Radiative transfer

[xiongyuup]

This pull request adds three new modules to the src/RadiativeTransfer/ directory:

✨ New modules:

• radar_simulation.jl: Simulates backscattering amplitude matrix elements for spheroidal raindrops based on the Invariant Imbedding T-Matrix (IITM) method. The results can be used for polarimetric radar variable synthesis and scattering property analysis.
• brightness_temperature.jl: Computes brightness temperature (TB) lookup tables using a plane-parallel radiative transfer model based on extinction coefficients.
• water_ice_refractive.jl: Implements temperature- and frequency-dependent complex refractive index models for both liquid water and ice.

📘 References:

• H. Li, Y. Xiong and Y. Chen, "Simulation of Complex Meteorological Target Echoes for Airborne Dual-Polarization Weather Radar Based on Invariant Imbedding T-Matrix," IEEE TGRS, vol. 62, pp. 1–17, 2024, Art. no. 5105817.
• G. Zhang, Weather Radar Polarimetry, China Meteorological Press, 2018, pp. 39–40.
• B. R. Brown, M. M. Bell, and A. J. Frambach, 2016: "Validation of Simulated Hurricane Drop Size Distributions using Polarimetric Radar", Geophys. Res. Lett., 42, doi:10.1002/2015GL067278.

🛠 Notes:

• Large WRF .nc files are excluded from version control. A wrf_data/ folder is provided for local testing.
• A sample WRF NetCDF file can be downloaded manually from the following link:
  🔗 Download: https://pan.baidu.com/s/1xFomb3d1TYG66LD_NZMghw
  🔐 Extraction code: hbub
  After downloading, place the file into the wrf_data/ directory.
• The current implementation supports single-species rain simulation; multi-hydrometeor and ice-phase support is under development.

🔧 Planned enhancements:

• Integration with WRF output fields for radar variable generation
• Brightness temperature simulation with surface emissivity modeling
• Dual-polarization radar variable synthesis (Z_DR, K_DP, ρ_HV, etc.)