Look up table and interpolation methods for radiative transfer calculations in the infrared application to balloon and satellite spectra
Résumé
Remote sensing of atmospheric state parameters (such as temperature and mixing ratio profiles, trace gas column amounts) requires fast and accurate forward model radiative transfer calculations. Application of commonly used line-by-line (LBL) models to calculate atmospheric spectra and simulate actual measurements provides high accuracy but is often too much time consuming. One way to decrease the calculation time is to use pre-calculated absorption cross sections performing a satisfactory accuracy on an optimized grid of wavenumber, temperature and pressure. This pre-calculated database method (known as Look-up table method) is becoming more and more widely used for processing spectra of satellite-borne instruments like IMG and IL AS, of the balloon-borne instruments like LPMA (Limb Profile Monitor of the Atmosphere), and of future satellite instruments (IASI, ACE, etc.). However, the use of Look-up table on a wide wavenumber interval and for a large number of trace gases is efficient when a good interpolation scheme and an appropriate storage of the data are chosen. Comparison of data processing results obtained using Look-up table technique with results obtained by direct line-by-line LPMA algorithm is presented and discussed. Retrievals of CFC-12 and CO2 from solar occultation limb FTIR balloon spectra are exemplified.