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DOI	10.5194/acp-20-8709-2020
	Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6
	Fritsch F.; Garny H.; Engel A.; Bönisch H.; Eichinger R.
发表日期	2020
ISSN	1680-7316
起始页码	8709
结束页码	8725
卷号	20 期号:14
英文摘要	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 SF6 measurements in Engel et al. (2009, 2017). To correct for the non-linear increase in SF6 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. For this, we use the EMAC (ECHAM MESSy Atmospheric Chemistry) climate model as a test bed, where AoA derived from a linear tracer is available as a reference and modelled age spectra exist to diagnose the actual spatial age spectra widths. The comparison of mean AoA from the linear tracer with mean AoA from a SF6 tracer shows systematic deviations specifically in the trends due to the selection of the parameters. However, for an appropriate parameter selection, good agreement for both mean AoA and its trend can be found, with deviations of about 1 % in mean AoA and 12 % in AoA trend. In addition, a method to derive mean AoA is evaluated that applies a convolution to the reference time series. The resulting mean AoA and its trend only depend on an assumption about the ratio of moments. Also in that case, it is found that the larger the ratio of moments, the more the AoA trend gravitates towards the negative. The linear tracer and SF6 AoA are found to agree within 0.3 % in the mean and 6 % in the trend. The different methods and parameter selections were then applied to the balloon-borne SF6 and CO2 observations. We found the same systematic changes in mean AoA trend dependent on the specific selection. When applying a parameter choice that is suggested by the model results, the AoA trend is reduced from 0.15 to 0.07 years per decade. It illustrates that correctly constraining those parameters is crucial for correct mean AoA and trend estimates and still remains a challenge in the real atmosphere. © 2020 Author(s).
语种	英语
scopus关键词	atmospheric circulation; concentration (composition); Northern Hemisphere; sensitivity analysis; stratosphere; trace gas; tracer; trend analysis
来源期刊	ATMOSPHERIC CHEMISTRY AND PHYSICS
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/247627
作者单位	Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft-und Raumfahrt (DLR), Oberpfaffenhofen, Germany; Meteorological Institute Munich, Ludwig Maximilians University of Munich, Munich, Germany; Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt Am Main, Frankfurt am Main, Germany; Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
推荐引用方式 GB/T 7714	Fritsch F.,Garny H.,Engel A.,et al. Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6[J],2020,20(14).
APA	Fritsch F.,Garny H.,Engel A.,Bönisch H.,&Eichinger R..(2020).Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6.ATMOSPHERIC CHEMISTRY AND PHYSICS,20(14).
MLA	Fritsch F.,et al."Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6".ATMOSPHERIC CHEMISTRY AND PHYSICS 20.14(2020).