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SpectralFiles
Prev: Prescribed optical properties for clouds and aerosols
The discretization in frequency within the radiation code is not fixed. This is set externally in the spectral file which contains the band breakdown to be used for the radiative transfer calculations and the optical properties for all the radiative processes determined for these bands. A number of standard spectral files are available in $RAD_DIR/data/spectra/ including those used for previous Hadley Centre climate configurations:
ls $RAD_DIR/data/spectra/
The files consists of a number of different blocks of data which are presented in a fairly readable format. The latest are those for the Met Office Global Atmosphere 9 configuration which will be used for CMIP7:
less $RAD_DIR/data/spectra/ga9/sp_sw_ga9
less $RAD_DIR/data/spectra/ga9/sp_lw_ga9
Full details are given in the Socrates Technical Guide (chapter 3), but to summarise, the main blocks provide:
- BLOCK 0: Summary of spectral data including the gases and aerosols parametrised in the file.
- BLOCK 1: Limits of the spectral bands.
- BLOCK 2: Normalized solar flux in each spectral interval.
- BLOCK 3: Rayleigh mass scattering coefficients.
- BLOCK 4: List of gaseous absorbers in each band.
- BLOCK 5: k-terms (absorption coefficients) for each gas.
- BLOCK 6: Thermal source function for each band.
- BLOCK 8: List if continuum absorbers in each band.
- BLOCK 9: Continuum absorption coefficients.
- BLOCK 10: Parametrized scattering data for droplets.
- BLOCK 11: Scattering parameters for aerosols.
- BLOCK 12: Parametrized scattering data for ice crystals.
The blocks of the spectral file are created or modified using the routine prep_spec. For example, a skeleton version of the SW GA7 spectrum can be created using the inputs to prep_spec found in $RAD_DIR/examples/sp_sw_jm/mk_sp_sw_6_skel:
cd $RAD_DIR/examples/sp_sw_jm/
emacs mk_sp_sw_6_skel&
prep_spec
The answers to the prompts can be pasted in from mk_sp_sw_6_skel. Once information for blocks 0, 1, 4 and 8 has been gathered the basic structure of the file is complete and further blocks are added as required from the prep_spec menu:
Select from the following types of data:
2. Block 2: Solar spectrum in each band.
3. Block 3: Rayleigh scattering in each band.
5. Block 5: k-terms and p, T scaling data.
6. Block 6: Thermal source function in each band.
9. Block 9: Continuum extinction and scaling data.
10. Block 10: Droplet parameters in each band.
11. Block 11: Aerosol parameters in each band.
12. Block 12: Ice crystal parameters in each band.
17. Block 17: Spectral variability data in sub-bands.
-1. To write spectral file and exit.
-2. To quit without writing spectral file.
The mk_sp_sw_6_skel continues to add block 2 using a file containing a high resolution solar spectrum (lean_12). For longer wavelengths than those listed in the file a Rayleigh-Jeans (blackbody) tail is added for the solar temperature (as given in the file or defaults to 5785K). The program will then report:
Total irradiance of solar spectrum = 1366.059915637
Total irradiance of Rayleigh-Jeans tail = 0.000345791
Sum of normalised flux = 1.000000000
Rayleigh coefficients for each band (block 3) are then calculated by weighting with the same solar spectrum. The skeleton file produced can then be viewed:
less sp_sw_6_skel
The LW file is produced in a similar way with $RAD_DIR/examples/sp_lw_jm/mk_sp_lw_9_skel except that a polynomial fit to the thermal source function is added for block 6 instead of blocks 2 and 3.
Next: Parametrised optical properties for clouds and aerosols