Dear anonymous referee #1 many thanks for the generally positive feedback about our manuscript. Your comments are really appreciated. We are preparing a review of the document but as a preview  please find below our point-by-point responses to your suggestions and concerns.

Specific comments

Anonymous Referee #1: Interesting to note that unstable situations are found in the 330º-350º direction sector, which is a predominant one (page 8). If possible, please add a possible explanation for that.

Answer: As can be seen in figure 2, and it is explain in line119, the North face of Alaiz Mountain has a steep slope (the RIX value in the north sector in MP5 position is 22.4%) that empties into a large valley at around 700 m lower altitude. According to the reference book "An Introduction to Boundary Layer Meteorology" (Stull 1998) this topography causes ascending hillside/valley winds that generate convective turbulence and therefore situations of instability that could explain some of the instability found in the 330º-350º direction sector. We will clarify it in the revised version of the manuscript.

Anonymous Referee #1: At page 10, please clarify that the bulk Richardson number is also calculated on a 10-min interval, i.e. the same period used for the calculation of the Obukhov length.

Answer: Yes, the bulk Richardson number is also calculated on 10-min interval.

Anonymous Referee #1: As stated in the paper, sonic anemometers are not commonly used in the wind industry, so the Bulk Richardson number method would be of great interest for the industry. As the author mentions, one of the problems here for using that method is that the mast does not have a surface temperature sensor. It would be very useful if the authors can give their opinion on whether having the surface temperature sensor would improve the accuracy of the Bulk Richardson number method and thus the consistency between the 2 analyzed methods. If possible, please also give some guidance for future studies to estimate atmospheric stability using the Bulk Richardson number.

Answer: According to the good results with Richardson Bulk in offshore sites where the bulk Richardson method seems to be a robust approach to characterize stability offshore  (refrernce in the article "Atmospheric stability assessment for the characterization of offshore wind conditions",Sanz et al. 2015) and where it is calculated using sea temperature, we think that having the surface temperature sensor would improve the accuracy of the Bulk Richardson number method in onshore sites. In future experiments in Alaiz, we would like, on the one hand, to use measurements of the surface temperature and, on the other, keeping the heights close to the ground, of two meters or less, and use differential temperature sensors instead absolute ones to see if the results improve. With this information it would be possible to give clear guidelines on how to proceed.

Corrections:

Answer: We will correct them in the revised version of the manuscript.

Dear Llorenç,

Thank you very much for reviewing our manuscript and the many valuable comments and suggestions to improve the quality and clarity of the paper. Your feedback is really appreciated. We have carefully considered all your comments and, where necessary, will made changes to the manuscript accordingly. Before it please find below our point-by-point responses to your suggestions and concerns.

General comments

RC2:The research is very relevant to both the wind energy industry and the ABL scientific community around it. Further understanding of best practices for measuring atmospheric stability in complex terrain can yield important energy cost reductions. The research is well described and reproducible. However, I lack clear recommendations at the conclusions, e.g.:

• Which of the three studied levels provides the best Obukhov length for estimating turbulence intensity and wind shear at hub height?
• Bulk Richardson number cannot be safely computed if the lower level is not at the surface (or very close, e.g. 2m temp as in this case).
• Which of the two methods (Obukhov length or bulk Richardson number) is recommended? What is the benefit of investing in sonic anemometers?

Answer: Yes some recommendations as you said are:

Related with Bulk Richardson as it is explained in line 382 to 384: “The Rib number relies on smaller temperature differences for estimation of the mean gradient and its accuracy is therefore dependent on the sensor precision, calibration and measurement heights” so to obtain consistent results with bulk Richardson method it is required:

• Use high precision temperature sensors
• Calibrate all the temperature sensors at the same time
• Calibrate the temperature sensors in the operation range to guarantee better calibration in the temperatures of interest
• Have a reference temperature sensor below 2 meters, as close to the ground as possible.

We recommend the sonic method because with it we can obtain a local measurement of atmospheric stability that can be associated to a certain height above the ground. So the sonic method produces the most reliable estimate of stability since it is based on the local measurement of turbulent fluxes.

RC2: Additionally, I miss an analysis of how turbulence intensity and wind shear relate to bulk Richardson number (as in Figs. 9 & 10). It might be that although the stability description does not match the sonic one, it provides good predictive power of TI and shear at hub height, which is the ultimate goal of the stability characterization?

Answer: We had analyzed the behavior of the vertical profile and the intensity of turbulence with the bulk Richardson number (see in Rib_figures.pdf atached the figure 1 and 2) and, yes it provides some predictive power of TI and shear at hub height, but in order to characterize the stability dependency of the wind profile it is also important to have the percentages well defined in each stability class. This is important, for instance, when assigning stability-based frequencies to a discrete number of microscale flow simulations to compute a weighted-averaged wind conditions for a wind direction sector.   

RC2: In my opinion, the number of figures could be reduced without loss of informativeness. Also, some figures are difficult to read due to size (see proposed improvements below).

Answer: About the figures, we are checking how optimize them for a revised article.

RC2: The English language used and grammar is sometimes not very clear, and I recommend a revision of the language.

Answer: We will do it.

Specific comments  

RC2: L50: Please give more details on the cost and complexity of using sonic anemometers. It would also be good to quantify the benefits of this extra effort.

Answer: In economic terms, the cost of sonic anemometers, without calibration, is one order of magnitude higher than the cup anemometers. The sonics models we usually use cost around 5000€, while the cup anemometers cost approximately 500€. Sonic sensor calibrations are also more expensive. About complexity, sonic sensors do not have moving parts but they carry more electronics and are therefore more sensitive to disturbances in the network, lightning, etc. Moreover the sonic is very sensitive to impacts (when handling, by hail ...) since they misadjust the distance between the transducers. However the big advantage, for stability purpose, is that using a sonic anemometer we are able to measure the turbulent fluxes and derive z/L directly. Also as is explained in (Cuerva et al., 2006) the sensors have other advantages.

RC2: L58-59: what are the other challenges? Please describe them even if not addressed in this work.

Answer: The main challenges in complex terrain, which are affected by atmospheric interactions with the orography at different spatial scales, are those described in the article, the fact that the MOST is developed for horizontally homogeneous and flat terrain and in complex terrain vertical wind speed can be due to stability or sloping terrain, therefore, vertical fluxes will be “contaminated” by terrain effects. Another one, also mentioned in the article, is that small potential temperature gradients are difficult to measure accurately.

RC2: L63: Can this study be extended to the other towers in Alaiz? Or even to other instrumented sites in complex topography? How do these results for one site are expected to generalize to other sites in complex terrain?

Answer: Yes, we would like to have more data in complex terrain to consolidate the conclusions but, at this time, the other masts in Alaiz are not as well instrumented as the MP5. We would like, on the one hand, to use measurements of the surface temperature and, on the other, keep the heights close to the ground (below two meters)and use differential temperature sensors instead absolute ones to see how the results improve.

RC2: Table 1: please, provide the classes for the unstable range as well, with their names and abbreviations. The near-neutral class ranges from -0.02 to 0.02 I guess? This is unclear in the current description.

Answer: Yes the table with unstable range is: