LOTUS – Long-term Ozone Trends and Uncertainties in the Stratosphere

Twenty years after the peak of ozone depleting substances in the stratosphere, a clear understanding of ozone trends and their significance as a function of altitude and latitude is still needed. Following the SI2N activity (sponsored by SPARC, IO3C, IGACO-O3/UV and NDACC) that evaluated ozone profile trends from long-term satellite and ground-based records, the LOTUS activity aims at providing a clear evaluation of uncertainties in trend studies and in particular the consideration of errors linked to the sampling and stability of (merged) data sets that could not be achieved by the end of SI2N. Sponsored by APARC, WMO and IO3C, LOTUS has for objectives:

(a) to update and extend stratospheric ozone observations to recent years,

(b) to improve our understanding of crucial yet poorly known sources of uncertainties in trend retrieval,

(c) to investigate how uncertainties interact and propagate through the different stages of analysis chain,

and (d) to re-evaluate current best practice(s) and possibly establish more suitable alternatives.

The 1^st phase of LOTUS (2016-2018) was committed to  updating and refining stratospheric ozone trend analyses in time for the WMO/UNEP 2018 Ozone Assessment. Due to time constraints, it focused on the evaluation of global ozone trends from the most recent versions of merged satellite data sets, using a specially designed LOTUS trend model, and comparison of these results to trends from ground-based records and simulations from the CCMI initiative. Special attention was given to the combination of trends in broad latitude regions and evaluation of their significance as a function of altitude. Results from the first phase are published in the LOTUS Report .

The 2^nd phase of LOTUS activity (2018-2022) used newly updated ozone profile records, in particular new homogenized ozonesonde records that were not available for the 1^st phase and thus were not included in the LOTUS Report. The focus was on the open issues left unresolved after the first phase, namely:

• Development of techniques to directly assess uncertainties in the merged records resulting from small relative drifts, transformation between geometric altitude and pressure-based vertical coordinates and sampling frequency differences.
• Evaluation of long-term stability in satellite and ground-based data records.
• Optimization of the LOTUS trend model for analyses of the ground-based records in order to improve interpretation of dynamical processes that impact ozone changes over regions of limited spatial extent.
• Explore trends in polar regions and in the lower stratosphere. The latter will be done in conjunction with the OCTAV-UTLS APARC activity dedicated to the assessment of atmospheric composition and its decadal changes in the UTLS region.
• Comprehensive evaluation of the coherence between stratospheric and total column ozone trends.