On the Long-term Stability of Satellite and Ground-based Ozone Profile Records

Abstract : In recent years, many analyses of space- and ground-based data records reported signs or evidence of increasing ozone concentrations in the extrapolar upper stratosphere since the late 1990s. However, the magnitude and significance of the trend estimates vary from one study to another, prompting the ozone research community to further investigate the causes of these differences. A broader consensus has emerged in the past year, placing the positive trend in the upper stratosphere on solid ground and heralding the start of an observation-based exploration of the recovery of stratospheric ozone. More accurate trend estimates are needed to identify the geophysical processes contributing to the recovery and their relative importance. Uncovering seasonal and spatial trend patterns will be key in reaching this objective, not just in the extrapolar upper stratosphere but elsewhere as well. However, at the moment, it remains unclear whether current ozone profile observing systems are able to provide this information. We address this question with an exploration of the capabilities and limitations of current data records in space (limb/occultation sounders) and on the ground (NDACC/GAW/SHADOZ-affiliated sonde, stratospheric lidar and microwave radiometer sites) to infer decadal trends and their vertical, latitudinal and seasonal patterns. We focus on long-term stability, one of the key drivers of the ability to detect trends. We present updated results of a comprehensive analysis that allowed us to quantify the drift of satellite data relative to the ground-based networks (Hubert et al., 2016). In a companion analysis we exploited the satellite data to uncover temporal and spatial inhomogeneities in the ground-based time series, some of which were traced to known changes occurring at different moments across the network. These changes add to the challenge to derive unbiased ozone trends from ground-based observations and they impede our ability to constrain satellite drift to the level required for current and future ozone trend assessments. We conclude that ongoing efforts to homogenise the ground-based data records are essential.