Chapter 4: The LOTUS regression model

One of the primary motivations of the LOTUS effort is to attempt to reconcile the discrepancies in ozone trend results from the wealth of literature on the subject. Doing so requires investigating the various methodologies employed to derive long-term trends in ozone as well as to examine the large array of possible variables that feed into those methodologies and analyse their impacts on potential trend results. Given the limited amount of time, the LOTUS group focused on the most common methodology of multiple linear regression and performed a number of sensitivity tests with the goal of trying to establish best practices and come to a consensus on a single regression model to use for this study. This chapter discusses the details and results of the sensitivity tests before describing the components of the final single model that was chosen and the reasons for that choice

The site-specific primary calibration conditions for the Brewer spectrophotometer

The Brewer ozone spectrophotometer (the Brewer) is one of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW)'s standard ozone-monitoring instruments since the 1980s. The entire global Brewer ozone-monitoring network is operated and maintained via a hierarchical calibration chain, which started from world reference instruments that are independently calibrated via the primary calibration method (PCM) at a premium site (National Oceanic and Atmospheric Administration's (NOAA) Mauna Loa Observatory, Hawaii). These world reference instruments have been maintained by Environment and Climate Change Canada (ECCC) in Toronto for the last 4 decades. Their calibration is transferred to the travelling standard instrument and then to network (field) Brewer instruments at their monitoring sites (all via the calibration transfer method; CTM). Thus, the measurement accuracy for the entire global network is dependent on the calibration of world reference instruments. In 2003, to coordinate regional calibration needs, the Regional Brewer Calibration Center for Europe (RBCC-E) was formed in Izaña, Spain. From that point, RBCC-E began calibrating regional references also via PCM instead of CTM. The equivalency and consistency of world and regional references are then assured during international calibration campaigns. In practice, these two calibration methods have different physical requirements, e.g., the PCM requires a stable ozone field in the short term (i.e., half-day), while the CTM would benefit from larger changes in slant ozone conditions for the calibration periods. This difference dictates that the PCM can only be implemented on Brewer instruments at certain sites and even in certain months of the year. This work is the first effort to use long-term observation records from 11 Brewer instruments at four sites to reveal the challenges in performing the PCM. By utilizing a new calibration simulation model and reanalysis ozone data, this work also quantifies uncertainties in the PCM due to short-term ozone variability. The results are validated by real-world observations and used to provide scientific advice on where and when the PCM can be performed and how many days of observations are needed to achieve the calibration goal (i.e., ensure the calibration uncertainty is within a determined criterion, i.e., ≤5 R6 units; R6 is a measurement-derived double ratio in the actual Brewer processing algorithm). This work also suggests that even if the PCM cannot be used to deliver final calibration results for mid- or high-latitude sites, the statistics of the long-term PCM fitting results can still provide key information for field Brewer instruments as stability indicators (which would provide performance monitoring and data quality assurance).</p

An update on ozone profile trends for the period 2000 to 2016

Ozone profile trends over the period 2000 to 2016 from several merged satellite ozone data sets and from ground-based data measured by four techniques at stations of the Network for the Detection of Atmospheric Composition Change indicate significant ozone increases in the upper stratosphere, between 35 and 48 km altitude (5 and 1 hPa). Near 2 hPa (42 km), ozone has been increasing by about 1.5 % per decade in the tropics (20° S to 20° N), and by 2 to 2.5 % per decade in the 35 to 60° latitude bands of both hemispheres. At levels below 35 km (5 hPa), 2000 to 2016 ozone trends are smaller and not statistically significant. The observed trend profiles are consistent with expectations from chemistry climate model simulations. This study confirms positive trends of upper stratospheric ozone already reported, e.g., in the WMO/UNEP Ozone Assessment 2014 or by Harris et al. (2015). Compared to those studies, three to four additional years of observations, updated and improved data sets with reduced drift, and the fact that nearly all individual data sets indicate ozone increase in the upper stratosphere, all give enhanced confidence. Uncertainties have been reduced, for example for the trend near 2 hPa in the 35 to 60° latitude bands from about ±5 % (2σ) in Harris et al. (2015) to less than ±2 % (2σ). Nevertheless, a thorough analysis of possible drifts and differences between various data sources is still required, as is a detailed attribution of the observed increases to declining ozone-depleting substances and to stratospheric cooling. Ongoing quality observations from multiple independent platforms are key for verifying that recovery of the ozone layer continues as expected

Stratospheric Ozone Vertical Distribution at Select NOAA Global Monitoring Laboratory Dobson Monitoring Stations and Updated Trends of the Based on the LOTUS Regression Model

International audienceNOAA’s ground-based (GB) remote sensing and in situ instruments continuously track stratospheric ozone recovery in response to the Montreal Protocol and its amendments. Long-term records of daily Dobson total column ozone, daily Dobson/Umkehr vertical ozone distribution (with a recent new homogenization), and weekly ozonesonde profiles are well-calibrated. Regular intra-instrument comparisons aid in the tracking of instrumental changes. Additionally, NOAA’s homogenized satellite record (COH) from SBUV, SBUV/2 and OMPS provides information on ozone vertical distribution in zonal averages allowing the study of large-scale ozone variability. Overpass datasets provide further GB validation including verification of the homogenization of the GB records. The NOAA AC4 funded project assesses the consistency of the trends derived from the different records. We present an updated evaluation of stratospheric ozone profile trends from the long-term ozone record with focus on the 2000–2020 period. Analyses were performed using the updated version (0.8.0) of the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) Independent Linear Trend regression model. We primarily focus on Boulder, Colorado (40.0N, 105.3W), Haute Provence, France (43.9N, 5.8E), Lauder, New Zealand (45.04S, 169.68E), and Mauna Loa and Hilo, Hawaii (19.5N, 155.58W) which have both sonde and Umkehr measurements, and add other stations of opportunity (i.e. Arosa, Switzerland (46.8N, 9.68E). The Northern Hemispheric sites of Arosa, Haute Provence, and Mauna Loa all show positive trends in the mid to upper Stratosphere with trends peaking at ~3%/decade. Trends in the upper stratosphere at Boulder and Lauder are positive, but not statistically significant. In the lower stratosphere, trends are mostly negative, but trend uncertainties here are quite large. The causes of differences in ozonesonde trends at Lauder and OHP as compared to other datasets are being explored. Additionally, the Equivalent Latitude was investigated as an additional dynamical proxy in the LOTUS model at these sites, and as selection criteria for the overpass records. We will discuss the addition of the Eq Lat proxy and its impact on the trends and uncertainties

Stratospheric Ozone Vertical Distribution at Select NOAA Global Monitoring Laboratory Dobson Monitoring Stations and Updated Trends of the Based on the LOTUS Regression Model

International audienceNOAA’s ground-based (GB) remote sensing and in situ instruments continuously track stratospheric ozone recovery in response to the Montreal Protocol and its amendments. Long-term records of daily Dobson total column ozone, daily Dobson/Umkehr vertical ozone distribution (with a recent new homogenization), and weekly ozonesonde profiles are well-calibrated. Regular intra-instrument comparisons aid in the tracking of instrumental changes. Additionally, NOAA’s homogenized satellite record (COH) from SBUV, SBUV/2 and OMPS provides information on ozone vertical distribution in zonal averages allowing the study of large-scale ozone variability. Overpass datasets provide further GB validation including verification of the homogenization of the GB records. The NOAA AC4 funded project assesses the consistency of the trends derived from the different records. We present an updated evaluation of stratospheric ozone profile trends from the long-term ozone record with focus on the 2000–2020 period. Analyses were performed using the updated version (0.8.0) of the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) Independent Linear Trend regression model. We primarily focus on Boulder, Colorado (40.0N, 105.3W), Haute Provence, France (43.9N, 5.8E), Lauder, New Zealand (45.04S, 169.68E), and Mauna Loa and Hilo, Hawaii (19.5N, 155.58W) which have both sonde and Umkehr measurements, and add other stations of opportunity (i.e. Arosa, Switzerland (46.8N, 9.68E). The Northern Hemispheric sites of Arosa, Haute Provence, and Mauna Loa all show positive trends in the mid to upper Stratosphere with trends peaking at ~3%/decade. Trends in the upper stratosphere at Boulder and Lauder are positive, but not statistically significant. In the lower stratosphere, trends are mostly negative, but trend uncertainties here are quite large. The causes of differences in ozonesonde trends at Lauder and OHP as compared to other datasets are being explored. Additionally, the Equivalent Latitude was investigated as an additional dynamical proxy in the LOTUS model at these sites, and as selection criteria for the overpass records. We will discuss the addition of the Eq Lat proxy and its impact on the trends and uncertainties

Validation of SAGE III - ISS Ozone with NOAA OMPS and Ground -based Instruments in the Context of Prior Satellites

International audienceSAGE III (Stratospheric Aerosol and Gas Experiments) on ISS has provided limb measurements of ozone profiles since late 2017. SAGE III joins its predecessors SAGE I (1979-1981), SAGE II (1984-2005) and SAGE III Meteor-3M (2002-2005) which are often the backbone of several merged ozone profile data records and associated trend studies such as discussed in the SPARC/LOTUS [2018] report. The focus of this study will be on long-term stability. The increasing duration of the measurements opens the opportunity for review of instrument stability. This effort will additionally characterize the compatibility of SAGE III ISS with SAGE II using SBUV/2 and OMPS and available ground-based instruments as transfer standards. SBUV/2 measurements are available from the NOAA Polar Orbiters from 1978 to present, measuring nearly globally with 15 orbits per day. The OMPS Nadir Profiler (NP) on Suomi-NPP and NOAA-20 provide a follow on to SBUV with similar measurement traits. The vertical resolution of the SBUV/OMPS is smoothed as compared to the SAGE Limb, but abundant coverage of the NOAA satellites provides good opportunity for frequent comparisons. NOAA's ground-based instruments (GB) include vertical distribution of ozone from Dobson Umkehr and ozonesonde profiling with a long record. Microwave and lidar datasets from NDACC additionally provide opportunities to study the compatibility of the SAGE III instruments. Microwave specifically allows the exploration of the diurnal properties of SAGE as seen with sunrise/sunset sampling. SPARC/IOC/GAW (2018): SPARC/IOC/GAW report on Long-term Ozone Trends and Uncertainties in the Stratosphere. SPARC Report No. 9, WCRP-17/2018, GAW Report No. 241, doi: 10.17874/f899e57a20b

Validation of SAGE III - ISS Ozone with NOAA OMPS and Ground -based Instruments in the Context of Prior Satellites

International audienceSAGE III (Stratospheric Aerosol and Gas Experiments) on ISS has provided limb measurements of ozone profiles since late 2017. SAGE III joins its predecessors SAGE I (1979-1981), SAGE II (1984-2005) and SAGE III Meteor-3M (2002-2005) which are often the backbone of several merged ozone profile data records and associated trend studies such as discussed in the SPARC/LOTUS [2018] report. The focus of this study will be on long-term stability. The increasing duration of the measurements opens the opportunity for review of instrument stability. This effort will additionally characterize the compatibility of SAGE III ISS with SAGE II using SBUV/2 and OMPS and available ground-based instruments as transfer standards. SBUV/2 measurements are available from the NOAA Polar Orbiters from 1978 to present, measuring nearly globally with 15 orbits per day. The OMPS Nadir Profiler (NP) on Suomi-NPP and NOAA-20 provide a follow on to SBUV with similar measurement traits. The vertical resolution of the SBUV/OMPS is smoothed as compared to the SAGE Limb, but abundant coverage of the NOAA satellites provides good opportunity for frequent comparisons. NOAA's ground-based instruments (GB) include vertical distribution of ozone from Dobson Umkehr and ozonesonde profiling with a long record. Microwave and lidar datasets from NDACC additionally provide opportunities to study the compatibility of the SAGE III instruments. Microwave specifically allows the exploration of the diurnal properties of SAGE as seen with sunrise/sunset sampling. SPARC/IOC/GAW (2018): SPARC/IOC/GAW report on Long-term Ozone Trends and Uncertainties in the Stratosphere. SPARC Report No. 9, WCRP-17/2018, GAW Report No. 241, doi: 10.17874/f899e57a20b

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

International audienceIn 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