Review of wcd-2021-8

This study presents a climatological investigation of vertical wind shear zones at tropopause levels in a 10-year dataset of ERA5 reanalyses. The authors systematically study the frequency of occurrence of high vertical wind shear (based on a threshold criterion) in a framework relative to the lapse rate tropopause. They find that high wind shear mostly occurs within the lowest kilometres above the tropopause. The geographical distribution, vertical structure and temporal variability is examined for different latitudes and jet systems, and the physical processes associated with the shear layer are discussed, as well as its potential impact on the tropopause structure, stratosphere-troposphere exchange and turbulent mixing. The paper is interesting and well-written, it is put into context of existing literature, and the study makes a relevant and new contribution to the understanding of the UTLS structure. I recommend publication subject to a few minor revisions.

as well as its potential impact on the tropopause structure, stratosphere-troposphere exchange and turbulent mixing. The paper is interesting and well-written, it is put into context of existing literature, and the study makes a relevant and new contribution to the understanding of the UTLS structure. I recommend publication subject to a few minor revisions.

General comments:
1) Generally, the existence of wind shear above the jet core and in the lower stratosphere is not a surprise as is expected from balanced dynamics, the exact structure of the shear zones however are more involved. The authors mention the relation of the shear layer to the thermal wind balance at several instances in the manuscript along with other mechanisms. How much of the structure of the shear layer can be explained by the thermal wind relation? It should be possible to quantify this based on the ERA5 fields. The possible role of gravity waves is mentioned in several sections and maybe this way the magnitude of their contribution could be narrowed down. Furthermore, I would suggest to emphasize more clearly in the introduction, perhaps in a single summarizing sentence, what the main unknown aspects of the shear layer are (e.g. detailed structure, strength, vertical extent and occurrence in a statistical sense, formation mechanism) and which of these aspects are addressed in the study.
2) I think the authors should reconsider some expressions and definitions related to the shear layer phenomenon. -The words "enhanced" or "exceptional" are used frequently. In what sense is the wind shear "enhanced", compared to what reference? The study shows that the layers of strong wind shear above the tropopause occur rather frequently and strong wind shear is certainly not exceptional near jet streaks.
-The tropopause shear layer (TSL) is defined based on an occurrence frequency criterion. In this sense, it is a purely statistical feature. Since a layer of strong wind shear also seems to be physically present and nicely visible in instantaneous synoptic situations with a strong jet stream (see Fig. 13b), I find it unfortunate to define the "TSL" in a statistical sense rather than as a synoptic feature. It would be more intuitive to call the regions indicated by the red contours in Fig. 13b "TSL".
3) The authors have chosen a well-considered threshold S_t^2 and the choice is sufficiently explained. However, it would be interesting to test how sensitive the results are with respect to the threshold. How would the pattern of the occurrence frequencies change if S_t^2 was even higher? 4) I would be curious if the statistical analysis (for the midlatitudes) has also been done relative to the dynamical tropopause and whether there are any differences compared to the LRT-relative framework. This would be interesting e.g. in the context of many STE studies which focus on transport across the 2-PVU surface.
5) The introduction is quite long, the authors might consider shorten it a bit if possible.

Specific comments, suggestions and typos:
L13ff: Throughout the manuscript, the term "tropopause-based" vertical wind shear is used frequently. This expression is not very clear to me; does it mean "tropopauserelative", "near-tropopause" or "tropopause-level"?  Fig. 5b is very helpful and easy to understand, I find the explanation in the text rather unclear. The authors might consider rewriting these sentences.
L298: barclinic --> baroclinic L299: remove "is" L315-316: I assume your background state is still latitude-dependent? From this sentence it is not clear if you also average over latitudes.