How is the optical depth calculated ?

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How is the optical depth calculated ?

The aerosol types in OPAC each come with a certain height profile, allowing for calculating optical depths. In case you defined your own aerosol mixture, you are also free to create a specific height profile. These profiles are restricted to 5 layers (the 5th is for clouds), the meaning of which is explained in the standard opac input file. The meaning of the parameters controlling the optical depth in these layers are explained below:

With exception of the mineral aerosol layer, only exponentially decreasing extinction coefficients are assumed. The optical depth "tau" of a certain layer is thus:

tau = sig_e(0) * [ Z * (exp(-H1/Z)-exp(-H2/Z)) ]

where H1 and H2 are the lower and upper limit of the layer in [km] and Z is the scale height in [km]. sig_e(0) is the extinction coefficient at sea level.

In case of aerosols, 4 layers are assumed:

- the first one describes the aerosol in the boundary layer with   sig_e(0) being the extinction coefficient of the selected aerosol   type.

- the second one is a mineral aerosol layer, which describes transported   desert aerosol. The default thickness of this layer is 0. km. It   allows to introduce a mineral transported aerosol layer in each aerosol type. The default particle number density is 11 [cm^-3], which corresponds to an optical thickness of 0.1 for an assumed layer  thickness of 1 km. In this case a constant height profile is assumed,  which leads to an optical thickness:

  tau = sig_e(MITR) * N * H (min. transp.)

  sig_e(MITR) is the extinction coefficient for 1 [particle / cm^3]. N is the particle number density in [cm^-3], and H = H2-H1.

- the third one is the free tropospheric background aerosol. It consists of a continental average aerosol with mixing ratios somewhat changed against the aerosol type "continental average". An exponential profile is assumed, where sig_e(FT) is the extrapolated extinction coefficient at sea level. The resulting optical depth corresponds to the value given in the Standard Radiation Atmosphere (SRA).

- the stratospheric background aerosol consists of sulfate with 3 particles / cm^3 and a constant height profile. This number density together with a layer thickness of 23 km corresponds to the optical thicknesses at 500 nm as given in the Standard Radiation Atmosphere.

The cloud types all consist of 1 layer with constant extinction (Z=99) and a thickness of 1 km.