Review of paper : ‘Description and evaluation of the new UM-UKCA (vn11.0) double extended stratospheric-tropospheric (DEST vn1.0) scheme for comprehensive modelling of halogen chemistry in the stratosphere’, by Bednarz et al.

The paper provides a description and an evaluation of the new version of the extended stratospheric-tropospheric scheme in the UM-UKCA model (DEST). The main updates are an explicit treatment of 14 of the long-lived ODS, the inclusion of bromine VSLS emissions and chemistry and the inclusion of chlorine VSLS emissions/lower boundary conditions and chemistry.

The paper fits well within the scope of the journal. It is well written and well organized. The strategy for the evaluation of the changes linked to the new developments is clear. However, the paper could be improved by providing more justification and a more in-depth analysis on a few points as detailed below.

Major comments

L210. The 3-member ensemble uses time-varying Cl-VSLS LBCs. Why Cl-VSLS and/or Br-VSLS ? Is the variability more important for Cl-VSLS ? Are there larger uncertainties on Cl-VSLS LBC/emissions than on Br-VSLS LBC/emissions ?

L214-L222. Two problems were identified in the code used for the DEST 20-year long simulations run in the time slice year 2000. The authors acknowledge that these issues are important. They also say that they should not have a dominant impact on the evaluation. This statement needs to be supported by a more detailed argumentation. Regarding the photolysis cross-section of CFC-113, how is the update expected to modify the stratospheric chlorine budget (Cly) ? For COCl2 + O(1D), it would be interesting to quantify the loss of chlorine associated to this problem, possibly based on a test experiment  over one year.  

L238-L240. In Fig. 1b, there are lower bromine levels in DEST compared to StratTrop in the southern hemisphere (Fig 1b). Since there are Br-VSLS included in DEST and not in StratTrop, one would think that the bromine levels in the troposphere would be greater in DEST everywhere. What could be the cause of this negative anomaly in DEST with respect to StratTrop ?

Section 4.2.1. To support the analysis of the results discussed in this section, would it be possible to have an estimation of the differences in the LBC concentrations for the ODS between DEST and StratTrop ? This will help to know if the ODS levels are expected to be lower or higher in DEST compared to StratTrop.  

Sections 4.2.2 and 4.2.3 shows that the changes of stratospheric ozone affect stratospheric climate (temperature, H2O, age of air). Apart from the effect of chlorine and bromine emissions, is there any significant impact on PSCs between the DEST and the StratTrop simulations?

Section 4.3. Comparison with the ACE-FTS products.  The uncertainty on the ACE-FTS stratospheric ozone concentrations is mentioned L315-317. More generally, it would be useful to know what is the uncertainty on each of the ACE-FTS products used to assess the DEST simulations. This would give an indication of the ability of the DEST simulation to be within these uncertainty ranges.

Bromine monoxide is also a compound of interest that can be estimated from satellites observations and that shows differences between DEST and StratTrop. I would recommend looking for such data to evaluate the DEST simulation.

Minor comments

L248. “differences in in” à “differences in”

L254-L256. Could you add a short explanation on how the heterogeneous bromine reactions are linked to BrO?

L270. I think the reference to Fig. 4 should be Fig. 3c and the reference to Fig3f and Fig. 3c should be Fig 4f and Fig. 4c

This paper describes an extension of the standard chemistry scheme of the UM-UKCA chemistry-climate model and a detailed evaluation of the performances of the new chemistry scheme (called DEST) using comparisons of present-day model simulations against multiple observational datasets, notably satellite data.  The model updates are an explicit treatment of key long-lived ozone-depleting substances, of bromine-containing very short-lived species emissions and chemistry, of chlorine-containing very short-lived species emissions. The scheme also includes additional inorganic halogen tracers, and changes to the photolysis, gas-phase and heterogeneous reaction rates.  The paper is well written, clear, and a valuable contribution to the stratospheric ozone numerical modelling community. The new model is better suited for studies of stratospheric ozone depletion/recovery associated with both controlled long-lived ozone-depleting substances and, more importantly, uncontrolled very short-lived halogen substances. Its scope fits perfectly with those of GMD model description. Therefore, I recommend publication with very minor comments that the authors may consider.

-p3, l76: ...chemistry-climate model...

-p3, l84: why ‘double’?

-p4, l110: add ‘(Lower Boundary Conditions)’

-p7, l194: It could be interesting for a follow-on study to rerun the time-varying simulations but forced with meteorological analyses. This alternative set-up could make the testing of the chemistry scheme simpler, less dependent on the model dynamical response to chemical composition changes.

-p7, l216: replace , by .

-p8, l248: remove one ‘in’.

-p9, l257: ?? rephrase

-p9, l262: add ‘zonal mean’ and define reactive chlorine.

-p9, l283: ‘consequence...stability’. not sure to have understood this comment.

-p10, l293: Perhaps, the authors should highlight the differences between the analysis/objective by Bernarz et al, 2022 and the present one.

- p10, l307: As shown by Bernarz et al, 2022, the inclusion...

-p11, l330: the too cold/persistent Antarctic vortex bias is common to several chemistry-climate models.

-p11, l336: It is not totally surprising. Dynamical parametrisations (e.g. orographic and non-orographic waves) of chemistry-climate models are tuned in order to reproduce climatologies. When a scheme, here the chemistry scheme, is changed/improved, this tends to degrade initially the model performances. Dynamical model parameters might need to be tuned again with the new chemistry scheme.