SASKTRANIF- a New Engine for the Radiative Transfer Modeling of Solar Occultation Measurements.

Abstract:

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 necessary for estimating volume mixing ratio quantities of these gases. One particular forward model is the SASKTRAN Inter-Face (or SASKTRANIF), 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 engine has now been implemented, designed to model solar occultation based measurements. Solar rays are traced through each atmospheric layer using an algorithm that accounts for refraction of the atmosphere. The extinction is calculated along the line of sight for a penetrating ray intersecting multiple layers of the atmosphere given a known chemical composition. By default, the engine 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 a priori information (such as input trace gas concentrations, temperature, and pressure). The new engine is currently in a testing phase. Here, we firstly compare synthesized spectra from SASKTRANIF with spectra derived from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) forward model. Secondly, we retrieve vertical volume mixing ratio profiles of various atmospheric gases by performing a global fit to ACE-FTS measured spectra where model parameters are determined using a Marquardt-Levenberg nonlinear least squares algorithm. Resulting vertical profiles are compared to those derived using the ACE-FTS retrieval system.