The GOME-type Total Ozone Essential Climate Variable (GTO-ECV) is a level-3
data record, which combines individual sensor products into one single
cohesive record covering the 22-year period from 1995 to 2016, generated in
the frame of the European Space Agency's Climate Change Initiative Phase II.
It is based on level-2 total ozone data produced by the GODFIT (GOME-type
Direct FITting) v4 algorithm as applied to the GOME/ERS-2, OMI/Aura,
SCIAMACHY/Envisat and GOME-2/Metop-A and Metop-B observations. In this paper
we examine whether GTO-ECV meets the specific requirements set by the
international climate–chemistry modelling community for decadal stability
long-term and short-term accuracy. In the following, we present the
validation of the 2017 release of the Climate Research Data Package Total
Ozone Column (CRDP TOC) at both level 2 and level 3. The inter-sensor
consistency of the individual level-2 data sets has mean differences
generally within 0.5 % at moderate latitudes (±50°),
whereas the level-3 data sets show mean differences with respect to the OMI
reference data record that span between −0.2 ± 0.9 % (for GOME-2B)
and 1.0 ± 1.4 % (for SCIAMACHY). Very similar findings are reported
for the level-2 validation against independent ground-based TOC observations
reported by Brewer, Dobson and SAOZ instruments: the mean bias between GODFIT
v4 satellite TOC and the ground instrument is well within 1.0 ± 1.0 %
for all sensors, the drift per decade spans between −0.5 % and 1.0 ± 1.0 % depending on the sensor, and the peak-to-peak seasonality of the
differences ranges from ∼ 1 % for GOME and OMI to
∼ 2 % for SCIAMACHY. For the level-3 validation, our first goal was to show that the level-3 CRDP produces findings consistent with the level-2 individual sensor comparisons.
We show a very good agreement
with 0.5 to 2 % peak-to-peak amplitude for the monthly mean difference
time series and a negligible drift per decade of the differences in the Northern Hemisphere
of −0.11 ± 0.10 % decade−1 for Dobson and +0.22 ± 0.08 % decade−1
for Brewer collocations. The exceptional quality
of the level-3 GTO-ECV v3 TOC record temporal stability satisfies well the
requirements for the total ozone measurement decadal stability of
1–3 % and the short-term and long-term accuracy requirements of 2
and 3 %, respectively, showing a remarkable inter-sensor consistency, both
in the level-2 GODFIT v4 and in the level-3 GTO-ECV v3 datasets, and
thus can be used for longer-term analysis of the ozone layer, such as decadal
trend studies, chemistry–climate model evaluation and data assimilation
applications
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