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Abstract

Tropopause folds are the key process underlying stratosphere-to-troposphere transport (STT) of
ozone, and thus they affect tropospheric ozone levels and variability. In the present study we perform a process-
oriented evaluation of Copernicus Atmosphere Monitoring Service (CAMS) reanalysis (CAMSRA) O3 during
folding events over Europe and for the time period from 2003 to 2018. A 3-D labeling algorithm is applied to
detect tropopause folds in CAMSRA, while ozonesonde data from WOUDC (World Ozone and Ultraviolet Radi-
ation Data Centre) and aircraft measurements from IAGOS (In-service Aircraft for a Global Observing System)
are used for CAMSRA O3 evaluation. The profiles of observed and CAMSRA O3 concentrations indicate that
CAMSRA reproduces the observed O3 increases in the troposphere during the examined folding events. Never-
theless, at most of the examined sites, CAMSRA overestimates the observed O3 concentrations, mostly at the
upper portion of the observed increases, with a median fractional gross error (FGE) among the examined sites
> 0.2 above 400 hPa. The use of a control run without data assimilation reveals that the aforementioned overes-
timation of CAMSRA O3 arises from the data assimilation implementation. Overall, although data assimilation
assists CAMSRA O3 to follow the observed O3 enhancements in the troposphere during the STT events, it intro-
duces biases in the upper troposphere resulting in no clear quantitative improvement compared to the control run
without data assimilation. Less biased assimilated O3 products, with finer vertical resolution in the troposphere,
in addition to higher IFS (Integrated Forecasting System) vertical resolution, are expected to provide a better
representation of O3 variability during tropopause folds.