Single Scatter Sources

Single scattering in the atmosphere is defined as the direct solar beam attenuated to a point in the atmosphere, and then scattered into a specific direction, e.g., back into an instrument’s line of sight. We also consider direct reflection of the Earth’s surface when viewing in the nadir direction to be considered single scattering for convenience.

Generally the recommended way to enable single scattering is with

import sasktran2 as sk

config = sk.Config()

config.single_scatter_source = sk.SingleScatterSource.Exact

here Exact refers to how the solar attenuation is calculated. In the Exact mode, a ray is traced towards the sun everytime the single scatter source is required at a new point, i.e., the solar attenuation is calculated “exactly”.

The other available single scatter source,

config.single_scatter_source = sk.SingleScatterSource.Table

differs in that the solar attenuation is pre-computed on an appropriate grid, and then interpolated whenever it is requested. This can reduce accuracy in some cases, but also offer computational efficiency improvements. Generally the advantages of using Table over Exact are minimal, and we only recommended experimenting with the table option if you have a situation where many lines of sight are requested (>100) and the majority of the calculation time is suspected to be inside the single scatter source.

Single scattering can be explicity disabled with

config.single_scatter_source = sk.SingleScatterSource.NoSource

Additonal Notes

• All available single scatter sources are perfectly linearized, capable of producing weighting functions to machine precision

• All single scatter sources support polarized calculations (nstokes=3)

Relevant Configuration Options

sasktran2.Config.single_scatter_source	Sets which (if any) single scatter source is to be used inside the calculation.
sasktran2.Config.num_stokes	Sets the number of Stokes parameters used in the calculation.