SUBROUTINE GREY_EXTINCTION

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 What it does
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Calculates grey extinctions

1. For each activated optical process, excluding gaseous absorption,
   • increments are calculated for the total and scattering extinctions
   • the products of the asymmetry factor and the forward scattering factor in clear and cloudy regions.
2. These increments are summed, and the grey total and scattering extinctions and the asymmetry and forward scattering factors are thus calculated.

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 Structure of Routine
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1. Many variables passed into subroutine
2. Many variables defined
3. INITIALIZE THE EXTINCTION COEFFICIENTS, K_EXT_TOT_FREE and K_EXT_SCAT_FREE, AND THE ASYMMETRY PRODUCT, ASYMMETRY_FREE, to zero.
4. IF (L_RESCALE) THEN set FORWARD_SCATTER_FREE(L, I)=0.0E+00
5. IF (L_RAYLEIGH) THEN INCLUDE RAYLEIGH SCATTERING (alters K_EXT_SCAT_FREE).
6. IF (L_AEROSOL) THEN INCLUDE THE EFFECTS OF AEROSOL, which modifies K_EXT_TOT_FREE, K_SCATTER,K_EXT_SCAT_FREE, ASYMMETRY_FREE and FORWARD_SCATTER_FREE.
7. IF (L_CONTINUUM) THEN INCLUDE CONTINUUM ABSORPTION (alters K_EXT_TOT_FREE).
8. ADD THE SCATTERING ON TO THE TOTAL EXTINCTION (alters K_EXT_TOT_FREE).
9. IF (.NOT.L_CLOUD) THEN
   • alter ASYMMETRY_FREE
   • IF (L_RESCALE) THEN alter FORWARD_SCATTER_FREE
   • RETURN
10. ADD IN BACKGROUND CONTIBUTIONS
    • alter K_EXT_TOT_CLOUD, K_EXT_SCAT_CLOUD, ASYMMETRY_CLOUD and FORWARD_SCATTER_CLOUD
11. ADD ON THE TERMS REPRESENTING PROCESSES WITHIN CLOUDS - DO K=1, N_CONDENSED
    • CALL OPT_PROP_WATER_CLOUD or
    • CALL OPT_PROP_ICE_CLOUD
    • INCREMENT THE ARRAYS OF OPTICAL PROPERTIES
12. CALCULATE THE FINAL OPTICAL PROPERTIES.
13. REPEAT FOR CLOUDS

Called by	Calls
flux_calc (summ)	opt_prop_ice_cloud (summ) opt_prop_water_cloud (summ)

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