Evidence of gravity wave contribution to vertical shear and mixing in the lower stratosphere: a WISE case study

Umbarkar, Madhuri; Kunkel, Daniel; Miltenberger, Annette; Lachnitt, Hans-Christoph; Kaluza, Thorsten; Schwenk, Cornelis; Hoor, Peter

doi:10.5194/egusphere-2025-5142

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https://doi.org/10.5194/egusphere-2025-5142

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19 Dec 2025

| 19 Dec 2025

Evidence of gravity wave contribution to vertical shear and mixing in the lower stratosphere: a WISE case study

Madhuri Umbarkar, Daniel Kunkel, Annette Miltenberger, Hans-Christoph Lachnitt, Thorsten Kaluza, Cornelis Schwenk, and Peter Hoor

Abstract. Evidence is presented which illustrates the role of atmospheric gravity wave (GW) induced shear as a mechanism for the occurrence of clear air turbulence and exchange of air masses with different chemical composition in the lower stratosphere. This study investigates the characteristics of GWs and their impact on the distribution of trace species in the lowermost stratosphere during an extratropical cyclone over the North Atlantic using airborne in-situ observations, ERA5 reanalysis data as well as IFS and ICON forecast data. Tracer observations as well as model simulations reveal fine scale structures around the tropopause which are embedded in a region influenced by the inertia gravity waves, warm conveyor belt ascent and mesoscale modifications of the tropopause structure. The GWs propagate through highly sheared flow above the jet stream maximum, perturbing background wind shear and static stability, and thereby creating conditions conducive to turbulent mixing in the lowermost stratosphere. The observed significant correlation between GW-induced momentum flux and enhanced shear perturbations confirms the role of GWs in driving potential turbulence and facilitating trace gas exchange in the lower stratosphere. Further analysis of turbulence diagnostics suggests that GWs produce shear which leads to the occurrence of clear air turbulence.

Received: 16 Oct 2025 – Discussion started: 19 Dec 2025

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Madhuri Umbarkar, Daniel Kunkel, Annette Miltenberger, Hans-Christoph Lachnitt, Thorsten Kaluza, Cornelis Schwenk, and Peter Hoor

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-5142', Anonymous Referee #1, 06 Jan 2026
Review of "Evidence of gravity wave contribution to vertical shear and mixing in the lower stratosphere: a WISE case study" by Umbarkar et al.
The manuscript investigates the role of gravity-wave (GW) induced shear in generating turbulence and associated irreversible mixing of air masses and chemical tracers in the UTLS, based on observations from the WISE campaign over the North Atlantic. Campaign observations, reanalysis data, and forecasts from two NWP models -- ICON at ~3 km horizontal resolution over the studied domain and IFS at ~9 km -- are used to examine the role of GWs. Attributing turbulence and clear-air turbulence (CAT) to GWs in a real-world setting is inherently challenging compared to idealised case studies. While the authors make a reasonable effort to disentangle these processes, and the topic is of clear interest to ACP, I find parts of the analysis unconvincing and the interpretation at times confusing. I believe substantial revisions are required before this manuscript can be recommended for publication. My major comments are listed below (in no particular order).
Major Comments:
Introduction / representation of turbulence: It should be stated more clearly that Kelvin-Helmholtz instability and turbulence are not explicitly resolved in ICON, IFS or ERA5 even if the gravity waves themselves are; instead, these processes are represented through subgrid-scale parameterizations. It would therefore be useful to analyse the contributions from these subgrid-scale processes explicitly. This distinction should be discussed, and where possible the relevant tendencies should be diagnosed. For example, in ERA5 these can be obtained from MARS using short forecasts on model levels, with fields such as the time-mean eastward and northward wind tendencies due to parameterizations.

Origin of the gravity waves: The source of the turbulence-inducing GWs is unclear from the analysis and discussion. Do the authors attribute these waves primarily to baroclinic instability, or to flow over orography? From the figures, flow over orography appears at least as plausible as baroclinic instability, particularly given that the GW signal occurs near the transition between ocean and Icelandic topography. The presence of a front does not necessarily imply GW generation via baroclinic instability; instead, the front may simply provide favourable background wind conditions allowing surface-generated GWs to propagate into the UTLS. It would therefore be useful to extend Fig. 6d–f down to the surface and to explicitly mark the underlying topography.

Comparison of ICON with IFS and ERA5: The ICON forecasts analysed here are at lead times of 3–4 days, whereas the IFS forecasts are at lead times of 0–1 day and therefore much closer to the analysis/ERA5. This difference should be explicitly acknowledged when comparing the two models. A major reason for the discrepancies between ICON and IFS is likely the different background flows, as ICON will have drifted further from the analysis. This is already evident in the large-scale fields shown in Figs. 1, 6, 7, 9, 12, and 13, and the discussion of these figures should reflect this. For a more direct one-to-one comparison, the two models should ideally be compared at the same forecast lead time.

In Sect. 2.4, please also discuss the vertical resolution of ICON in the analysed region, as this is important for comparing vertical wavelengths with IFS (L331–332).

Regarding Fig. 6, it is notable that ICON and IFS show very similar GW amplitudes, even though ICON’s horizontal resolution in this region is about three times finer than that of IFS (3.3 km versus 9 km). One might expect ICON to produce larger-amplitude resolved GWs, consistent with the statement in L323 that nominal resolution controls resolved GW amplitude. This does not appear to be the case in Fig. 6. This could again be related to differences in the background flow. However, later in Figs. 8 and 11, ICON does show larger GW momentum fluxes than IFS, as expected given its higher resolution. Please comment on this apparent inconsistency. I am also somewhat confused by the ICON figures in the supplement and the discussion on L409-410: It was my understanding that you are analysing the nested simulations for ICON throughout the manuscript with the region of interest having 3km horizontal resolution. Is this not the case? If not you need to clarify section 2.4.

Scale separation in the UTLS (L159–161):

The authors state that their scale-separation approach follows commonly used GW separation methods and cite several previous studies. While this is indeed true in the stratosphere, where there is a clear scale separation between planetary waves and gravity waves (and where studies such as Gupta et al. and Stephan et al. apply these methods), the situation is less clear in the UTLS. In this region, the separation between GWs and other mesoscale structures is more ambiguous. Please comment on the applicability and limitations of this approach in the UTLS.

Figure 1 (PV field): Are regions of negative potential vorticity present, as suggested by the colour-bar range? If so, could these be highlighted using a distinct colour rather than shades of blue? Negative PV is dynamically significant and would indicate regions of instability.

Wavelength estimates (L302–303): Please state the estimated vertical and horizontal wavelengths explicitly here, so that they can be clearly compared with model-resolved scales later in the paper.

Interpretation of Fig. 6d–f and GW breaking: If irreversible mixing is attributed to GW-induced KH instability, why does the GW appear to continue propagating beyond the UTLS in Fig. 6d–f? This would suggest that the GW has not undergone saturation or breaking, which would normally be required for GW-induced CAT as argued in Sect. 4.4. Alternatively, this may not be the intended interpretation. The discussion in L361–368 is somewhat unclear and appears to suggest that only reversible GW propagation contributes to shear generation. Please clarify this point.

Figure 7 and GW contribution to shear: Since the authors argue that the GW contribution to shear is important for mixing, it would be very useful to show an analogue of Fig. 7 for S', i.e. the component of the shear associated with gravity waves. This would help to quantify how much GWs actually contribute to the shear and would also allow one to verify that this contribution is confined to regions where GWs can be clearly identified.

Section 4.5 and momentum-flux deposition: To complement Figs. 12 - 13, it would be useful to show the vertical gradient of the resolved GW momentum flux without any additional modulation. One would expect momentum-flux deposition in the region where GW-induced KH instability, CAT, and turbulence occur. Is this observed?

Minor comments and typos are provided in the marked-up PDF. This annotated PDF also re-iterates the major comments listed above in the relevant part of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-5142-RC1
RC2: 'Comment on egusphere-2025-5142', Anonymous Referee #2, 30 Jan 2026

see attached PDF

Citation: https://doi.org/10.5194/egusphere-2025-5142-RC2

Madhuri Umbarkar, Daniel Kunkel, Annette Miltenberger, Hans-Christoph Lachnitt, Thorsten Kaluza, Cornelis Schwenk, and Peter Hoor

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https://doi.org/10.5194/egusphere-2025-5142-supplement

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Data used for "Evidence of gravity wave contribution to vertical shear and mixing in the lower stratosphere: a WISE case study" Madhuri Umbarkar https://doi.org/10.5281/zenodo.17227439

Madhuri Umbarkar, Daniel Kunkel, Annette Miltenberger, Hans-Christoph Lachnitt, Thorsten Kaluza, Cornelis Schwenk, and Peter Hoor

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Short summary

We present an extratropical cyclone case study over the North Atlantic, focusing on the role of atmospheric gravity waves (GWs) in the generation of enhanced vertical wind shear and (clear-air) turbulence, as well as their impact on tracers distribution in the lowermost stratosphere. Our research suggests the GW related processes should be considered as a key for UTLS transport and mixing and as an important candidate for the interpretation of CAT, that appear to emanate from GW-induced shear.


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