Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The long-term record of Umkehr measurements from four NOAA Dobson spectrophotometers was reprocessed after updates to the instrument calibration procedures. In addition, a new data quality-control tool was developed for the Dobson automation software (WinDobson). This paper presents a comparison of Dobson Umkehr ozone profiles from NOAA ozone network stations – Boulder, the Haute-Provence Observatory (OHP), the Mauna Loa Observatory (MLO), Lauder – against several satellite records, including Aura Microwave Limb Sounder (MLS; ver. 4.2), and combined solar backscatter ultraviolet (SBUV) and Ozone Mapping and Profiler Suite (OMPS) records (NASA aggregated and NOAA cohesive datasets). A subset of satellite data is selected to match Dobson Umkehr observations at each station spatially (distance less than 200 km) and temporally (within 24 h). Umkehr Averaging kernels (AKs) are applied to vertically smooth all overpass satellite profiles prior to comparisons. The station Umkehr record consists of several instrumental records, which have different optical characterizations, and thus instrument-specific stray light contributes to the data processing errors and creates step changes in the record. This work evaluates the overall quality of Umkehr long-term measurements at NOAA ground-based stations and assesses the impact of the instrumental changes on the stability of the Umkehr ozone profile record. This paper describes a method designed to correct biases and discontinuities in the retrieved Umkehr profile that originate from the Dobson calibration process, repair, or optical realignment of the instrument. The Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) Global Modeling Initiative (M2GMI) and NASA Global Modeling Initiative chemistry transport model (GMI CTM) ozone profile model output matched to station location and date of observation is used to evaluate instrumental step changes in the Umkehr record. Homogenization of the Umkehr record and discussion of the apparent stray light error in retrieved ozone profiles are the focus of this paper. Homogenization of ground-based records is of great importance for studies of long-term ozone trends and climate change.

Highlights

  • The success of 30-years of international collaborations since the implementation of the Montreal Protocol and its amendments were celebrated at the Symposium for the 30th Anniversary of the Montreal Protocol that brought together leading scientists, policymakers, and the public at the French Academy of Sciences in Paris, France (Godin40 Beekmann et al, 2018) on September 22-23, 2017

  • This paper presents a comparison of Dobson Umkehr ozone profiles from NOAA ozone network stations (Boulder, OHP, MLO, Lauder) against several satellite records, including Aura Microwave Limb Sounder (MLS; ver. 4.2), and combined Solar Backscatter Ultraviolet (SBUV) and OMPS records (NASA AGG and NOAA COH)

  • The previous approach of standardised stray light corrections helped with reduction of biases but did not eliminate the step changes in the station record associated with 585 instrumental changes

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Summary

Introduction

The success of 30-years of international collaborations since the implementation of the Montreal Protocol and its amendments were celebrated at the Symposium for the 30th Anniversary of the Montreal Protocol (http://www.montreal30.io3c.org/) that brought together leading scientists, policymakers, and the public at the French Academy of Sciences in Paris, France (Godin Beekmann et al, 2018) on September 22-23, 2017. 95 Some global locations that host a Dobson instrument have been providing routine, morning and afternoon, Umkehr observations to the World Ozone and UV Radiation Center, WOUDC, database, including a number of stations hosting Umkehr time series that start in the late 1950s This renders the Umkehr ozone profiles the longest ozone profile time series (Bojkov et al, 2002) and is central in validating other observational methods (Petropavlovskikh et al, 2005a), as well as numerical models that simulate and forecast ozone content changes (Zanis et al, 2006). 105 The Umkehr retrieval algorithm relies on the “self-calibration” technique that applies normalization of a set of morning or afternoon measurements to a single measurement selected at the smallest SZA This process removes the majority of the instrumental artifacts and homogenizes time series.

NOAA Dobson total ozone and Umkehr ozone profiles
Ozonesonde data
SBUV and OMPS ozone profile records
Aura MLS profiles
SAGE II ozone record
GMI CTM and M2GMI simulated ozone profiles
Combined MLS and ozonesonde record The
Standardised stray light corrections
Empirical correction methodology
Discussion of optimization results
Comparisons of optimized Umkehr time series against reference records
Changes in mean and seasonal biases
Findings
Temporal changes in optimized Umkehr time series
Conclusions
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