Evaluation of ozone and water vapor fields from the ECMWF reanalysis ERA‐40 during 1991–1999 in comparison with UARS satellite and MOZAIC aircraft observations

The European Centre for Medium‐Range Weather Forecasts (ECMWF) 40‐year Re‐analysis (ERA‐40) ozone and water vapor reanalysis fields during the 1990s have been compared with independent satellite data from the Halogen Occultation Experiment (HALOE) and Microwave Limb Sounder (MLS) instruments on board the Upper Atmosphere Research Satellite (UARS). In addition, ERA‐40 has been compared with aircraft data from the Measurements of Ozone and Water Vapour by Airbus In‐Service Aircraft (MOZAIC) program. Overall, in comparison with the values derived from the independent observations, the upper stratosphere in ERA‐40 has about 5–10% more ozone and 15–20% less water vapor. This dry bias in the reanalysis appears to be global and extends into the middle stratosphere down to 40 hPa. Most of the discrepancies and seasonal variations between ERA‐40 and the independent observations occur within the upper troposphere over the tropics and the lower stratosphere over the high latitudes. ERA‐40 reproduces a weaker Antarctic ozone hole, and of less vertical extent, than the independent observations; values in the ozone maximum in the tropical stratosphere are lower for the reanalysis. ERA‐40 mixing ratios of water vapor are considerably larger than those for MOZAIC, typically by 20% in the tropical upper troposphere, and they may exceed 60% in the lower stratosphere over high latitudes. The results imply that the Brewer‐Dobson circulation in the ECMWF reanalysis system is too fast, as is also evidenced by deficiencies in the way ERA‐40 reproduces the water vapor “tape recorder” signal in the tropical stratosphere. Finally, the paper examines the biases and their temporal variation during the 1990s in the way ERA‐40 compares to the independent observations. We also discuss how the evaluation results depend on the instrument used, as well as on the version of the data.