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

Abstract

[1] The European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (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.

Long-term climatology of air mass transport through the Tropical Tropopause Layer (TTL) during NH winter

“…The effect of using the ECMWF standard ozone climatology is illustrated by the green and orange lines, which maximize at 1 and 1.25 K/day with lower frequencies. In the tropical tropopause region there is slightly more ozone in the standard climatology compared to the assimilated fields (Fortuin and Langematz, 1995;Oikonomou and O'Neill, 2006), which leads to a stronger heating, hence upwelling in the TTL. For the rest of this study we are using the prognostic ECMWF ozone fields as input data.…”

Comparison of Fast In situ Stratospheric Hygrometer (FISH) measurements of water vapor in the upper troposphere and lower stratosphere (UTLS) with ECMWF (re)analysis data

“…Oikonomou and O'Neill (2006) found for 1991 to 1999 that the ERA-40 mixing ratios of water vapor are considerably larger than observed by MOZAIC, typically by 20 % in the tropical upper troposphere, and by more than 60 % in the lower stratosphere at high latitudes. The moist bias with an overestimation of the extratropical lower stratospheric specific humidity in the ECMWF operational analysis and forecast system has been also intensively studied with CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) in situ measurements between 2005 and 2012 (Dyroff et al, 2014).…”

“…It is thus important to improve the moist bias in atmospheric models for the calculation of radiative fluxes, particularly in the stratosphere, where absolute humidities are small but relative errors can be large. The moist model bias analyzed with the MOZAIC measurements may be due to limitations in the ECMWF model, as described by Oikonomou and O'Neill (2006). The exact difference between the model and MOZAIC measurements may also be influenced by the bias in the MOZAIC water vapor observations in the lower stratosphere.…”

“…A more climatological evaluation of water vapor fields based on the multiyear MOZAIC (Measurements of Ozone and Water Vapour by Airbus In-Service Aircraft) program was presented for the former ERA-40 data set by Oikonomou and O'Neill (2006) and for operational ECMWF analysis fields by Luo et al (2007). Oikonomou and O'Neill (2006) found for 1991 to 1999 that the ERA-40 mixing ratios of water vapor are considerably larger than observed by MOZAIC, typically by 20 % in the tropical upper troposphere, and by more than 60 % in the lower stratosphere at high latitudes.…”

Middle atmosphere water vapour and dynamical features in aircraft measurements and ECMWF analyses

“…Simmons et al (1999) examined a development version of the ERA-40 system, finding a broadly realistic distribution of water vapour in the stratosphere. Oikonomou and O'Neill (2006) compared analysed ERA-40 humidities to observations from the Upper Atmosphere Research Satellite (UARS) and to aircraft measurements. ERA-40 specific humidities were found to be too low by 10-20% in the mid-and upper-stratosphere (50 hPa upwards).…”