====== Rainbows with libRadtran ======

To calculate a rainbow with libRadtran is relatively straightforward. It is computationally expensive but to generate the input is quite simple. Rainbows form when we have large water droplets. For this reason we first need to calculate optical properties for a size distribution with large droplets. We may use the mie tool to do that. The input file is very short:

  mie_program MIEV0
  refrac water
  mimcut 0.0000000001
  nmom 10000
  wavelength 380 780
  wavelength_step 5.
  
  output_user netcdf
  
  r_mean 100    # ignored
  
  size_distribution_file ./sizedist_1.010

In order to get a realistic result, we need to define a size distribution on a reasonable grid (if a monodisperse distribution is assumed - that is, all droplets are same size - the result would show may spikes and individual features which cannot be observed in nature because a real cloud always includes a distribution of different droplet sizes). For our example we used a gamma distribution where the radius grid is incremented by a factor of 1.01 from wavelength to wavelength:

{{:user_area:sizedist.png|}}

The mie tool provides a number of optical property files in netcdf format for wavelengths between 380 and 780nm with a step of 5nm. These can be directly used as input to uvspec: 

  atmosphere_file ../data/atmmod/afglus.dat
  solar_file ../data/solar_flux/atlas_plus_modtran
  
  wavelength 380 780
  
  output rgb   # translate radiance to colors, red/green/blue
  
  wc_file ./wc.dat   # simple one-layer cloud, 1-2km, LWC = 1g/m2, r_eff=100 micron
  wc_layer
  wc_properties ./wc.   # use the previously calculated Mie files
  
  zout 3 
  
  nstr 64
  
  umu 0.017452 0.034899 ...   # equidistant in theta, from 0 to 90 degree
  phi 0 

And the output will be colors which can be directly converted to a picture. In this example we have the sun at the zenith and look straight downward - hence we only need the viewing zenith angle dependence and don't need to care about the azimuth. Voila!

{{:user_area:rainbow.png|}}