Harmonisation of Free Tropospheric Ozone Satellite Data Records in Support of TOAR Phase II

The first TOAR assessment encountered several observational challenges that limited the confidence in estimates of the burden, short-term variability and long-term changes of ozone in the free troposphere. One of these challenges is the difficulty to interpret tropospheric observations from space, especially when combining data records from multiple satellites with differences in vertical sensitivity, prior information, resolution and spatial domain. Additional confounding factors are time-varying biases and the lack of harmonisation of geophysical quantities, units and definition of the tropospheric top level. All together, these increased the uncertainty on the distribution and trends of tropospheric ozone, impeding firm assessments relevant for policy and science. These challenges motivated the Committee on Earth Observation Satellites (CEOS) to initiate a coordinated activity on improving assessments of tropospheric ozone measured from space. Here, we report on work that contributes to this CEOS activity and to various Working Groups (SOWG, OPT, HEGIFTOM, TOP, ROSTEES, ...) of the ongoing second TOAR assessment.

Our primary objective is to harmonise the vertical perspective of different satellite data records. A first class of tropospheric ozone products is obtained through an inversion of spectral measurements by nadir-viewing sounders into a vertical profile. We describe two complementary approaches (Prior Replacement and Complete Data Fusion) to harmonise the differing profile retrieval set-up for GOME-2, IASI and other UV-visible and infrared nadir sensors, using information conveyed in the prior and the averaging kernels. A second class of products is obtained through subtraction of the stratospheric component from total column retrievals. The stratospheric column is derived with various methods, resulting in differing spatial coverage, tropospheric top level, sampling frequency, etc…  We present how all tropospheric ozone products, from both classes, are harmonised to a common tropospheric top level. We then intercompare all harmonised satellite records, and report on the differences and how these reduce upon harmonisation. Finally, we reflect on the importance of the vertical harmonisation process to improve constraints of the spatial distribution and trends in tropospheric ozone.

Acknowledgements : We are grateful to the sustained effort and committment of the teams, institutes and agencies that collect and provide satellite and ozonesonde data records of high quality.