==== Case E4: US standard atmosphere and two aerosol layers ====

In this case we include the US standard atmosphere, the corresponding Rayleigh scattering optical thickness and molecular absorption profiles are provided at 450 nm.
In addition two aerosol layers are added: desert dust in the troposphere and sulfate in the stratosphere.

//Setup://

  * model atmosphere US-standard, scattering optical thickness provided in file {{:intercomparisons:phase_e:tau_rayleigh_450nm_usstd.dat|}}, molecular absorption optical thicknes in {{:intercomparisons:phase_e:tau_absorption_450nm_usstd.dat|}}. The file includes 2 columns where the first is the altitude of the layer boundaries in km, and the second is the layer optical thickness (e.g. the value for 0 km corresponds to the optical thickness for the layer from 0 to 1 km).
  * desert dust aerosol between 0 and 3 km altitude with an optical thickness of 0.5. The aerosol optical properties are given in the file {{:intercomparisons:phase_e:desert.cdf}}. The format of the file is the same as in case D4.
  * sulfate aerosol between 20 and 21 km altitude with an optical thickness of 0.05 with optical properties from {{:intercomparisons:phase_e:sulfate.cdf}}.
  * wavelength: 450 nm 
  * Rayleigh depolarization factor: 0.03
  * surface albedo: 0
  * Earth radius: 6371 km
 

  * Sun-observer geometries as defined in general settings:
      * //viewing zenith angles// (deg): [0, 9, 18, 26, 34, 41, 48, 54, 60, 65, 70, 74, 78, 81, 84, 86, 88, 89, 90] (up-looking at bottom of atmosphere, down-looking at top of atmosphere]
      * //viewing azimuth angles// (deg): 0-180, 10° increment 
      * //solar zenith angles// (deg): [30, 60, 80, 87, 90, 93, 96, 99]
      * //solar azimuth angle// (deg): 0
      * //sensor altitude// (km): 0, 120