|This manuscript discusses the growth of differences that arise in an ensemble of Large Eddy Simulations of an idealised squall line. The authors identify and discuss different stages of growth in error (assuming simulation differences represent error growth). They identify two stages of error growth in their simulations, and argue the first of these is associated with differences in gravity waves generated by the initial convection. These gravity waves lead to the initiation of secondary cells ahead of the squall line in some of the simulations, which impact on cold pool development later on. The second stage of error growth is associated with differences in cold pool propagation.|
Although the findings of this study may not be groundbreaking, the topic is certainly of interest to the atmospheric dynamics community. The authors clearly show a good knowledge of the literature on squall lines and error propagation in numerical models. However, in its current form the manuscript has a number of issues that need to be addressed before it can be published. Most of these are only textual, but resolving them will require restructuring the results section.
1) The key messages of the study and the motivation for it aren’t made sufficiently clear at the onset.
Although there is a discussion of the mechanisms that will be studied, it is not clear at the onset what motivated the study (what questions were left open by previous studies). A clearer focus and more guidance would also help to streamline the results section.
2) Several sections are difficult to read/interpret. Some sections could be reduced as they contain too much detail. A previous reviewer has already given some very helpful feedback here, which could be exploited further.
- Section 2.4 is hard to interpret without more context.
- Section 3 as a whole contains many numbers and details, which make it hard to read. The key messages are given in section 4. It would be better to only retain what is needed to support section 4. It is probably best to integrate these sections, so that results and their interpretation are presented together.
- The supplementary material could likewise be reduced further (at this point, I have focused on the main text).
3) The figures need some improvement (both in terms of presentation and clarity)
It would be clearer to plot the tracer in each simulation (and only keep the difference plot if the differences are not clear from a direct comparison) in figures 4 and 5. In figure 5, I wonder if both tracers are needed to tell the story.
For figure 6, why not simply show the trajectory over time for each of the simulations and use that as a starting point for discussion? The lag-correlation is harder to interpret.
For figure 8 and 9, using equations rather than words in the text would make it clearer what precisely is plotted. In terms of presentation style, some axis labels are missing, and some text overlaps. Time units switch between minutes and hours.
4) Details about the differences between the simulations that form the ensemble are unclear.
As I understand it, the simulations differ by the height of the zonal shear layer, but it is not clear which member corresponds to which interface height. Though the authors argue the interface height does not monotonically relate to e.g. “w_loc”, it would still be good to order the simulations by it.
5) In section 3.3.2, the precursors may not always be driving the target.
For example, a higher precipitation flux may cause higher evaporation and then faster cold pool propagation. That said, faster propagation could indeed also lead to more intense convection. In this context, it may be worth looking at a paper by Alfaro (2017) in JAS “Low-Tropospheric Shear in the Structure of Squall Lines: Impacts on Latent Heating under Layer-Lifting Ascent”.
- Check the text for compound (multi-word) adjectives, and hyphenate these: e.g. “three dimensional” → “three-dimensional”; “high resolution simulations” → “high-resolution simulations”.
- Remove/replace words that can be left out with no loss of information, e.g: “Presented diagnostics” → “diagnostics” ; “used scheme” → “microphysics scheme”; “The applied initial conditions” → “The initial conditions”.
- The subject “One” is overused in the text. I realise some authors try to avoid “we”, but the use of the first person makes it clearer whether the authors agree with a line of thought or not.
- Where two references are given outside parentheses, replace “;” by “and” (e.g. line 605).
In several places in the introduction, the text is vague/general/unclear, for example:
- Line 17: “Given the increasing computational resources”
- Line 21: “It also includes the aspect of representation”
- Line 25: “How squall lines depend on microphysics, shear and instability has been investigated rigorously by now, (e.g. Morrison et al., 2009; Grant et al., 2018; Adams-Selin, 2020a, b).” (also note the comma here)
- Line 35: “This was the core feature of both sensitivity studies.”
- Line 69: “A sensitivity of these discrete convective cell was identified, which lead to a dependence of initiation on the active treatment of radiation.”
- Line 1: Remove the first occurrence of “ensemble”.
- Line 4: “cold pool acceleration within the ensemble envelope” → “differences in cold pool propagation within the ensemble”
- Line 18: “augmented” → Probably this is not the right word. Do you mean that these systems have been studied in CRM, and more recently also in LES?
- Line 19: “high resolutions”
- Line 24: “true convergence” → note that there may be convergence of bulk properties (see e.g. work by Wolfgang Langhans and others), even if there is no numerical convergence.
- Line 39: Mentioning the work of Lorenz (1969) here already would be beneficial.
- Line 43: “Despite this focus they have compared the error growth of divergent and rotational wind components and found that divergent winds are mostly affecting larger scale errors.” → No need to start this sentence with “despite”.
- Line 51: These 4 points could be shortened, especially the ones that are not of interest here.
- Line 98: “, but by targeting at an inflow layer” → “, and by targeting at an inflow layer”
- Line 107: “are pointed out” → rephrase
- Line 119: “using the model version of Bryan (2019)” → is there a version number?
- Line 120: “All grid cells have a 200 m horizontal grid spacing and 100 m in the vertical.” → “All grid cells have a 200 m horizontal grid spacing and 100 m vertical grid spacing.”
- Line 123: “2 moment” “two-moment”
- Line 125: “Radiation is not actively resolved.” → are any tendencies prescribed?
- Line 141: “criticality” → “importance” (or explain what is meant)?
- Line 146: Even local noise would generate some 3D behaviour, though this may be more confined to small scales.
- Line 166: “Furthermore, the ensemble members all have slightly different boundary conditions, as controlled by their own evolution nearby/at the boundaries. The boundary conditions are solely based on their conditions, with the first derivatives set to zero right at the boundary.” → This is unclear, possibly what is meant is that the values at the boundary are different, even though the same type of boundary conditions is applied.
- Line 184: “valid” → “valid for”
- Line 209: y-variations may not be “smoother”, but on smaller scales?
- Line 234: “analysed in Section 3.2.2 for further analysis” → rephrase
- Figure 3: the upper levels could be left out.
- Line 247: “ocurs” → “occurs”
- Line 255: “about a couple of km” → “several km”
- Line 268: “time of installation until given output time” → rephrase
- Line 279: “peculiar patterns” → rephrase
- Line 289: “massively different” → rephrase
- Line 310: “as it has been” → “as has been”
- Line 312: “in the former section” → “in the previous section”
- Line 342: The use of the word “uncertainty” here is confusing, as it refers to simulations with small differences.
- Section 3.2.2: w_loc is always the value after 30 minutes, is this correct?
- L 384: “many more faster gravity wave signals” → the three comparatives make it hard to interpret this sentence.
- L 385: “would definitely pass the statistical significance test” → why not simply check it passes.
- Line 396: “The signal of the this stage” → ??
- Line 416: “(positive for updraft detection)” → remove
- Line 439: remove second occurrence of “causes”
- Line 466: “a process leading to highly unbalanced tendencies dominates error tendencies.” → explain
- Line 468: the word “climatological” is confusing in the context of the present paper.
- Line 471: “The zonal wind approach is differs” → differs
- Line 493: remove “do”
- Line 567: “doing” → “making”
- Line 587: “halfway of” → “half of”
- Section 4.2: Start the discussion with outcomes, rather than limitations (e.g. “This study identifies a highly relevant window for squall line error growth in the first 80 minutes of the
development, in which systematic variability in the squall line relative flow has fully developed and also decays.”)
- Line 599-630: This section can be shortened to focus on the (new) results presented in the manuscript.
- Line 614: “In spite of their analysis mostly carried out in spectral space” (“being” missing)
- Line 633: “apart from the depth of the shear layer” → put at the end of the sentence.
- Line 631: The idealisation certainly has the advantage that it simplifies the analysis, but also means that the spin-up of convection happens everywhere across the y-axis at the same time, which may not be representative of the way differences develop in an NWP ensemble.
- Line 634: “The high degree of 2D” → rephrase
- Line 640: the correlations look plausible, but the strength of the evidence seems overstated here. It can be questioned if the ensemble members can be treated as fully independent realisations given the small differences between them. Here, it would be good to show that w_loc is indeed poorly correlated with the interface height.
- Line 690: “Noteworthy” → “Notably”? Can you clarify this sentence?
Commendations to the authors on an interesting computation, and congratulations to the student on a PhD well earned.