Ozone assimilation in the ERA‐40 reanalysis project

In this paper we outline the ozone model and analysis system used in the ECMWF 45‐year reanalysis project, and take a first look at the quality of the resulting ozone fields. The analysed ozone, both profiles and total‐column ozone field, generally compares well with independent observations. However, there are certain problematic situations when the profile is not correct even if the analysed total ozone is close to observations. These situations appear when there is a bias between the model and the observations used in the assimilation. Suggestions are made for improvements of the ozone assimilation and modelling, including the introduction of a bias correction scheme for ozone data.