====== Outgoing longwave radiation spectrum ======

//Task:// Compute the emitted irradiance spectrum at the Earth surface and at the top of the atmosphere from 3.5 µm to 100 μm. Plot the result together with the Planck function at the surface temperature.

<file bash thermal_irr.inp>
                         # specify libRadtran data path
data_files_path  /home/claudia/libRadtran/data

                         # Location of atmospheric profile file. 
atmosphere_file          us-standard

mol_abs_param             lowtran

albedo 0.0               # Surface albedo

rte_solver disort2       # Radiative transfer equation solver

wavelength 3500 100000.0 # Wavelength range [nm]

                         # calulcation wavelengths
transmittance_wl_file    trans_wl.dat

source thermal           # Emission from Earth surface is source of radiation

zout toa                 # surface or toa

output per_nm            # Output unit
</file> 

The following file includes wavelengths for which the radiative transfer calculation is performed:
{{:lectures:radiative_transfer:trans_wl.dat|}}

<file python plot_olr.py>
from pylab import *

def planck(lam, T):
    
    c= 2.99792458e8
    k= 1.380662e-23
    h= 6.626180e-34
    return 2*pi*h*c*c/(lam**5 * (exp(h*c/(lam*k*T))-1.))

# irradiance from 300 to 500 nm
lw_spectrum_sur=loadtxt('thermal_sur.out')
lw_spectrum_toa=loadtxt('thermal_toa.out')

figure(1, figsize=(10,7))

plot(lw_spectrum_sur[:,0], lw_spectrum_sur[:,3], label='surface')
plot(lw_spectrum_toa[:,0], lw_spectrum_toa[:,3], label='TOA')
plot(lw_spectrum_toa[:,0], planck(lw_spectrum_toa[:,0]*1e-9, 288.2)*1e-9, label='Planck')
xlim(3500,60000)
title('LW spectrum')
xlabel('wavelength [nm]')
ylabel('irradiance [W/ (m^2 nm)]')
legend(loc=1)

savefig('olr.png')
</file>

[{{ :teaching:radiative_transfer:olr.png?500 |Upwelling irradiance at top of atmosphere (green). The upwelling irradinace at the surface corresponds exactly to the Planck function for 288.2 K (surface temperature).}}]