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:

1. 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.
2. 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.
3. 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.
   
   
   
   
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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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. 

see attached PDF

The manuscript describes a case study of gravity wave generation and induced turbulence and mixing in the 

extratropical tropopause region. It combines airborne observations from a field campaign and several model outputs 

(reanalysis, and two high-resolution runs or forecasts). The combination illustrates convincingly a case of 

turbulence induced in the lowermost stratosphere, in a highly sheared region above the jet, by low-frequency 

gravity waves. There are some points which can be improved (see major concerns below and a series of minor 

points regarding the framing of the motivation, in the introduction). These different concerns likely 

require only minor revisions. 

Major concerns 

1. The link between gravity waves and the occurence of turbulence is expected from dynamical reasons, with 

the GW fluctuations enhancing the shear and modifying stability. However, the choice to use the Absolute

Momenum Flux (AMF) to quantify GW when relating turbulence to gravity waves (l140-141). There is not necessarily 

a better choice for quantitatively relating the two. Nonetheless, the choice to use AMF should be better explained. 

What is the theoretical justification to use absolute momentum fluxes (AMF) in order to investigate 

"vertical transport due to small-scale processes"? For conservative waves (i.e. in the absence of dissipation), it is 

possible to have large fluxes associated to displacements which are reversible, and hence do not lead to net transport.

AMF is a quadratic quantity, and it depends on the frequency of the waves, with the low-frequency waves carrying 

less momentum than high-frequency waves. The impacts of waves on stratification and on shear will also involve both 

their amplitudes, and their frequency. 

2. About the discussion and conclusions: the authors benefit from a case which has been documented by airborne observations. 

Regarding the results that concern turbulence and mixing, how much of the conclusions are supported by evidence from 

the measurements? It is not sufficiently clear, in the concluding section, which statements are based on inferences 

based on modelled diagnostics (Richardson number, Turbulence Indices...) and which statements (regarding mixing 

and turbulence) are based on the measurements. 

Minor concerns

l16-18: 'Large amounts of momentum and energy can be propagated and transferred..' The sentence should be rephrased. 

There are two concerns: one should not overstate the role of gravity waves in the circulation, and the formulation 

is clumsy.

Gravity waves matter for the general circulation. Their vertical propagation induces momentum fluxes that contribute 

to driving the mean circulation in the middle atmosphere (stratosphere and mesosphere). In the mid-latitudes, 

this contribution can be important but is secondary relative to that of Rossby waves, in particular planetary 

waves. In the mesosphere, the contribution of gravity waves is essential. The reference cited rather emphasizes 

the contribution of large-scale, balanced waves. 

l32-34: moist processes turn out to play an important role in the generation of GW from mid-latitude and 

high-latitude weather systems (e.g. Wei and Zhang 2014, Plougonven et al 2015, Holt et al 2017)

l36-38: The sentence on baroclinic lifecycles is ambiguous: is this a statement on the contribution of 

weather systems (i.e. synoptic activity)? While weather systems are essential to the tropospheric circulation 

and zonally-averaged zonal flow, their influence does not necessarily penetrate very far into the stratosphere; 

the contribution from larger-scale Rossby waves is more important for the Brewer-Dobson Circulation (line 37). 

However, it seems that the topic of these two sentences should rather be 'Gravity waves excited from 

tropospheric weather systems (or synoptic eddies)...' given that the references given on line 38 are 

reviews on gravity waves. With gravity waves in mind, one should more clearly separate the different impacts: 

1. the vertical transfer of momentum; regarding this, what is in fact essential is the ultimate dissipation 

of the momentum fluxes associated to gravity waves, leading to forcing of the background flow at upper levels. 

Models to not take into account a 'depletion' of momentum at the launch level of GW parameterization. In this 

sense, what matters really is the forcing of the gravity waves in the middle atmosphere, not a 'transfer of 

momentum', since the source region is not affected. This forcing of the middle atmosphere's circulation is 

the main motivation and main purpose of parameterizations. 

2. there are local impacts due to gravity waves, such as enhancement of local shear, and onset of turbulence 

where the waves break. This has implications for vertical mixing. These effects are known to exist, but are 

not as high on the list as the forcing of the circulation in the stratosphere and mesosphere. This should be 

reflected in the way these implications are put forward in the introduction. 

l48-49: the effect of GWs on the thermodynamic structure near the tropopause should be made more precise. 

GWs that do not dissipate induce fluctuations, which are reversible. Hence, although they modify locally 

the shear and stratification, this is averages out in a time average, a priori. The situation is different 

when dissipation is present: mixing due to breaking induces irreversible transport. Dissipation of momentum 

or heat fluxes induce a forcing of the background flow. What the authors have in mind here should be better 

explained. 

l75-78: There is a problem in the discussion of the motivations of this study. The different processes and different 

scales through which gravity wave may have impacts are stated in a way that includes repetition and which could be 

better organized, here and in other parts of the introduction. For example, in lines 75-78, the first sentence 

of the paragraph highlights the impact of gravity wave breaking for "vertical mixing of 

key tracers". The third sentence begins by "Beyond its relevance to aviation", although aviation has not been 

highlighted in the previous sentences, and "cross-tropopause transport" is highlighted - although this is very 

related to "vertical mixing of key tracers"...

l86-87: the status of the idealized baroclinic lifecycles is ambiguous from the phrasing: it may be understood that 

the impacts of gravity waves will be investigated in idealized baroclinic lifecycles. The next sentence clearly 

indicates that observations and a real case study will be considered, in apparent contradiction. 

l91: "whether ERA5 is good enough resolved to study GWs, shear and mixing in the UTLS": to be rephrased, e.g. 

"which aspects of the gravity wave field can be reliably investigated in ERA5 fields, given their resolution." 

l139: "ourself" -> "ourselves"?

l139-141: not sure to understand the logic of restraining to the consideration of PDFs; could the authors explain? 

l155-161: while the methodology appears reasonable, and indeed consistent with what has been done in other studies, 

it does mix different methods (spectral approach to obtain the perturbation quantities, then lowpass Gaussian filter 

to average over several wavelengths and obtain a smooth estimate of the fluxes. 

l176: "to account [FOR] this limitation"? 

l178-179: the structure of the sentence is not clear, to be rewritten

l209: word missing? "between two equatorward [?] reaching..." or placed later in the sentence ("streamers")? 

l229: "ascend" -> "ascent"? 

l276-277: the paragraph discusses the measurements. Suddenly, 

l292-293: "which rotates anticyclonically (cyclonically) for an upward (downward) propagating wave" -> "which rotates

anticyclonically  for an upward propagating wave, and cyclonically for a downward propagating wave."? 

l334-335: the conclusion that the IFS manages to capture the appearance of inertia-gravity waves excited 

by jet-front systems, despite a limited resolution, is consistent with findings of previous studies that 

have investigated gravity waves in ECMWF products and reanalyses in particular (see Jewtooukoff et al, 2O15, and 

references therein). 

l343: the authors emphasize the shear in Fig. 7, "in particular in ICON". This is somewhat at odds with the 

figure, in which the shear in ICON seems weaker. 

l371-391: as far as I understand, the relation between AMF (absolute Momentum Flux) and the shear is somewhat 

opportunistic, rather than a fundamental relation. Momentum fluxes are used as a proxy for the presence 

of waves, and these contribute to shear. Figure 8, perhaps because of the logarithmic axis and logarithmic colorbar, 

does not seem so compelling regarding the correlation between shear and AMF. The text and comments around 

this figure should clearly recognize that this relation is tentative. 

l535-537: it is worth noting that there are several different uses that can be made of ERA5 for studies of GWs. It 

is very challenging and demanding to expect the representation of small-scale shear to be accurate, let alone the 

representation of the resulting turbulence. Providing insights about regions favorable to the occurence of 

gravity waves is more reasonable. In the assessment of the value of ERA5 for GW studies, it should be mentionned that 

there are different levels of information that may be targeted. 

l540: "small-scale gravity waves": it is worthwhile being more precise here. It is possible to have low-frequency 

waves which are small-scale in the vertical (vertical wavelength of a few hundred meters) yet retain horizontal 

wavelength on the order of several tens to a hundred of kilometers. It is possible to hve higher frequency waves 

that have horizontal wavelengths only of a few kilometers (and similar vertical wavelengths). The frequency (which 

can be estimated from the tilt of the phase lines) matters very much in the description of the waves.  

l541-544: the relationship highlighted here is not clear: do the authors highlight GWs producing pockets of turbulence, through

local enhancement of shear? Or do they have evidence for secondary generation of smaller-scale gravity waves where a 

primary wave-packet breaks? The second interpretation does not appear very likely, but the structure of the sentence is 

ambiguoous (~ "Turbulence (...) correlates with appearance of (...) small-scale gravity waves"..

l560-561: on the difficulty of convergence with resolution, the study by Kruse et al 2022 is also relevant. 

Holt, L. A., Alexander, M. J., Coy, L., Liu, C., Molod, A., Putman, W., & Pawson, S. (2017). An evaluation of gravity waves and gravity wave sources in the Southern Hemisphere in a 7 km global climate simulation. Quarterly Journal of the Royal Meteorological Society, 143(707), 2481-2495.

Wei, J., & Zhang, F. (2014). Mesoscale gravity waves in moist baroclinic jet–front systems. Journal of the Atmospheric Sciences, 71(3), 929-952.

Plougonven, R., Hertzog, A., & Alexander, M. J. (2015). Case studies of nonorographic gravity waves over the Southern Ocean emphasize the role of moisture. Journal of Geophysical Research: Atmospheres, 120(4), 1278-1299.