the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
12 Jan 2021
12 Jan 2021
The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis
- 1Institute for Energy and Climate Research: Stratosphere (IEK–7), Forschungszentrum Jülich, Jülich, Germany
- 2Institute for Atmospheric and Environmental Research, University of Wuppertal, Wuppertal, Germany
- 3Laboratoire de Météorologie Dynamique, UMR8539, IPSL, UPMC/ENS/CNRS/Ecole Polytechnique, Paris, France
- 4Institute for Atmospheric and Environmental Sciences, Goethe-University Frankfurt, Frankfurt, Germany
- 1Institute for Energy and Climate Research: Stratosphere (IEK–7), Forschungszentrum Jülich, Jülich, Germany
- 2Institute for Atmospheric and Environmental Research, University of Wuppertal, Wuppertal, Germany
- 3Laboratoire de Météorologie Dynamique, UMR8539, IPSL, UPMC/ENS/CNRS/Ecole Polytechnique, Paris, France
- 4Institute for Atmospheric and Environmental Sciences, Goethe-University Frankfurt, Frankfurt, Germany
Abstract. This paper investigates the global stratospheric Brewer–Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA–Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA–Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30–40 % weaker in ERA5, likely correcting a known bias in ERA–Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989–2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002–2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA–Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA–Interim.
Felix Ploeger et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2020-1253', Anonymous Referee #1, 09 Feb 2021
Summary:
The authors investigate the representation of the Brewer-Dobson circulation (BDC) in the new ERA5 reanalysis, as compared to the previous ERA-Interim product as well as observational estimates. To do so, they use a Lagrangian transport model (CLaMS) in order to calculate stratospheric age of air. ERA5 is found to show significantly older mean ages than ERA-Interim, by as much as 2 yr (50-75%) in the lower stratosphere, indicative of a slower mean BDC. In terms of trends, ERA5 is found to exhibit a decreasing age trend (strengthening circulation) throughout the stratosphere over the 1979-2018 period studied. In contrast, ERA-Interim shows a decreasing age trend in the SH but increasing in the NH. A closer look at the trends reveals an apparent step-change decrease in extratropical ERA5 age in the early-mid 1990s, which is less prominent in ERA-Interim. Comparison with observationally inferred ages suggests that the mean age values might be in slightly closer agreement for ERA-Interim, though the latitudinal structure may be better for ERA5. Overall, the authors conclude it is unclear whether the ERA5 stratospheric age of air climatology and trends are any better (or worse) than the previous ERA-Interim.
I found this to be a clear, interesting paper, which I very much enjoyed reading. The age of air calculations which the authors present will provide a useful comparison with the transformed Eulerian mean calculations from Diallo et al. 2020, as well as within the wider context of the S-RIP exercise. The results and their wider interpretation are very nicely discussed. I am therefore happy to recommend this paper for publication in ACP, following just a few minor comments below, which I hope the authors find useful.
General comment:
I think that the apparent step-change extratropical in age around the early-mid 1990s is an interesting result and one of the clearer differences between ERA5 and ERA-Interim. The fact that this does not appear clearly in observational data (e.g. Fig. 10) may bring into question the reliability of ERA5 trends. It’s also interesting that this step-change is not apparent in the tropical upwelling (Diallo et al. 2020, their Fig 12). I might encourage the authors to elaborate a little more on this in the paper as the result is not very prominent. In particular, do any TEM diagnostics of the circulation show this step change, or is this only seen in age calculations? Is so, could the authors speculate as to why? If this step-change is not thought to be real, do the authors have any suggestions for what change in the assimilation scheme may be responsible?
Minor comments:
L7: ‘Above’: it wasn’t clear to me what this was referring to as being above. Maybe ‘in the mid-upper stratosphere’ would be clearer?
L94: theta (potential temperature) should be defined.
L105: along -> along with?
Fig. 2: I think this would benefit if a difference plot were also shown (ERA-Int minus ERA5) to aid with comparison of the two reanalyses and with Fig 1 e-g. It is quite difficult to pick out the differences without such a plot.
Fig. 3: This is a minor point, but I would encourage the authors to consider using a ‘perceptually uniform’ color scale for the age plots (a,b,d,e), such as grayscale, viridis etc. The rainbow scheme used here can introduce the appearance of false boundaries (where the yellow color are) in the date. The same goes for Figs 4 and 5.
L391: ‘strongly overly’. I’m not sure what this means? Perhaps ‘decadal variations are significant compared to potential long-term trends’?
L427: ‘steplike change around the year 2000’: To me it looks like the main steplike change in ERA5 is over ~1992-1997 rather than around 2000.
References in this review:
Diallo, M., Ern, M., and Ploeger, F.: The advective Brewer-Dobson circulation in the ERA5 reanalysis: variability and trends, Atmos. Chem. Phys. Discuss., 2020, 1–39, https://doi.org/10.5194/acp-2020-881, https://acp.copernicus.org/preprints/acp-2020-881/, 2020.
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AC1: 'Reply on RC1', Felix Ploeger, 07 Apr 2021
Reply to Reviewer 1
We thank the Reviewer for the positive evaluation of the manuscript and the good comments. In the following, we address all comments and questions raised (Reviewer's comments in italics). Text changes in the manuscript are highlighted in color (except minor wording changes).
General comments:
{\it I think that the apparent step-change extratropical in age around the early-mid 1990s is an interesting result and one of the clearer differences between ERA5 and ERA-Interim. The fact that this does not appear clearly in observational data (e.g. Fig. 10) may bring into question the reliability of ERA5 trends. It’s also interesting that this step-change is not apparent in the tropical upwelling (Diallo et al. 2020, their Fig 12). I might encourage the authors to elaborate a little more on this in the paper as the result is not very prominent. In particular, do any TEM diagnostics of the circulation show this step change, or is this only seen in age calculations? Is so, could the authors speculate as to why? If this step-change is not thought to be real, do the authors have any suggestions for what change in the assimilation scheme may be responsible?}
This is indeed a very good question - and not easy to answer. At first glance, the change in age of air in the mid-nineties appears as a sudden step-change, but a closer look shows that the change occurs over a few years, between about 1991-1995. This change is clearly evident in ERA5 age, and to a weaker degree also seen in ERA-Interim. The age time series in Fig. 9 show that the clearer step change in ERA5 in the mid-nineties is mainly a result of the positive age trend in the eighties and the age increase around 1991 which is likely related to the Pinatubo aerosol. The main difference to ERA-Interim is the trend over the eighties.
As suggested by the Reviewer, we further investigated basic meteorological variables for similar changes, and we considered both the residual circulation vertical velocity and the diabatic heating rate (new Fig. 12 in the revised manuscript). At upper stratospheric levels the heating rates show abrupt changes related to changes in the assimilation system (e.g., in 1998), as discussed for ERA-Interim e.g. by Abalos et al. (2015). However, in the lower stratosphere none of the two variables shows a step-like change in the mid-nineties which could be related to the age of air change. On the other hand, the heating rates show a decrease after the Pinatubo eruption (1991) in both reanalysis, related to the age increase during the same period. The difference in the strength of this effect between the two reanalysis, and also between heating rates and residual circulation velocity, are not clear to us.
We agree with the Reviewer that a more thorough discussion of these issues clearly improves the paper and we included a new figure (Fig. 12) showing the vertical velocity and heating rate changes and extended the discussion section 6 in this regard.
Minor and Technical comments:
L7: {\it ‘Above’: it wasn’t clear to me what this was referring to as being above. Maybe ‘in the mid-upper stratosphere’ would be clearer?}
Indeed, our wording here was not precise. We actually meant that ERA5 age appears somewhat high-biased outside the TTL at all locations where we compared to observations. However, we compared only at 20km (aircraft data, Fig. 10a), in NH middle latitudes above 24km (balloon data, Fig. 10b), and in the NH lower stratosphere between about 350-480K (aircraft and balloon data, Fig. 11). Hence, this statement should not be considered as too strong. To be more precise, we changed the wording to:
``At 20\,km and in the NH stratosphere, ERA5 age values are at the upper edge of the uncertainty range from tracer observations, indicating a comparatively slow BDC.'' Note that we also changed parts of the comparison to observations in Sect. 5, as suggested by Reviewer 2 (we replaced the age-F11 correlation analysis with an analysis of the age difference distributions between model and observations).L94: {\it theta (potential temperature) should be defined.}
Sentence has been changed accordingly.
L105: {\it along $\rightarrow$ along with?}
We just deleted ``along''.
Fig. 2: {\it I think this would benefit if a difference plot were also shown (ERA-Int minus ERA5) to aid with comparison of the two reanalyses and with Fig 1 e-g. It is quite difficult to pick out the differences without such a plot.}
Thanks for this suggestion, which eases the comparison! We just added difference plots to all cases (ERA5 and ERA-Interim, DJF and JJA) in Figs. 1 and 2.
Fig. 3: {\it This is a minor point, but I would encourage the authors to consider using a ‘perceptually uniform’ color scale for the age plots (a,b,d,e), such as grayscale, viridis etc. The rainbow scheme used here can introduce the appearance of false boundaries (where the yellow color are) in the date. The same goes for Figs 4 and 5.}
We agree that the used color scheme could probably be improved. However, most publications we are aware of show age of air usig a similar blue--green--red color scheme. As readers therefore are mostly used to that we would stay with it here.
L391: {\it ‘strongly overly’. I’m not sure what this means? Perhaps ‘decadal variations are significant compared to potential long-term trends’?}
We changed the sentence as suggested.
L427: {\it ‘steplike change around the year 2000’: To me it looks like the main steplike change in ERA5 is over ~1992-1997 rather than around 2000.}
This is totally correct, and the ``2000'' was just wrong - Thanks for pointing this out! We changed the text to ``in the mid-nineties''.
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AC2: 'Reply on RC1', Felix Ploeger, 07 Apr 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2020-1253/acp-2020-1253-AC2-supplement.pdf
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AC1: 'Reply on RC1', Felix Ploeger, 07 Apr 2021
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RC2: 'Timely and useful paper - comparisons with obs should be better discussed', Simon Chabrillat, 21 Feb 2021
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AC3: 'Reply on RC2', Felix Ploeger, 07 Apr 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2020-1253/acp-2020-1253-AC3-supplement.pdf
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AC3: 'Reply on RC2', Felix Ploeger, 07 Apr 2021
Felix Ploeger et al.
Felix Ploeger et al.
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