Quality assessment of the Ozone-cci Climate Research Data Package (release 2017)-Part 1: Ground-based validation of total ozone column data products

Abstract

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 0.5ĝ€% at moderate latitudes (±50°), whereas the level-3 data sets show mean differences with respect to the OMI reference data record that span between ĝ'0.2ĝ€±ĝ€0.9ĝ€% (for GOME-2B) and 1.0ĝ€±ĝ€1.4ĝ€% (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 1.0ĝ€±ĝ€1.0ĝ€% for all sensors, the drift per decade spans between ĝ'0.5ĝ€% and 1.0ĝ€±ĝ€1.0ĝ€% depending on the sensor, and the peak-To-peak seasonality of the differences ranges from ĝ1/4 1ĝ€% for GOME and OMI to ĝ1/4 2ĝ€% 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 2ĝ€% 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 ĝ'0.11ĝ€±ĝ€0.10ĝ€%ĝ€decadeĝ'1 for Dobson and +0.22ĝ€±ĝ€0.08ĝ€%ĝ€decadeĝ'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 1-3ĝ€% and the short-Term and long-Term accuracy requirements of 2 and 3ĝ€%, 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.