On the quality of the ERA-Interim ozone reanalyses: comparisons with satellite data

This article presents an assessment of the quality of the ERA-Interim ozone analyses produced by the European Centre for Medium-Range Weather Forecasts. Ozone retrievals from a number of satellite instruments and an ad hoc generated mean total column ozone (TCO) reference are used to assess the quality of the ERA-Interim ozone analyses during the period from January 1989 to December 2008. The ERA-Interim TCO is typically within +/- 5 DU (about +/- 3%) from the TCO reference, while showing up to 2% lower values than the Ozone Monitoring Instrument TCO between 50 degrees S and 50 degrees N. Comparisons with SAGE, HALOE and (UARS and Aura) MLS data show consistent results both in the Tropics and Extratropics, with mean residuals typically within +/- 5% around 5 hPa and within +/- 10% in the region of the ozone mixing ration maximum at 10 hPa. However, the comparisons with POAM II and III show mean relative residuals ranging from a few percent in summer to about -40% in winter at high latitudes, partly confirming the known problems of accurately modelling the ozone field during the polar night. Mean residuals of about +10% (but up to +20% at times) and within +/- 20% are found both in the Tropics and Extratropics for all instruments near 30 hPa and in the lower stratosphere around 65 hPa, respectively. The quality of the ERA-Interim ozone analyses is also compared with that of ERA-40. The study shows that, until December 1995, the ERA-Interim ozone analyses are in better agreement with the independent observations than their ERA-40 equivalent in the upper troposphere and lower stratosphere, but slightly degraded on average in the middle stratosphere. With the start of the assimilation of GOME ozone profiles (January 1996 -December 2002), the agreement between the independent data and the co-located ERA-Interim analyses improves and exceeds that calculated for ERA-40 also in the middle stratosphere. Copyright (C) 2011 Royal Meteorological Society