The present work regard the use of  one of the most promising new techniques for measuring the structure of the surface layer: the fibre-optic distributed temperature sensing (DTS), which has the potential to provide to study thermal turbulence with high spatial and temporal resolution.

The authors after a detailed description of the methodology used and the calibration of the DTS system present the results obtained during two meteorological events: a study of the erosion of a near-ground cold layer during the passage of a warm front, and a comparison of the near-ground thermal structure with and without the presence of a sea-breeze cell during springtime convective boundary layer development. An eddy covariance system was used to compare and validate the DTS data.

The results demonstrate the utility of DTS measurements in revealing the internal surface layer structure, beyond the predictions of traditional surface layer theories.

The possibility of using this system allows for high frequency and high spatial resolution temperature profiles and to improve surface layer theories and parameters.

The limits and errors in using this system are widely and well reported and I think it can be published in the present form.

 Review of “Quantifying the coastal urban surface layer structure using distributed temperature sensing in Helsinki, Finland”

Authors: S. Karttunen, E. O’Connor, O. Peltola, L. Järvi

The manuscript focuses on observing the urban surface layer using distributed temperature sensing (DTS) in Helsinki, Finland. The main focus was on showing the performance of DTS measurements compared to standard eddy-covariance (EC) measurements during different conditions. The manuscript shows through careful and convincing analysis that the DTS system can have a low signal-to-noise ratio especially during near-neutral conditions. Nevertheless, as shown in two different case studies, even under these conditions DTS measurements can give insights into internal boundary layers beyond the capability of EC or tower measurements. The main benefit of DTS measurements are the (mostly) spatially-continuous profiles which in combination with at least one EC instrument can give useful insights into the development of the near-surface boundary layer during condition when MOST can not be applied. Accordingly, DTS will help in developing parametrisations for surface layer flows which are required for improving the prediction of near-surface temperatures by numerical weather forecast and climate models.

The manuscript is well structured, explains the analysis accurately, leads the reader nicely, and is well written. Figures are well explained and give good insights into the results. I enjoyed reading the manuscript. Thank you!

I only have very minor suggestions below. Accordingly, I recommend accepting as is.

Minor suggestions:

• L2: the word “coastal” is used twice within the sentence, maybe use “such” for the second one 
• L91: “...above sea level (ASL).” → since a lot of parameter are abbreviated using capital letters, I would recommend using lowercase letters: asl or a.s.l., but I am unsure if the journal recommend using ASL, in this case, I of course have no saying here.
• L100: Written sectors are also fine, but I always like visualizations: Maybe a pie chart? But this is only a suggestion, the written absolutely works.
• L477: “...This was the first time thatthe EC mesaurements…” → there is a space missing between “that” and “the”