A comparative analysis of UV nadir-backscatter and infrared limb-emission ozone data assimilation

Rossana Dragani

doi:10.5194/acp-16-8539-2016

Abstract

Abstract. This paper presents a comparative assessment of ultraviolet nadir-backscatter and infrared limb-emission ozone profile assimilation. The Meteorological Operational Satellite A (MetOp-A) Global Ozone Monitoring Experiment 2 (GOME-2) nadir and the ENVISAT Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) limb profiles, generated by the ozone consortium of the European Space Agency Climate Change Initiative (ESA O3-CCI), were individually added to a reference set of ozone observations and assimilated in the European Centre for Medium-Range Weather Forecasts (ECMWF) data assimilation system (DAS). The two sets of resulting analyses were compared with that from a control experiment, only constrained by the reference dataset, and independent, unassimilated observations. Comparisons with independent observations show that both datasets improve the stratospheric ozone distribution. The changes inferred by the limb-based observations are more localized and, in places, more important than those implied by the nadir profiles, albeit they have a much lower number of observations. A small degradation (up to 0.25 mg kg−1 for GOME-2 and 0.5 mg kg−1 for MIPAS in the mass mixing ratio) is found in the tropics between 20 and 30 hPa. In the lowermost troposphere below its vertical coverage, the limb data are found to be able to modify the ozone distribution with changes as large as 60 %. Comparisons of the ozone analyses with sonde data show that at those levels the assimilation of GOME-2 leads to about 1 Dobson Unit (DU) smaller root mean square error (RMSE) than that of MIPAS. However, the assimilation of MIPAS can still improve the quality of the ozone analyses and – with a reduction in the RMSE of up to about 2 DU – outperform the control experiment thanks to its synergistic assimilation with total-column ozone data within the DAS. High vertical resolution ozone profile observations are essential to accurately monitor and forecast ozone concentrations in a DAS. This study demonstrates the potential and limitations of each dataset and instrument type, as well as the need for a balanced future availability of nadir and limb sensors and long-term plans for limb-viewing instruments.

Highlights

Since the discovery of its global decline in early 1980s (Farman et al, 1985; Solomon et al, 1986), ozone has attracted the interest of both the scientific community and policy makers (e.g. WMO, 2014a, b, and earlier assessments), as well as of the general public
The 2014 WMO assessment of ozone depletion stresses that recent studies agree that the start of the 21st century represented a turning point in the global total-column ozone trend, which sees a slow increase in ozone abundance, it is not yet clear whether the current global increase can be attributed to a reduction in the amount of ODSs (WMO, 2014a)
This study focuses on the assimilation of the Global Ozone Monitoring Experiment 2 (GOME-2) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) ozone profiles retrieved by the ozone consortium (O3-CCI hereafter) created as part of the ESA Climate Change Initiative (CCI, Plummer, 2009, http://cci.esa.int)

Summary

Introduction

Since the discovery of its global decline in early 1980s (Farman et al, 1985; Solomon et al, 1986), ozone has attracted the interest of both the scientific community and policy makers (e.g. WMO, 2014a, b, and earlier assessments), as well as of the general public. Such an interest is driven and justified by the crucial role ozone plays in the chemistry and in the thermal structure of the atmosphere: a change in the amount of ozone could lead to a warming or cooling of the Earth (depending on the altitude at which the change occurs), and it could affect the Earth’s climate The attention to closely monitoring the ozone evolution remains high

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