Abstract

Abstract. Atmospheric ozone plays a key role in air quality and the radiation budget of the Earth, both directly and through its chemical influence on other trace gases. Assessments of the atmospheric ozone distribution and associated climate change therefore demand accurate vertically resolved ozone observations with both stratospheric and tropospheric sensitivity, on both global and regional scales, and both in the long term and at shorter timescales. Such observations have been acquired by two series of European nadir-viewing ozone profilers, namely the scattered-light UV–visible spectrometers of the GOME family, launched regularly since 1995 (GOME, SCIAMACHY, OMI, GOME-2A/B, TROPOMI, and the upcoming Sentinel-5 series), and the thermal infrared emission sounders of the IASI type, launched regularly since 2006 (IASI on Metop platforms and the upcoming IASI-NG on Metop-SG). In particular, several Level-2 retrieved, Level-3 monthly gridded, and Level-4 assimilated nadir ozone profile data products have been improved and harmonized in the context of the ozone project of the European Space Agency's Climate Change Initiative (ESA Ozone_cci). To verify their fitness for purpose, these ozone datasets must undergo a comprehensive quality assessment (QA), including (a) detailed identification of their geographical, vertical, and temporal domains of validity; (b) quantification of their potential bias, noise, and drift and their dependences on major influence quantities; and (c) assessment of the mutual consistency of data from different sounders. For this purpose we have applied to the Ozone_cci Climate Research Data Package (CRDP) released in 2017 the versatile QA and validation system Multi-TASTE, which has been developed in the context of several heritage projects (ESA's Multi-TASTE, EUMETSAT's O3M-SAF, and the European Commission's FP6 GEOmon and FP7 QA4ECV). This work, as the second in a series of four Ozone_cci validation papers, reports for the first time on data content studies, information content studies and ground-based validation for both the GOME- and IASI-type climate data records combined. The ground-based reference measurements have been provided by the Network for the Detection of Atmospheric Composition Change (NDACC), NASA's Southern Hemisphere Additional Ozonesonde programme (SHADOZ), and other ozonesonde and lidar stations contributing to the World Meteorological Organisation's Global Atmosphere Watch (WMO GAW). The nadir ozone profile CRDP quality assessment reveals that all nadir ozone profile products under study fulfil the GCOS user requirements in terms of observation frequency and horizontal and vertical resolution. Yet all L2 observations also show sensitivity outliers in the UTLS and are strongly correlated vertically due to substantial averaging kernel fluctuations that extend far beyond the kernel's 15 km FWHM. The CRDP typically does not comply with the GCOS user requirements in terms of total uncertainty and decadal drift, except for the UV–visible L4 dataset. The drift values of the L2 GOME and OMI, the L3 IASI, and the L4 assimilated products are found to be overall insignificant, however, and applying appropriate altitude-dependent bias and drift corrections make the data fit for climate and atmospheric composition monitoring and modelling purposes. Dependence of the Ozone_cci data quality on major influence quantities – resulting in data screening suggestions to users – and perspectives for the Copernicus Sentinel missions are additionally discussed.

Highlights

  • Climate studies related to atmospheric composition and the Earth’s radiation budget require accurate monitoring of the horizontal and vertical distribution of ozone on the global scale and in the long term (WMO, 2010)

  • This work, the second in a series of four Ozone_cci papers, reports for the first time on data content studies, information content studies, and comparisons with co-located groundbased reference observations for all 13 nadir ozone profile data products that are part of the Climate Research Data Package (CRDP) on atmospheric ozone of the European Space Agency’s Climate Change Initiative. These products consist of five L2 UV–VIS instrument retrieval datasets, two L2 Thermal infrared (TIR) retrieval datasets, four UV– VIS L3 monthly gridded data series, a merged UV–VIS L4 product, and a 0 to 6 km integrated tropospheric L3 product based on Infrared Atmospheric Sounding Interferometers (IASI)-A data

  • To verify their fitness for purpose and especially their compliance with the requirements identified for the Global Climate Observing System (GCOS), these ozone datasets were subjected to a comprehensive quality assessment system developed in several heritage projects

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Summary

Introduction

Climate studies related to atmospheric composition and the Earth’s radiation budget require accurate monitoring of the horizontal and vertical distribution of ozone on the global scale and in the long term (WMO, 2010). Over the past decades these retrievals have been frequently quality-checked and often improved in order to meet climate research user requirements like the Global Climate Observing System (GCOS) targets (WMO, 2010) Both the verification of retrieval algorithm updates and the validation of their outputs against fiducial reference measurements (FRM) are still essential parts of the climate monitoring process, to be performed by specialized independent groups (Donlon and Zibordi, 2014; Loew et al, 2017). These are namely the Rutherford Appleton Laboratory (RAL, UK) version 2.14 for the backscatter UV–VIS instruments and the FORLI (Fast Optimal Retrievals on Layers for IASI) version 20151001 for the thermal infrared mission instruments, developed at the RAL and by the cooperation of the Belgian ULB (Université Libre de Bruxelles, Belgium) and the French LATMOS (Laboratoire Atmosphères, Milieux, Observations Spatiales, Paris, France), respectively. Molec m−2 climate modelling, e.g. in view of CCI contributions to the Tropospheric Ozone Assessment Report (TOAR)

CRDP overview
L2 UV–VIS retrieval algorithm
L2 TIR retrieval algorithm
L3 monthly gridded data
L4 data assimilation
Quality assessment of atmospheric satellite data
Ground-based reference data selection
Co-location and harmonization of satellite and reference data
Data content
Information quantities
Degrees of freedom in the signal
Height-resolved information content
Comparison statistics
L2 UV–VIS nadir ozone profiles
L3 UV–VIS monthly gridded ozone product
UV–VIS L2 and L3 drift studies
L4 assimilated data
L2 TIR nadir ozone profiles
L3 TIR monthly gridded tropospheric ozone product
Findings
Discussion
Conclusions
Full Text
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