Stratosphere / Ozone

The creation of homogenized ozone datasets based on limb and occultation measurements from ENVISAT sensors (GOMOS, MIPAS, SCIAMACHY) as well as from ESA Third Party Missions (OSIRIS, SMR and ACE-FTS) is one of the objectives of the on-going ESA ozone-CCI project.

In the framework of the ozone-CCI project, different datasets are created. They include the HARMOZ (HARMonized dataset of Ozone profiles), monthly zonal mean datasets, semi-monthly mean dataset with the resolved longitudinal structure, spatio-temporal distributions in the UTLS.  In this presentation, we introduce these datasets and show data analyses based on them. The special focus of our presentation is the merged long-term dataset for analyses of trends in the vertical distribution of ozone.

The evolution of the vertical distribution of atmospheric ozone, an Essential Climate Variable (ECV), has been monitored over the past few decades by various instruments in space. Ozone research activities depend more and more on global, stable and consistent multi-decennial ozone profile data records to make further progress in understanding the interactions between changes in ozone, ultraviolet radiation and climate. This need led to several international initiatives to improve the synergistic use of complementary data records by satellite missions. ESA’s Climate Change Initiative project on ozone (Ozone_cci) represents a significant contribution to this effort. It is aimed at collecting, improving and merging observations from instruments aboard the European platforms ERS-2 (GOME), Envisat (GOMOS, MIPAS and SCIAMACHY) and MetOp (GOME-2, IASI), and aboard Third Party Missions Odin (OSIRIS, SMR) and SCISAT (ACE-FTS). Several prototype data records were developed in the first phase of the project: (1) a homogenized L2 data set of screened ozone profiles from each individual sensor, and (2) temporally and zonally averaged (at various resolutions) L3 data sets for each sensor separately and for all sensors combined. In the current second phase of the project, the Climate Research Data Package (CRDP) is being improved and expanded with new instruments (e.g. IASI, SAGE II and Aura-MLS) and new data products (focusing e.g. on the UT/LS and the mesosphere).

Validation is one of the cornerstones of the CCI programme, with a threefold objective: identifying the optimal retrieval algorithm for each instrument, characterising all products in the CRDP, and assessing their compliance with GCOS requirements and specific research needs. Here, validation results are reported for the new and updated ozone profile products developed during the second phase of the Ozone_cci project. Validation analyses include the exploration of data content (spatio-temporal coverage and sampling) and information content (vertical averaging kernels), and comparison of the satellite data to co-located ground-based observations collected by the NDACC/GAW/SHADOZ ozonesonde and NDACC stratospheric ozone lidar networks. Preliminary estimates of the decadal stability of the GOME and GOME-2A nadir ozone profile data sets will be reported, together with patterns in bias and spread. The agreement of 2008 IASI nadir ozone profile data with correlative ground-based measurements will be presented as well. The tropospheric ozone column data derived by vertically integrating GOME-2A and IASI profiles will be compared to tropospheric ozone columns derived from co-located ozonesonde data. In the second part of this paper two areas of progress in the limb/occultation CRDP will be discussed, i.e. the new algorithm versions developed for most ESA and TPM instruments, and the addition of the latest ozone records of several US sounders (SAGE II, HALOE, Aura-MLS). Each of the limb/occultation data sets has been compared to ground-based observations by ozonesonde and lidars. For each data record, we report the latest results of decadal stability, overall bias and short-term variability, and their dependence on altitude, latitude and geophysical or instrument parameters. The evolution in data quality compared to the previous release of the CRDP is evaluated, and an overview is given of the overall consistency between the different satellite missions.

Within the framework of the first phase of the European Space Agency's Climate Change Initiative Ozone project (ESA Ozone_cci), a global comprehensive total ozone level-3 data record covering the period from 1995 to 2011 has been generated. The so-called GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record has been compiled from European satellite sensors GOME/ERS-2, SCIAMACHY/ENVISAT, and GOME-2/MetOp-A, thereby taking advantage of the high inter-sensor consistency and excellent long-term stability, which has been achieved through the application of the common retrieval algorithm GODFIT_V3 and a soft-calibration approach. The data record has been used to analyze the past evolution of the ozone layer on both global and regional scales. The various aspects of ozone change and variability have been investigated, in which a key issue was the detection of the expected onset of ozone recovery and its spatial fingerprint as a consequence of the 1987 Montreal Protocol.

Within the scope of the second phase of ESA's Ozone_cci project the GTO-ECV data record is further extended until 2015 with GOME-2/MetOp-B as well as OMI/Aura observations. Due to its excellent temporal stability OMI follows GOME as the new long-term reference for the other instruments and ensures the high quality of the merged total ozone data record. The extended GTO-ECV then provides global total ozone information for the last 20 years. This enables us to update and to reinvestigate the long-term evolution of the ozone layer and to gain new insights on the impact of the various natural and anthropogenic forcings on total ozone. Of particular interest is the ozone variability in the middle latitudes - due to complex feedback mechanisms between chemical and dynamical processes - which still masks the expected onset of recovery. Ground-based measurements at various locations are used to confirm the satellite-based observations and findings. Furthermore, two long-term Chemistry-Climate Model simulations will be confronted with the GTO-ECV data record in order to address further scientific questions related to changes in the ozone distribution.

Atmospheric ozone is an Essential Climate Variable which impacts the radiation budget of the Earth, interacts with atmospheric dynamics and climate, and influences chemically other radiatively active species. As part of the Ozone Climate Change Initiative (Ozone_cci) project, a large number of ozone data sets have been generated from a full suite of atmospheric chemistry satellite missions. Following a first phase of 3 years during which new and improved algorithms and data products have been demonstrated and assessed against well-defined user requirements, the ongoing second phase of the Ozone_cci concentrates on extending and further improving these data sets with the ambition to realize the full potential of the existing archive of satellite ozone sensors. We present an overview of the main realizations of the project. This covers long-series of consistent ozone columns and profiles derived from nadir UV sensors and the thermal infrared IASI instrument. Also addressed is the generation of a large scale coherent data base of vertically resolved ozone measurements derived from a full suite of limb and occultation sensors, optimised for accuracy in a broad range of altitudes extending from the UT/LS to the mesosphere.

RAL’s ozone profile retrieval scheme for the GOME-class of solar uv/vis backscatter spectrometer has unique sensitivity to tropospheric ozone, which led to its selection for ozone profile retrieval from this class of sensor in ESA‘s Climate Change Initiative. The JASMIN/CEMS computing facility at RAL is enabling production of full-mission global data sets from GOME-1, SCIAMACHY, OMI and GOME-2A & 2B, resulting in a 20 year height-resolved dataset for ozone from 1995-2015, spanning both stratosphere and troposphere.

We present some highlights of this dataset, including comparisons with coupled chemistry climate models, chemical transport models and MACC/CAMS analyses.

The RAL ozone profile scheme has been transferred to a near-real time chain for MetOp on the JASMIN/CEMS facility at RAL, and data are under assessment at ECMWF for the Copernicus Atmospheric Monitoring Service.  In the coming year, it is intended to apply the RAL scheme to Sentinel-5 Precursor data, to compare with ESA’s operational S5P scheme and with our concurrent retrievals from GOME-2.

Land surface reflectivity is higher and Rayleigh scattering lower in the visible (Chappuis) band than the uv (Hartley-Huggins) bands used in our standard ozone retrieval scheme, offering a potential increase in near-surface sensitivity. There are however challenges to overcome to exploit the visible band, due to ozone absorption features being shallower and broader than those in the Huggins bands and therefore less easily separated from surface spectral reflectance features and instrumental artefacts. We shall discuss progress on the addition of the visible band to GOME-2 retrievals, in anticipation also of NASA’s TEMPO mission.