' side of the layer is seen.
  3. In the limit of optically thin layers the two methods of calculating radiance converge and the Flag has only a small effect.
  4. Similarly to the LAY flag, this flag can be used as a check that the user-supplied profile is sufficiently fine; in this case, for the assumption that the layer's effective emission temperature is the same as the Curtis-Godson temperature.

Bugs
[none reported]