2-stream solver for CRTM

[chengdang]

Adding 2-stream adding-doubling solver for CRTM

[chengdang]

Design:

• A two-stream solver based on the adding–doubling method used in SNICAR-AD, with potential updates informed by: (1) the recent revisit of two-stream methods by Ho et al. (2024), and (2) the two-stream treatment implemented in RTTOV.
• The two-stream solver will represent a coupled atmosphere–surface system, where the surface may consist of snow/ice, ocean, or land. Snow and ice will be represented using a multi-layer structure, analogous to the treatment of atmospheric layers.
• Two coupling approaches will be considered:
  • Coupled atmosphere–surface approach: Treat the atmosphere and surface as separate components, similar to ESMs. Direct and diffuse downwelling radiation from the atmosphere is passed to the surface component, while the surface-reflected/upwelling radiation is passed back to the atmospheric component. This approach facilitates direct comparison with ESM implementations, and is probably more practical for Jacobians.
  • Unified-column approach: Treat the atmosphere and surface as a single radiative column within the adding–doubling framework. This approach is more radiatively self-consistent, although the differences between the two approaches may be small in practice.