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
Abstract. The paper describes the development and performance of the Double Extended Stratospheric-Tropospheric (DEST vn1.0) chemistry scheme, which forms a part of the Met Office’s Unified Model coupled to the United Kingdom Chemistry and Aerosol (UM-UKCA) chemistry-climate model, the atmospheric composition model of the United Kingdom Earth System Model (UKESM). The scheme extends the standard Stratospheric-Tropospheric Chemistry scheme (StratTrop) by including a range of important updates to the halogen chemistry. These allow process-oriented studies of stratospheric ozone depletion and recovery, including the impacts from both controlled long-lived ozone-depleting substances (ODSs) and emerging issues around uncontrolled very short-lived substances (VSLS). The main updates in DEST are (i) an explicit treatment of 14 of the most important long-lived ODSs; (ii) an inclusion of Br-VSLS emissions and chemistry; and (iii) an inclusion of Cl-VSLS emissions/lower boundary conditions and chemistry. We evaluate the scheme’s performance by comparing DEST simulations against analogous runs made with the standard StratTrop scheme, as well as against observational and reanalysis datasets. Overall, our scheme addresses some significant shortcomings in the representation of atmospheric halogens in the standard StratTrop scheme, and will thus be particularly relevant for studies of ozone layer recovery and processes affecting it, in support of future Ozone Assessment reports.
Ewa M. Bednarz et al.
Status: final response (author comments only)
- RC1: 'Comment on gmd-2022-215', Anonymous Referee #1, 16 Feb 2023
- RC2: 'Comment on gmd-2022-215', Anonymous Referee #2, 21 May 2023
Ewa M. Bednarz et al.
Ewa M. Bednarz et al.
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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.
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.
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