A new SASKTRAN module for the radiative transfer modeling of solar occultation measurements

Vertical distributions of atmospheric gases measured by satellite instruments can be retrieved by mathematical inversion algorithms involving a forward model of the radiative transfer equation. Hence, an accurate forward model to predict atmospheric spectra is imperative for estimating volume mixing ratio quantities of these gases. One particular forward model is SASKTRAN, which is a line by line radiative transfer model typically used to model atmospheric spectra arising from limb scattered sunlight at ultraviolet to near infrared wavelengths, using linear ray tracing and a three dimensional spherical shell atmosphere of homogeneous layers. An additional module has now been implemented in the SASKTRAN code that is designed for the modeling of solar occultation based measurements, using the same atmospheric model geometry as the scattering module. Solar rays are traced through each atmospheric layer using an algorithm that accounts for refraction of the atmosphere. For a known chemical composition associated with a given occultation ray that intersects multiple layers of the atmosphere, SASKTRAN calculates the extinction of the penetrating ray along its path. By default, the module uses the HITRAN (2008) spectral database to obtain information about the absorption cross sections of each modeled species, and also utilizes user defined climatologies for the purpose of a priori information (such as trace gas concentrations, temperature, and pressure). Furthermore, the user has the option of selecting various other parameters, such as, atmospheric species to be modeled, instrument line shape, and microwindow size. The new module is currently in a testing phase. Here, we compare synthesized spectra from SASKTRAN with spectra derived from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer forward model.