|The authors have revised the manuscript adding a procedure for correcting the SF6 AoA for depletion, and inserting more references. Otherwise the manuscript has undergone only minor changes. The majority of the questions raised in my previous review were not fully addressed.|
1. The main objective(s) of the study are still not clearly declared. A phrase has been added as a response to my first comment “… with the aim to understand the effects of SF6 sinks on tracer derived AoA and its long-term trends. Specifically, we calculate for the first time the effect of the sinks on the long-term trend of AoA.”
This phrase is by no means sufficient. The latter sentence should have and "apparent AoA" (or "SF6 apparent AoA") rather than AoA. SF6 sinks, probably, have a minor effect on AoA via radiative forcing and circulations, but it is well beyond the accuracies of current models.
I would be expecting formulation of the objectives in a way they can be clearly shown as achieved in the conclusions, and streamlining of the whole narrative to meet the objectives.
The way SF6 sinks affect apparent AoA and its trends have been shown earlier in several studies (though with theoretical considerations and somewhat shorter simulations), and the conclusions made are quite unequivocal: "The apparent over-ageing introduced by the sink is large and variable in space and time. Moreover, the over-ageing due to the sink increases as the atmospheric burden of SF6 grows."
The need for longer-time simulation has to be justified, and research questions to be addressed with the simulations have to be formulated.
2. The new version of the manuscript states that "a comprehensive understanding of what contribution the individual effects have on the AoA trend depending on altitude and latitude is still missing”. A comprehensive understanding is a subjective and poorly defined matter, and thus does not qualify for an objective. One could put as an objective to check if the conclusions of earlier studies are reproducible with longer simulations and more sophisticated model. A valid aim would be to quantify influence of the individual effects on the apparent AoA trend, or evaluate their relative importance. However, explicit list of the individual effects that are under consideration is mandatory in this case. The paragraph above the phrase lists them: acceleration of the BDC, concave growth rate of tropospheric SF6, sparse sampling of in-situ observations, differences in the changes between the deep and the shallow BDC branch, integrated effect of mixing, SF6 sink. However, of all those, the paper considers only SF6 sink (WS vs NS cases, CSS case) and acceleration of the BDC (REF vs TS2000). The correction for the concave growth rate of tropospheric SF6 has been just applied, and the parameters of the correction were chosen to get the ages from (SF6,NS) case matching those from (lin,NS), essentially making one of these tracers redundant, and leaving the effect of concave growth rate beyond the consideration. The effect of the changes in the reactant species has been considered in the paper, but not listed here. Besides those, a gravitational separation (Kovacs 2017, Kouznetsov 2020) has been considered earlier and shown to have an effect, though much smaller than one of the mesospheric sink.
3. The conclusions now admit that a major effect of sinks on the apparent SF6 AoA and its trends was shown earlier. The minor missing part now is the statement that the current study confirms/disproves those findings.
4. The new text states "previous studies ... showed that the chemical sinks can strongly influence SF6-derived AoA in terms of absolute values and decadal changes". Indeed it has been earlier shown that the chemical sinks do strongly influence SF6-derived AoA terms of absolute values and trends, not not just that that they can.
5. The only major change in the manuscript since the previous revision is re-writing and changing of the scope for the section 4. The section is now called "Theoretical considerations and concept for sink correction methods", and formulates a correction procedure for over-aging that (as Conclusions state) "can likely be applied to AoA values up to 4 years".
The section is weakly linked to the rest of the paper. The formulations there rely on obviously violated unrealistic assumptions (delta-function age spectrum and existence of the "effective lifetime" as a function of true AoA), the experimental evidence provided only for a linearly-growing SF6 and only for 30-50N latitude belt. The resulting conclusions are vague.
The feasibility of such a correction has been challenged (Kouznetsov 2020, see also notes about the 1D-model behind the correction for non-linear growth by Waugh and Hall, 2002). If one decides to propose such a correction, it deserves a separate paper with a consistent formulations and strong evidence. The applicability limits, uncertainties, and error-propagation through the correction have to be carefully considered there.
Therefore I repeat the request from the previous-stage review to discard this section from the present paper.
Sec 2.2: Now the implemented depletion mechanism is more clear. If I got it right, the model dynamics in the present study is not affected by any of the simulated tracers. Moreover, the reactant species and photon flux are prescribed and are not coupled to the rest of the model dynamics. This essentially means prescribed loss rate as a function of time and location. Is there any principal difference between an online CCM and an offline CTM driven by a climate model in this case?
Sec 2.3: The SF6 emissions (prescribed lower BC for SF6 concentrations) should be described here rather than in sec 2.5. Does "linear emission" mean linear growth of prescribed SF6 mixing ratio at the lower BC? I would note that it does not correspond to a constant or linearly-growing emission rate, which one might think of when reading "linear emission". How exactly the lower BC was prescribed? Was it just a uniform mixing ratio over the surface as a given function of time? A reference to the origin of the data for that function would be needed. How the lower BC were specified for the PROJ run?
Fig.1a. (Not fully addressed in reply #10 from previous review) Completeness of the dataset is a valid criterion. The request to justify the use of a specific latitude belt (comment 12 from previous review) has not been addressed.
The contribution of the noise error into the error of zonal-mean profile averaged over several years is negligible, contrary to errors arising from averaging kernel and systematic measurement errors. Looks like the terms "standard deviation of profiles over the averaging range" and "standard error of the mean profile" are confused. The latter is the right measure of the mean-profile uncertainty which should appear as the error bar.