the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2025
Wind profiles and low-level jet structures over the coastal waters of Japan
Abstract. Accurate characterization of coastal wind conditions is essential for offshore wind energy development; however, atmospheric structures in Japan's nearshore regions remain poorly understood. This study analyzed year-long vertical light detection and ranging (LiDAR) observations at closely located onshore and offshore sites along the Aomori coast to clarify the differences in wind profiles and their seasonal and directional dependence. Offshore wind speeds showed strong correlations (r >0.8) with onshore data, indicating that, although direct substitution is inappropriate, onshore observations can effectively serve as reference data for offshore extrapolation when using the measure–correlate–predict (MCP) method. Low-level jets (LLJs) were frequently observed in spring and summer, particularly when wind directions aligned with the coastline, with occurrence rates ≥20 % higher than in other seasons. Case analyses revealed that diurnal transitions associated with land–sea breeze circulation modulate vertical mixing and surface friction, promoting the development of LLJs. These results advance our understanding of nearshore boundary-layer dynamics and provide a basis for improving assessments of offshore wind resources, turbine designs, and LLJ forecasting strategies.
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RC1: 'Comment on wes-2025-239', Anonymous Referee #1, 09 Dec 2025
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The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2025-239/wes-2025-239-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/wes-2025-239-RC1 -
RC2: 'Comment on wes-2025-239', Anonymous Referee #2, 23 Dec 2025
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Wind profiles and low-level jet structures over the coastal waters of Japan
The presented manuscript considers the reliability of using onshore Lidar measurements to estimate nearby offshore conditions. The paper also addresses the formation of low-level jets off a Japanese coast.
Overall, the paper is interesting and can be beneficial for the wind-energy community. Given the nature of the considered dataset, I would suggest that the study be articulated as a case study since the drawn conclusions are local to the studied site and no generalizations were made. The paper would also benefit from some improvements to the writing to ensure clarity.
Below are my comments:
- Line 22: ‘Because wind-power generation is proportional to the third power of wind speed, wind conditions play a vital role in multiple aspects of offshore wind projects.’
Please remove this.
- Line 25: ‘assessing' should be ‘assess’
- Line 41: ‘Wagner et al. (2019) used FINO1 data and argued that diverse and context-dependent mechanisms are responsible for LLJ formation.’
Can you please be clearer on the conclusions of this study?
- Lines 55 & 61: It is a bit strange to read previous research that used vertical profiling Lidars could not ‘capture event-based phenomena such as LLJs or monotonic shear’, then a few lines after, a research gap is identified and addressed using the same tool: a vertical profiling Lidar. Can the authors further clarify what is the difference between what they did with the Lidar measurements vs what has been done before?
- Line 100: Is this the figure caption? Please put the figures description in the caption throughout the manuscript.
- Lines 120-122: Can you please comment on how and why would the formation of LLJs off the Japanese coast be different from their formation elsewhere with similar atmospheric conditions?
- Lines 128, 129: Can you please further elaborate on the definition? 25% of what? I think this sentence would be clearer if it was rewritten.
- Please correct m/s⁻¹ throughout the manuscript. It is either m/s or m s⁻¹.
- Line 138: Does this sentence belong here?
- Caption of Fig. 2 is confusing. The figure shown is just a single profile. Where is the method by Wagner et al?
- Equation 2: isn’t u_min set to 4 m/s in this study? Why not substitute 1.1 u_min with 4.4 m/s?
- Line 157: this is the second Eq. 2. It should be Eq. 3.
- Please re-structure section 2.3. What is the purpose of introducing the Obukhov length and the different methods to calculate it, then say it will not be used?
- Also in section 2.3, can the authors illustrate with examples (maybe in an Appendix) why the w’T’ signal was problematic?
- Line 175: Fig. 3 has not been introduced yet. The same applies to subsequent figures.
- Line 185: I think you mean it meets the criteria and can be used as a substitute for offshore data, right?
- Line 192: Please define what the normalized wind speed is.
- Line 194: Can this claim be supported by evidence?
- What’s the purpose of Fig. 8 if only the case of 2 May 2024 was considered in Fig. 9?
Citation: https://doi.org/10.5194/wes-2025-239-RC2 - Line 22: ‘Because wind-power generation is proportional to the third power of wind speed, wind conditions play a vital role in multiple aspects of offshore wind projects.’
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RC3: 'Comment on wes-2025-239', Anonymous Referee #3, 23 Dec 2025
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General comments:
This work provides an interesting and valuable analysis of on- and offshore wind patterns near the northern coast of Japan, making a case for the importance of doing so given the expanding development of offshore wind energy development. The authors make use of a unique (to this region) data set of lidar and sonic anemometer wind data covering a full annual cycle, September 2023-August 2024. They use 10-minute averaged data to investigate the frequency, seasonality, diurnal variation, and direction-dependence of LLJ occurrence, and connect their results primarily to land-sea contrasts and the resulting sea- and land-breezes. Their findings will be of interest to both wind plant developers and operators and to climate researchers.
The authors make some comparisons between LLJs in this area of Japan and LLJs that have been studied in other offshore regions. Despite the authors' description of the uniqueness of this region (lines 48-50), I am a little skeptical about these comparisons given that the authors use a specific definition of an LLJ (Eq. 2) that is not the same as what (apparently) is a more common definition (Eq. 1). I believe it will be helpful for readers - and that it will strengthen the paper - for the authors to provide an explanation of how the identification of LLJs is affected by their choice to use Eq. 2. For example, are you over- or underestimating LLJ occurrence with Eq. 2 vs. Eq. 1? And I am especially curious about how your results (figures 3-6 and table 3) might differ if you used Eq. 1 to identify LLJs rather than Eq. 2. I believe it is important to more fully explain and justify your choice of Eq. 2, given that the rest of the paper relies entirely on this definition.
Specific comments:
(1) Figure 2: This figure is difficult to interpret. How exactly does the figure compare the detection criteria? It seems more like the figure is identifying how umax and umin are identified from the profile, and then LLJ presence is based on Equations 1 and 2 rather than the figure itself.
(2) Line 156: You already have an Equation 2, so the equations starting here need renumbering.
(3) Lines 172-174: "Based on the NEDO Wind Observation Guidelines (2023), wind speeds 120 m above the onshore and offshore areas were compared for 16 wind directions. The classification of wind direction was determined using 120 m wind direction data from the offshore Doppler LiDAR." Does this mean you are using the 10-minute average wind speeds (lines 137-138) to identify LLJs every 10 minutes?
(4) Figure 3: Related to comment (3) above, what do the dots in this figure represent? Does each dot represent one 10-minute detection of a LLJ?
(5) Line 193: Why are land breeze directions WNW and WSW but sea breeze direction is only E?
(6) Figure 4:
- Did you look at the vertical profiles for all 16 directions for each season? Are these the only ones that show LLJs?
- Is the sample size the number of 10-minute averages from your one-year data set? Is it the number of hours in your one-year data set when LLJs are detected? This is not clear to me.
- Also, I don’t see season labels on this figure, but maybe they’re just small enough that I can’t find them?
(7) Table 3:
- It would be helpful to see this for all 16 directions, as justification for focusing your analysis on these specific directions.
- Is the sample size the number of 10-min wind speeds in your one-year data set? If so, this is a very small N compared to the number of 10-minute periods in a full year.
(8) Lines 233-234: E has a similarly small sample size, so why not the same caveat?
(9) Lines 308-311: I think you are saying your result actually is consistent with Blackadar's theory, given that "the thermal gradient between onshore and offshore areas" is what drives sea breezes?
(10) Line 329-330: "LLJs were frequently observed when winds were parallel to the coastline, particularly during spring and summer..." How frequent? How many hours per year or per season?
(11) Lines 331-332: "Detailed analyses..." What about the LLJs that you associate with N and S directions, which are parallel to the coast and thus not sea breezes? I note that the N and S occurrences are more frequent than E (sea breeze) occurrences, per your table 3.
Citation: https://doi.org/10.5194/wes-2025-239-RC3
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