Review in connection with the manuscript entitled

Impact of small-scale orography on deep boundary layer evolution and structure over the Tibetan Plateau

by Basic et al.

Manuscript: https://doi.org/10.5194/egusphere-2025-4302

General

In this paper the authors use high-resolution LES (CM1) in idealized experiments to study  the impact of ‘subgrid-scale orography’ (subgrid to typical global NWP or regional climate models) on the development of extremely deep convective boundary layers on the Tibetan Plateau. It is found that beyond the earlier identified conditions of weak stratification aloft and not too weak surface heating, ‘local orography’ indeed has an enhancing effect on CBL growth – thus leading to order 10% higher CBL. Overall, the paper is well designed (numerical experiment) and the results are presented in a consistent manner. I have, however, two ‘major comments’ addressing some details of the experiment. They  are called ‘major’ because they cannot be attributed to a line (as the minor comments) – but will easily be addressed. In this sense, I think , the paper can be published after having addressed the minor comments.

Major comment

• Why is mechanical forcing introduced in such an unrealistic way (i.e., the steep gradient from zero wind speed in the valley to 10 m/s over just 500 m)? If there are conditions of forced convection, i.e. nonzero mechanical turbulence, this will result in a more gradual decrease of mean wind speed towards the mean plateau level. Can the authors comment on that?
• The numerical experiment is placed over the TiP and the local orography is used to define ‘REAL’ case. However, the experiment has no other ingredient of an elevated plateau (with the exception of pressure, perhaps). The question is therefore, to which degree the conclusions can be transferred to lower plateaus. Clearly, an enhanced CBL height over a lower plateau would not lead to those record-high CBLs with the potential to even reach the stratosphere – but still, a several-km deep CBL has been observed either under conditions of strong convection at desert locations, or on the TiP. This aspect should be worked out to some detail in a revised version.

Minor comments

l.16             ‘TiP’: the abbreviation is introduced in the abstract – but this usually not counted. So, please introduce the abbreviation here as well.

l.70             ‘FLAT (no orography)’: this may be a matter of definition, but I consider the TiP to be part of ‘orography’, too. So, maybe the authors want to consider to re-label this to locally flat (‘no local orography’, no ‘subgrid-scale orography ‘) or alike.

l.91             this ‘delta20 m’ notation seems to be odd. Why not ‘the vertical grid spacing is set to 20 m below….’? Several occurrences on this and the next line.

l.94             ‘the vertical grid spacing…’: this is repeated (and this time the ‘delta’ is already removed….)

l.96             what is a semi-slip condition? Please explain.

l.98             ‘idealized sounding’: please specify whether this only refers to temperature, or also includes humidity and wind speed (the u10 simulation suggests that the reference has no wind?)

l.103           ‘…starting a 09LT….’: is this the start of the simulation? Or is there a spin-up period considered? Please specify.

Tab 1          ‘Flat orography’ seems to be a contradiction. In the text it is referred to as ‘no orography’, which also seems to be counter intuitive...(see comment to l.70). Again, I suggest to consider ‘no subgrid scale orography’, or ‘elevated plain’ or similar).

l.139           ‘to quantify’ is probably not appropriate here – consider ‘to diagnose’ or ‘to determine’ instead.

Fig.3, caption: what does ‘see 2.32.3.2‘ refer to? Also, the black contour line should be explained, as 800 m above plateau level (i.e., 4200 m ALS)

Fig.3           In the caption it says ‚at 1800 m AGL (6 km ASL)‘: for the ‚REAL‘ simulations the equivalence of these two statements is not given. Please specify, which applies.

Fig. 4, caption:  must specify the time, when the instantaneous fields are obtained.

Eqs (4) and (5): the lhs must be u’^2, v’^2 respectively. Furthermore, if using the according to (1) and using (2) the ‘difference between the time-averaged second and first moments’ is not obvious. I suggest to explicitly derive this (maybe in an appendix). However, in the nomenclature of the present work, it would, correspond to the resolved variance. This should be added.

Fig.7           you have introduced the tracers as ‘1’ and ‘2’ – and here, they are referred to as ‘surface-based’ and ‘upper-level, respectively. I suggest to introduce this convention where the tracers are introduced. Also, in the caption of Fig. 7, the units of the isolines must be specified.

Fig.8, caption: ‘horizontal distribution...’: if I understand the accompanying text correctly, these are the horizontal statistics (median, interquartile range) of the diagnosed mixing height (or CBL height) based on the surface emitted tracer and the elevated tracer (and not: tracer concentrations). I furthermore cannot understand how ‘the mean concentration’ can be marked (with an x) in a height domain (time) diagram. Please explain.

l.249           ‘…by 383 m…’: I don’t think that ‘meter resolution’ here is appropriate (I much more like the ‘about 10%’, even if in the summary then it should be indicated percent of what). Putting into context could also be done by expressing the change in percent of the terrain height.

Appendix 1:    this appendix is never mentioned in the manuscript, nor is the parcel method. So, either the appendix can be removed from the ms, or the parcel method is included into the analysis.

The authors conduct semi-idealized LES experiments under dry conditions to compare three scenarios: a flat plateau (FLAT), realistic terrain (REAL), and realistic terrain with added upper-level wind (REALu10). They analyze boundary layer depth, turbulence structure, tracer dispersion, and vertical mixing. Key findings include:

Small-scale orography accelerates early CBL growth and leads to localized deepening (up to ~500 m over mountains). Wind shear organizes convection into roll vortices, enhancing mixing and further increasing CBL depth (up to 9.4 km ASL). Terrain and shear effects are roughly additive, with the deepest and most uniform CBL occurring in REALu10. A tracer-based method for diagnosing CBL height is introduced and validated against a parcel method.

The manuscript in its current form has minor weaknesses that must be addressed before it can be considered for publication. These primarily concern a lack of methodological clarity, and occasional overstatement of conclusions.

This study makes an original contribution by quantifying the role of small-scale orography and shear in driving extreme CBL depths over the Tibetan Plateau. I am curious, compared with the stability aloft, the fine-scale terrain and shear effects are still the firs-order factor for the extreme high CBL? Please add more sensitivity simulation to compare their respective importance.

The description of the Terrain Processing methodology is vague and omits critical details necessary for reproducibility and full assessment. The statement that the terrain was "tapered" and then "smoothed with a Gaussian filter (200 m)" is insufficient. What was the standard deviation of the Gaussian kernel? Was the filter applied once or iteratively? A figure showing the original SRTM data, the tapered terrain, and the final smoothed terrain used in the REAL case would be immensely helpful to understand the degree of modification and its potential impact.

Flow Decomposition: The description of the Reynolds averaging procedure is confusing. The authors use a 2D Gaussian filter on time-averaged fields. This is a spatial filter, not a classic ensemble average. The justification for this hybrid approach over a pure temporal or spatial average needs to be strengthened. Furthermore, the choice of a 500 m standard deviation and a 4 km support width seems arbitrary; Please give an explanation why these values were used. A sensitivity analysis or a stronger justification based on the dominant turbulent length scales would be beneficial.

Initial Conditions and Forcing: The use of an idealized sounding with a constant weak lapse rate above a 300 m mixed layer is a major simplification. The authors must more explicitly discuss the limitations this imposes, as the real atmosphere over the TP likely has multiple layers of varying stability that could significantly modulate entrainment and CBL growth.

Local vs. Domain-Averaged Impacts: While the local enhancement over the ridge (R-TOPO) is well-quantified (~500 m), the domain-averaged impact of orography (REAL vs. FLAT) is surprisingly small (~80 m). The authors should discuss this dichotomy more thoroughly. Does this imply that the overall impact of small-scale orography on the plateau-scale CBL is minor, but it creates strong local heterogeneity? The current narrative emphasizes the importance of orography, but the domain-mean results tell a more nuanced story.

The REAL terrain is described as featuring slopes up to 30° and ridges up to 1400 m. It would be helpful to contextualize this within the broader Tibetan Plateau to justify its representativeness.

While the idealized setup is appropriate for isolating mechanisms, the authors should more explicitly discuss how the exclusion of moisture, clouds, and synoptic variability might affect the generalizability of the results.

The authors mention that unresolved orography may bias CBL representation in models. A more quantitative discussion—e.g., how much CBL growth is underestimated in models that smooth terrain—would enhance the impact of the study.

The results section frequently references figures (e.g., Fig. 3, 4, 7) before the reader has any context for what these figures show. The narrative would be much clearer if the description of each figure was immediately followed by the interpretation of its results.

The abstract and conclusions are somewhat repetitive. The conclusions could be strengthened by synthesizing the findings into a broader conceptual model of how terrain and shear interact, rather than just restating the results.

Line 6, Small-scale orography substantially accelerates early growth: by midday the CBL in REAL is ∼80 m higher than in FLAT, 80m higher than FLAT is substantially? Compare surface flux and stability in the free air, is it still substantially?

Line 7, locally above the mountain it is ∼500 m deeper. This sentence is confusing, please rephrase it.

Line 7, This terrain-induced advantage narrows later in the day, what is narrows? Please rephrase this sentence.

Line 12, Under clear-sky conditions, the plateau’s CBL can exceed 9 km within a single day given strong surface heating and weak stability aloft. This conclusion has been approved by Chen` study. It may not be proper to be included in the abstract and taken as a conclusion of this study.

Line 18, One distinguishing feature of the TiP’s ABL is its extraordinary

depth. It`s better to cite the reference here: X., C. and Y., M., 2025. The unique atmospheric boundary layer over the Tibetan Plateau (in Chinese). Chin Sci Bull, 70: 4180–4187.

Line 37, it is better to further point out the weak stability in the lower troposphere is associated to the tropopause folds activity (Chen et al. 2011 ACP).

Chen, X., Ma, Y., Kelder, H., Su, Z. and Yang, K., 2011. On the behaviour of the tropopause folding events over the Tibetan Plateau. Atmos. Chem. Phys, 11: 5113-5122.

P3, L75: Specify what "REAL terrain" represents in terms of maximum height AGL and horizontal scales.

P4, L105: The vertical grid spacing is described twice with slightly different wording. Consolidate this into a single, clear description.

P9, eq. 4-6, The equations for TKE components are incorrectly formatted. w'^2 is repeated for all three components. This must be corrected to u'^2, v'^2, and w'^2.

This manuscript investigates the impact that small-scale orography features have on the evolution and structure of the convective boundary layer over the Tibetan Plateau. The author used an LES model (CM1) and ran three idealized simulations for a special case using three different configurations that differ in the small-scale orography features included and in the presence/absence of upper-level wind. The results show these small-scale features and shear effects enhance the CBL growth by up to 15%. The paper is well designed, clearly states the objectives and methodology, and the results are properly presented and discussed. I recommend the paper for publication, although some minor comments should be addressed before that.

Minor corrections

• Line 4: replace “a upper-level” by “an upper-level”.
• Line 8: please explain what LT means (first time that appears in the text).
• Line 16: please explain what the “TiB” abbreviation means.
• Line 19: same for the abbreviation “CBL”.
• Line 94 to 95: sentence repeated (already explained in line 91).
• Line 96: please include a brief explanation of what semi-slip condition means.
• Line 116: I think “FLAT” is a bit confusing cause this simulation is not flat itself, but does not include small-scale features. Please consider changing the name, like NOSSO or something that states more clearly how the orography is represented in that simulation.
• Section “Experiments and Data”: could the authors include whether the simulations were run with a spinup period?
• Lines 145-147: these two sentences say the same in a slightly different way. Please consider keeping just one to avoid repetition.
• Line 153: include the symbol q after potential temperature, as this symbol will be used in the caption of several figures throughout the manuscript.
• Figure 2: this figure is just briefly discussed in lines 154 and 155. I would suggest moving it to the Appendix.
• Line 157: add “w” after vertical velocity as this will be used in the caption of the following figures.
• Figure 3: in the caption, the authors mentioned that the vertical cross-sections are calculated for y=0. To improve the clarity of the manuscript, I suggest including a line in figure 1 showing where this cross-section is located.
  
  In the caption of this figure, there is a reference to 2.32.3.2. Please correct.
• Line 182: the authors say they also analysed the instantaneous vertical velocity at lower levels and that those results are not shown. Perhaps the one at 500 m AGL could be included in the Appendix.
• Line 189 and 190: w’2 should be replaced by u’2 and v’2, respectively.
• Figure 7: please indicate the units used and explain the meaning of the lines shown.
• Line 202: FLAT should be replaced by R-FLAT if I am not mistaken.
• Figure 8: if I am not mistaken, y-axis is showing the altitude of the CBL height and not the surface-based tracer concentration. Please clarify this.
• Line 281: the Appendix is never mentioned throughout the manuscript. Perhaps, you could refer to it in section 2.3.2 when talking about the method used to diagnose the boundary layer height.

Dear authors

As a first note, I want to inform you that I already read through the three other review statements. In general, I agree with all of them, and henceforth, my comments might be less than a typical review, but please consider them as equally relevant.

General comments

This is a clear, well-written manuscript exploring the reasons for a very high-ranging boundary layer over the Tibetan Plateau. Given the Tibetan Plateau's role in the global climate system, this research is relevant for the community. The authors employ the CM1 model for idealized simulations and analyze the boundary-layer structure in simulations with/without orography, and imposing different background wind conditions. The manuscript is well-written, but additional analysis (TKE budget) and possibly one more simulation (FLAT/u10m) are necessary to strengthen the conclusions of the authors and to make the MS suitable for publication in ACP.

Major comments

You identified shear-induced turbulence as one of the major reasons for the high-ranging ABL over your plateau. However, although you mention shear throughout the manuscript, you never show its actual quantity, e.g., by calculating the shear production term in the TKE budget equation compared to buoyancy forcing. Creating a figure and the accompanying text of the TKE budget will strengthen your manuscript massively. 

As a second point, you talk about 'terrain forcing'. I understand that this conclusion is drawn from a comparison of flat vs. complex terrain. However, if I understood it right, the FLAT simulation is a sub-domain of the domain with real toporgaphy.

1) How can you make sure that the FLAT subdomain is not affected by a plain to mountain flow towards your plateau?

2) As a second point, it would be also advisable to add a simulation/analysis of a flat-terrain, but shear-induced simulation, i.e., FLATu10, so that you can make a direct comparison with the plateau simulation - and - especially relevant - to isolate the 'terrain' and 'shear' effects accodingly.

Minor comments

As other referees already pointed out minor formulation or speeling errors, I would only want to encourage the authors to avoid statements like 'typical development' or 'expected behaviour' without citing the relevant literature. Not all readers are aware of how boundary-layer development over orography is 'supposed to look like'. Furthermore, the last paragraph of the introduction might better fit in the methods Section.

Therefore, I recommend major revisions for the manuscript and look forward to the revised version.

Best regards