Mean age of air (AoA) is a diagnostic of transport along the stratospheric Brewer–Dobson circulation. While models consistently show negative trends, long-term time series (1975–2016) of AoA derived from observations show non-significant positive trends in mean AoA in the Northern Hemisphere. This discrepancy between observed and modelled mean AoA trends is still not resolved. There are uncertainties and assumptions required when deriving AoA from trace gas observations. At the same time, AoA from climate models is subject to uncertainties, too.

In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF$_{6}$ measurements in Engel et al. (2009, 2017). To correct for the non-linear increase in SF$_{6}$ concentrations, a quadratic fit to the time series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors in the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study.

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In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF$_{6}$ measurements in Engel et al. (2009, 2017). To correct for the non-linear increase in SF$_{6}$ concentrations, a quadratic fit to the time series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors in the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study.

... mehr

Zugehörige Institution(en) am KIT |
Institut für Meteorologie und Klimaforschung - Atmosphärische Spurenstoffe und Fernerkundung (IMK-ASF) KIT-Zentrum Klima und Umwelt (ZKU) |

Publikationstyp |
Zeitschriftenaufsatz |

Publikationsjahr |
2020 |

Sprache |
Englisch |

Identifikator |
ISSN: 1680-7316, 1680-7324 KITopen-ID: 1000123392 |

Erschienen in |
Atmospheric chemistry and physics |

Verlag |
European Geosciences Union (EGU) |

Band |
20 |

Heft |
14 |

Seiten |
8709-8725 |

Nachgewiesen in |
Dimensions Web of Science Scopus |

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