Task: Compute the direct and the diffuse solar irradiance spectrum at the surface in the spectral range from 200 to 3500 nm. Plot the result together with the extraterrestrial solar spectrum and the estimate of solar irradiance using the Planck function. Which spectral features result from absorption in the Earth atmosphere?

solar_irrad_spectrum.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
 
                         # Location of the extraterrestrial spectrum
source solar               kurudz_1.0nm.dat
 
albedo 0.0               # Surface albedo
 
sza 30.0                 # Solar zenith angle
 
rte_solver disort2       # Radiative transfer equation solver
 
wavelength 200.0 3500.0  # Wavelength range [nm]
 
zout sur                 # surface
 
transmittance            # use this option if you want transmittance instead of irradiance  

solar_irradiance_spectrum.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.))
 
 
# solar spectrum
solar_spectrum=loadtxt('/home/claudia/libRadtran/data/solar_flux/kurudz_1.0nm.dat')
 
d= 1.49e11      # sun-Earth distance
r= 1.3914e9/2.  # radius of the sun
 
 
# irradiance from 300 to 500 nm
sw_spectrum=loadtxt('irradiance.out')
 
figure(1, figsize=(10,7))
 
plot(sw_spectrum[:,0], sw_spectrum[:,1]+sw_spectrum[:,2], label='irradiance')
plot(solar_spectrum[:,0], solar_spectrum[:,1], 'k', label='TOA')
plot(sw_spectrum[:,0], planck(sw_spectrum[:,0]*1e-9, 6000)*1e-6*(r/d)**2, label='Planck')
xlim(200,2500)
title('irradiance spectrum at the surface')
xlabel('wavelength [nm]')
ylabel('irradiance [mW/ (m^2 nm)]')
legend(loc=1)
savefig('solar_irrad.png')
 
# transmittance from 300 to 500 nm
trans_spectrum=loadtxt('transmittance.out')
 
figure(2, figsize=(10,7))
plot(trans_spectrum[:,0], trans_spectrum[:,1]+trans_spectrum[:,2])
xlim(200,2500)
title('transmittance spectrum at the surface')
xlabel('wavelength [nm]')
ylabel('transmittance')
savefig('transmittance.png')

Solar irradiance spectrum at the surface.

Transmittance spectrum.