Improved H2O GOMOS profiles using a new algorithm based on a Levenberg-Marquardt method
Résumé
H2O plays a very important role in the upper troposphere and stratosphere. It has a strong radiative effect and it plays a key role in the ozone chemistry, being a source of HOx species involved in the catalytic destruction of ozone. The evolution of H2O in the lower stratosphere during the last decades is still not well determined. Contradictory results are obtained depending of the source of data (balloons, satellites).
H2O measurements by GOMOS (Global Ozone Monitoring by Occultation of Stars) on board Envisat can play a significant role in this area. The two advantages of the stellar occultations method are the self-calibration nature and the well-defined geometry. The IPF 6 algorithm provides greatly improved H2O profiles compared to IPF5 due to a correction of the intra-pixel PRNU. However there is still some room for improvement. A new algorithm has been developed in which the wavelength assignment is improved, the new HITRAN 2012 database is used for H2O absorption and a Levenberg-Marquartdt method is applied for spectrum fitting instead of using look-up tables for the estimation of H2O slant columns. This new algorithm provides improved H2O profiles to be used for studies on H2O variability and trends in the UTLS.
These studies have been performed in the framework of ESA-funded ALGOM (GOMOS Level 2 algorithm evolution studies) project.