The 4D‐Var assimilation of ozone‐sensitive infrared radiances measured by IASI

An analysis of ozone that is constrained only by observations of ultraviolet backscatter has an obvious limitation. These data are not available at night‐time. A system has been developed to exploit ozone information from infrared radiances measured by IASI which suffer no such sampling problems related to the position of the sun. It has been found that, relative to a baseline system that has no ozone observations, the use of IASI significantly improves the fit to independent ozone estimates from the Aura MLS. Indeed this improvement is, in some areas, comparable to or better than that obtained when more established ozone estimates from UV sensors are assimilated. One area where the assimilation of IASI is found to be clearly beneficial is in the winter high latitudes and southern polar night.