This article provides an investigation of the effects of compressibility on 2D airfoil flows in the context of wind turbine blade application at high Reynolds numbers and realistic Mach numbers for modern wind turbines.

The manuscript is clearly and well-written. The methodology and scientific content appear correct and error-free. Only in a few places, the reviewer would like some clarifications and small corrections, as reported below.

Introduction, l.35-40: It is reported that 1) Yan and Archer found that neglecting compressibility could result in wind farm power production overestimation, 2) Campobasso et al and Ortoloni et al found that compressibility produces an increase of peak rotor power. This appears contradictory, and the authors should comment here if there are different mechanisms at stake, or at least point out this contradiction.

Section 2.2, l. 133: The authors refer to "dynamic stall". There seems to be some confusion here. The present calculations are done at constant angle of attack, and the authors probably means "stall" or "separation". Dynamic stall refers to the dynamic behavior of airfoil separation and stall when the angle of attack is varying.

Fig 6, caption: "Twoo"

Section 3.3: The interpretation of Fig. 14 needs clarification. If the reviewer understands well, all symbols are URANS calculations. When a supersonic region is present, the grey symbols are replaced by red ones, which are in turn replaced by green ones when a shock is detected. It is not clear why the authors did not conduct calculations for an angle of attack of 2 degs, M=0.6 and 0.65, for all Reynolds numbers, in order to refine the threshold curve shapes in this region, as illustrated by the horizontal line in Fig. 13 in this region.

Furthermore, it is noticed (l.244-245) that increasing Reynolds number promotes the appearance of transonic flow. Couldn't it be surmised here that this is caused by the decreasing viscosity? Or could there be another explanation?

Finally, it could be mentioned somewhere in the manuscript that tip effects (where the compressibility effects are maximum) may also play a role in the scenarios presented in this paper.

In conclusion, the paper is worth publishing and only minor revisions are suggested.

The article addresses the possibility of transonic/supersonic flow occurrence in the blade tip region of large wind turbines. The manuscript is well organized and well written, and relevant to the field. An interesting cross comparison of high- and low-fidelity aerodynamic methods to foresee the possible occurrence of transonic/supersonic flow is also presented.

Some additional comments are as follows. I would recommend the authors addressing them in the revised manuscript.

Mesh refinement should be addressed, indicating if or providing evidence that the presented parametric analyses provide mesh-independent results.

Could authors indicate the turbulence intensity of the results in Fig. 1? I would imagine the probability of transonic flow occurrence increases with the turbulence intensity, that tends to be low offshore. This I would assume that the occurrence of transonic conditions is more likely onshore. Can authors comment on this in the paper?

Page 4. Article reads: ‘The aim of the first part of this study is to prove that transonic flow can appear even in normal operation conditions’. Please define ‘normal operating conditions’.

Page 4, article reads: ‘Also, to save computational resources, wall functions are used to model the boundary layer region’. AoAs of \pm 15 degrees are quite high. Is there not the chance that shock-induced separation due to boundary layer/shock interactions may occur? In this circumstance, use of wall functions may induce significant errors. Can authors please comment on this matter in regard to the presented results?

Fig. 3, Table 1. This type of analysis should be performed with the AoA pf \pm 15 degrees, since pressure perturbations on either airfoil size are more likely to extend farther away from the airfoil, potentially leading to spurious reflections. I am not asking to redo this analysis, but some comments on the choice of 7.99 degrees may be helpful.

Fig. 4c: is it not possible that additional reason for the discrepancy OpenFOAM/experiments could also be the use of wall functions in separated flow region?

Perhaps figures 11 and 12 could be slightly improved? For example, indicating LE is at s=1 (I suppose). Indication of upper and lower side in cf graphs may also help quicker readability.