the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Apr 2024
Observations of Tall-Building Wakes Using a Scanning Doppler Lidar
Abstract. High-rise buildings, increasingly a feature of many large cities, impact local atmospheric flow conditions. Tall building wakes affect air quality downstream due to turbulent mixing and require parametrization in dispersion models. Previous studies using numerical or physical modelling have been idealised and under neutral conditions. There has been a lack of data available in real urban environments due to the difficulty in deploying traditional wind sensors. Doppler wind lidars (DWLs) have been used frequently for studying wind turbine wakes but never building wakes. This study is a year-long deployment of a DWL in a complex urban environment studying tall building wakes under atmospheric conditions. A HALO Photonic Streamline DWL was deployed in a low- and mid-rise densely packed area in central London. From its roof-top position (33.5 m agl compared to mean building height 12.5 m), Velocity Azimuth Display (VAD) scans at zero-degree elevation intersected with two, taller nearby buildings of 90 and 40 m agl. Using an ensemble averaging approach, wake dimensions were investigated in terms of wind direction, stability and wind speed. Boundary-layer stability categories were defined using eddy covariance observations from the BT Tower (191 m) and mixing height estimations from vertical stare scans. A method for calculating normalised velocity deficit from VAD scans is presented. For neutral conditions, wake dimensions around both buildings for the prevailing wind direction were compared with the ADMS-Build wake model for a single, isolated cube. The model underpredicts wakes dimensions, confirming previous wind tunnel findings for the same area. Under varying stability, unstable and deep boundary layers were shown to produce shorter, narrower wakes. Typical observed wake lengths were 120–300 m and widths were 80–150 m and were reduced by 50–100 m downwind. Stable and shallow boundary layers were less frequent and produced an insignificant difference in wake dimensions to neutral conditions. The sensitivity to stability was weakened by enhanced turbulence upstream (i.e., due to other building wakes). Weakened stability dependence was confirmed if there were more obstacles upstream as the wind direction incident on the buildings changed. The results highlight the potential for future wake studies using multiple DWLs deploying both vertical and horizontal scan patterns. Dispersion models should incorporate the effect of a complex urban canopy within which tall buildings are embedded.
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RC1: 'Comment on egusphere-2024-937', Anonymous Referee #1, 23 Apr 2024
The manuscript entitled "Observations of Tall-building Wakes Using a Scanning Doppler Lidar" presents the results of field campaign of the urban flow over London. In particular, the study uses observation acquired by a scanning Dopple lidar to characterize the wake of two tall buildings under various stability conditions. The manuscript is well structured and written and the results are presented in a clear manner. However, I think that there are few points that need to be clarified. Please find below my comments and suggestions for improving the manuscript.
General Comments
- The manuscript in the 3rd paragraph introduces how Dopper lidars have been used in wind energy research studies and in the 4th paragraph they highlight results of research investigations of wind turbine power production and wakes using Doppler lidars. It is not clear how and if these references are relevant to the research objective of this study. Furthermore, there have a been a couple of already published studies that investigate the impact of buildings or bridges to the surrounding wind conditions. I suggest that the authors should consider mentioning such studies in the introduction if they think that it is relevant. For example:
- Kiros E.W. Lim, Simon Watkins, Reece Clothier, Raj Ladani, Abdulghani Mohamed, Jennifer L. Palmer, Full-scale flow measurement on a tall building with a continuous-wave Doppler Lidar anemometer, Journal of Wind Engineering and Industrial Aerodynamics, Volume 154, 2016, Pages 69-75, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2016.04.007.
- Xiaoying Liu, Hongwei Zhang, Songhua Wu, Qichao Wang, Zhiqiang He, Jianjun Zhang, Rongzhong Li, Shouxin Liu, Xi Zhang, Effects of buildings on wind shear at the airport: Field measurement by coherent Doppler lidar, Journal of Wind Engineering and Industrial Aerodynamics, Volume 230, 2022, 105194, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2022.105194.
- Mohammad Nafisifard, Jasna B. Jakobsen, Jonas T. Snæbjörnsson, Mikael Sjöholm, Jakob Mann, Lidar measurements of wake around a bridge deck, Journal of Wind Engineering and Industrial Aerodynamics, Volume 240, 2023, 105491, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2023.105491.
For the record, I am neither the author nor any of the co-authors of the above publications.
- To estimate the reference wind speed with the current measuring setup it was assumed that the mean wind speed and direction is the same over a circular area with a radius of 500 m. I understand that this is a necessary assumption, but probably it is not realistic. This is partially discussed in the lines 336 – 342. I think that this should be clearly stated as one the limitations of the used measuring setup.
- The scanning Doppler lidar measures the projection of the wind vector on each line-of-sight of the scanning pattern. The wind direction of the case study presented in Figs. 4, 5 and 6 was 220 degrees. For that wind direction the line-of-sight directions is almost perpendicular to the mean of the wake of the buildings A and B. This measuring configurations makes the estimation of the radial speed uncertain since small errors in either the pointing accuracy or in the radial speed estimation can result to large errors in the instantaneous radial speed when normalizing with a mean Vtheta that has almost a zero value. I think that this point should be highlighted in either Section 5 or in a “Discussion” section.
- It is not very clear how are the data of the ADMS-Build model used to compare to the measurements. Is the wind vector estimated from the ADMS-Build model projected to each line-of-sight? Section 5 is written now in a way that it could be understood that the mean radial speed is the input to the ADMS-Build model.
- I suggest adding in the caption of the figures that present cross-sections of the wake flow, that these profiles correspond to the normalised radial velocities in the wake.
- I think that the “Conclusions” section is quite lengthy. I suggest considering if it would make more sense splitting this section to a “Discussion” and a “Conclusions” section.
Specific Comments
- Line 73. What kind of data were used to estimate the mean canopy height and for plotting Fig. 1? Furthermore, please explain how the plan area index is calculated.
- Line 86. I think that the authors mean that the “focus distance” was set to infinity not the “focal length”.
- Line 87. The authors write: “The lidar was configured with a range resolution of 18 m with 6 points per range bin”. I guess that a range gate was estimated for every 18 m, but what was the length of each range gate? The length of the range gate determines the spatial resolution of a Doppler lidar. Also, what is meant when it is written that each range bin had 6 points?
- Lines 95 – 96. The “integration time per ray” mean the averaging time per line-of-sight in the staring mode?
- Line 97. It is written “The maximum range was changed to 555 gates…”. Do this mean that the maximum range was 555 x 18 m = 9990 m? Please clarify.
- Lines 100 – 102. I suggest moving this paragraph at Line 95. As it is written in the current version the duration of each scanning mode is described before they are introduced. Also, mention here that the total duration of the scanning is 30 minutes. Finally, when the authors write 6-point VAD and 72-point VAD do they refer to the azimuth angles or the range gates?
- Line 166. The lowest gate is at 98.5 m or at 108.5 m (36.5 + 4x18)?
- Line 191. The line-of-sight radial velocities measured by the scanning wind lidar shouldn’t be equal to Vtheta = Vmean Cos(theta-phi) ?
- Line 192. I that that here it should be written that the mean V should be projected to each line-of-sight, not the mean Vtheta.
- Line 197. How is the backscattering coefficient calculated in the Doppler lidar measurements? Also, please write the units with roman fonts.
- Lines 197 – 199. Are there trees in the measuring area that are high enough to block the Doppler lidar measurements?
- Line 205. How many VAD scans were averaged to plot Fig. 5?
- Lines 213 – 217. I suggest drawing the location of the building presented in this paragraph on Fig. 4d, so it can be become more straightforward to relate the features presented in the figure to the building discussed in the paragraph.
- Lines 235 – 244. In the ADMS-Build model is it considered that the inflow of the building A is affected by the wake of the building B?
- Lines 259 – 260. The observed asymmetry of the wake profile couldn’t be related to an inhomogeneous inflow due to the wake of the building B?
- Lines 273 – 274. What is meant here with the “sufficient” scans. How many scans were averaged?
- Lines 314 – 315. How is the length and width of the wake defined in the study? If I am not mistaken this the first time that numerical values of these two characteristics of the wake are mentioned. Please elaborate more about these characteristics in the main text of the manuscript.
Minor Corrections
The manuscript is very well written, so I have only three suggestions for minor corrections.
- Would it be possible to also add the information of the spatial dimension of Fig.1 (top) in meters? For example, by adding this information at the top x-axis and right y-axis. I think that it would help a lot in understanding the distance of the buildings in relation to the location of the wind lidar.
- Caption of Figure 6. Is it correct that are “depicted in 8”?
- Please write all units with roman fonts.
Citation: https://doi.org/10.5194/egusphere-2024-937-RC1 -
AC1: 'Reply on RC1', Natalie Theeuwes, 15 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-937/egusphere-2024-937-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-937', Anonymous Referee #2, 28 May 2024
It is a thorough investigation dealing with the ability of DWL to detect wakes from high-rise buildings in an urban environment. I have just a few comments that I ask the authors to consider.
Considering the manuscript in the introduction is partly focused on atmospheric dispersion in an urban area with high-rise buildings, I suggest adding a reference to “the Salt Lake City URBAN 2000 Tracer Experiment” which deals with the problems from a dispersion point of view of urban area with several high-rise buildings. A reference could be Allwine, K. J., J. H. Shinn, G. E. Streit, K. L.Clawson, and M. J. Brown, 2002: Overview of URBAN 2000: A Multi-Scale Field Study ofDispersion Through an Urban Environment, Bull.Amer. Meteor. Soc. 83(4), 521-536. Or Hanna Steven, Britter RFEX and Franzese (2003) Baseline urban dispersion model evaluated with Salt Lake City and Los Angeles data.Atmospheric Environment 37(36):5069-5082 DOI: 10.1016/j.atmosenv.2003.08.014
Line 114: Do you have a reference for the 30% lower threshold value for the number of data points available? I find the value rather low.
Line 117: what is the spread of the 21 values of the mixing height? Did you consider the effect of clouds and rain?
Line 152 How was the threshold values for the stability classes obtained?
Line 162. Same as in l152 but for the stability based on the mixing height.
Line 191 Check Eq. (1). Units on left hand side is m/s and on the right hand side s m2/s2. Additionally I am not sure that V has been defined.
Citation: https://doi.org/10.5194/egusphere-2024-937-RC2 -
AC2: 'Reply on RC2', Natalie Theeuwes, 15 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-937/egusphere-2024-937-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Natalie Theeuwes, 15 Sep 2024
-
EC1: 'Comment on egusphere-2024-937', Klara Jurcakova, 05 Jun 2024
Dear authors,
Your manuscript has been reviewed by two experts. Both have agreed that the manuscript is well written and the content is sound. They have raised questions and have comments. Please answer the questions and revise your manuscript accordingly.Citation: https://doi.org/10.5194/egusphere-2024-937-EC1
Status: closed
-
RC1: 'Comment on egusphere-2024-937', Anonymous Referee #1, 23 Apr 2024
The manuscript entitled "Observations of Tall-building Wakes Using a Scanning Doppler Lidar" presents the results of field campaign of the urban flow over London. In particular, the study uses observation acquired by a scanning Dopple lidar to characterize the wake of two tall buildings under various stability conditions. The manuscript is well structured and written and the results are presented in a clear manner. However, I think that there are few points that need to be clarified. Please find below my comments and suggestions for improving the manuscript.
General Comments
- The manuscript in the 3rd paragraph introduces how Dopper lidars have been used in wind energy research studies and in the 4th paragraph they highlight results of research investigations of wind turbine power production and wakes using Doppler lidars. It is not clear how and if these references are relevant to the research objective of this study. Furthermore, there have a been a couple of already published studies that investigate the impact of buildings or bridges to the surrounding wind conditions. I suggest that the authors should consider mentioning such studies in the introduction if they think that it is relevant. For example:
- Kiros E.W. Lim, Simon Watkins, Reece Clothier, Raj Ladani, Abdulghani Mohamed, Jennifer L. Palmer, Full-scale flow measurement on a tall building with a continuous-wave Doppler Lidar anemometer, Journal of Wind Engineering and Industrial Aerodynamics, Volume 154, 2016, Pages 69-75, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2016.04.007.
- Xiaoying Liu, Hongwei Zhang, Songhua Wu, Qichao Wang, Zhiqiang He, Jianjun Zhang, Rongzhong Li, Shouxin Liu, Xi Zhang, Effects of buildings on wind shear at the airport: Field measurement by coherent Doppler lidar, Journal of Wind Engineering and Industrial Aerodynamics, Volume 230, 2022, 105194, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2022.105194.
- Mohammad Nafisifard, Jasna B. Jakobsen, Jonas T. Snæbjörnsson, Mikael Sjöholm, Jakob Mann, Lidar measurements of wake around a bridge deck, Journal of Wind Engineering and Industrial Aerodynamics, Volume 240, 2023, 105491, ISSN 0167-6105, https://doi.org/10.1016/j.jweia.2023.105491.
For the record, I am neither the author nor any of the co-authors of the above publications.
- To estimate the reference wind speed with the current measuring setup it was assumed that the mean wind speed and direction is the same over a circular area with a radius of 500 m. I understand that this is a necessary assumption, but probably it is not realistic. This is partially discussed in the lines 336 – 342. I think that this should be clearly stated as one the limitations of the used measuring setup.
- The scanning Doppler lidar measures the projection of the wind vector on each line-of-sight of the scanning pattern. The wind direction of the case study presented in Figs. 4, 5 and 6 was 220 degrees. For that wind direction the line-of-sight directions is almost perpendicular to the mean of the wake of the buildings A and B. This measuring configurations makes the estimation of the radial speed uncertain since small errors in either the pointing accuracy or in the radial speed estimation can result to large errors in the instantaneous radial speed when normalizing with a mean Vtheta that has almost a zero value. I think that this point should be highlighted in either Section 5 or in a “Discussion” section.
- It is not very clear how are the data of the ADMS-Build model used to compare to the measurements. Is the wind vector estimated from the ADMS-Build model projected to each line-of-sight? Section 5 is written now in a way that it could be understood that the mean radial speed is the input to the ADMS-Build model.
- I suggest adding in the caption of the figures that present cross-sections of the wake flow, that these profiles correspond to the normalised radial velocities in the wake.
- I think that the “Conclusions” section is quite lengthy. I suggest considering if it would make more sense splitting this section to a “Discussion” and a “Conclusions” section.
Specific Comments
- Line 73. What kind of data were used to estimate the mean canopy height and for plotting Fig. 1? Furthermore, please explain how the plan area index is calculated.
- Line 86. I think that the authors mean that the “focus distance” was set to infinity not the “focal length”.
- Line 87. The authors write: “The lidar was configured with a range resolution of 18 m with 6 points per range bin”. I guess that a range gate was estimated for every 18 m, but what was the length of each range gate? The length of the range gate determines the spatial resolution of a Doppler lidar. Also, what is meant when it is written that each range bin had 6 points?
- Lines 95 – 96. The “integration time per ray” mean the averaging time per line-of-sight in the staring mode?
- Line 97. It is written “The maximum range was changed to 555 gates…”. Do this mean that the maximum range was 555 x 18 m = 9990 m? Please clarify.
- Lines 100 – 102. I suggest moving this paragraph at Line 95. As it is written in the current version the duration of each scanning mode is described before they are introduced. Also, mention here that the total duration of the scanning is 30 minutes. Finally, when the authors write 6-point VAD and 72-point VAD do they refer to the azimuth angles or the range gates?
- Line 166. The lowest gate is at 98.5 m or at 108.5 m (36.5 + 4x18)?
- Line 191. The line-of-sight radial velocities measured by the scanning wind lidar shouldn’t be equal to Vtheta = Vmean Cos(theta-phi) ?
- Line 192. I that that here it should be written that the mean V should be projected to each line-of-sight, not the mean Vtheta.
- Line 197. How is the backscattering coefficient calculated in the Doppler lidar measurements? Also, please write the units with roman fonts.
- Lines 197 – 199. Are there trees in the measuring area that are high enough to block the Doppler lidar measurements?
- Line 205. How many VAD scans were averaged to plot Fig. 5?
- Lines 213 – 217. I suggest drawing the location of the building presented in this paragraph on Fig. 4d, so it can be become more straightforward to relate the features presented in the figure to the building discussed in the paragraph.
- Lines 235 – 244. In the ADMS-Build model is it considered that the inflow of the building A is affected by the wake of the building B?
- Lines 259 – 260. The observed asymmetry of the wake profile couldn’t be related to an inhomogeneous inflow due to the wake of the building B?
- Lines 273 – 274. What is meant here with the “sufficient” scans. How many scans were averaged?
- Lines 314 – 315. How is the length and width of the wake defined in the study? If I am not mistaken this the first time that numerical values of these two characteristics of the wake are mentioned. Please elaborate more about these characteristics in the main text of the manuscript.
Minor Corrections
The manuscript is very well written, so I have only three suggestions for minor corrections.
- Would it be possible to also add the information of the spatial dimension of Fig.1 (top) in meters? For example, by adding this information at the top x-axis and right y-axis. I think that it would help a lot in understanding the distance of the buildings in relation to the location of the wind lidar.
- Caption of Figure 6. Is it correct that are “depicted in 8”?
- Please write all units with roman fonts.
Citation: https://doi.org/10.5194/egusphere-2024-937-RC1 -
AC1: 'Reply on RC1', Natalie Theeuwes, 15 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-937/egusphere-2024-937-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2024-937', Anonymous Referee #2, 28 May 2024
It is a thorough investigation dealing with the ability of DWL to detect wakes from high-rise buildings in an urban environment. I have just a few comments that I ask the authors to consider.
Considering the manuscript in the introduction is partly focused on atmospheric dispersion in an urban area with high-rise buildings, I suggest adding a reference to “the Salt Lake City URBAN 2000 Tracer Experiment” which deals with the problems from a dispersion point of view of urban area with several high-rise buildings. A reference could be Allwine, K. J., J. H. Shinn, G. E. Streit, K. L.Clawson, and M. J. Brown, 2002: Overview of URBAN 2000: A Multi-Scale Field Study ofDispersion Through an Urban Environment, Bull.Amer. Meteor. Soc. 83(4), 521-536. Or Hanna Steven, Britter RFEX and Franzese (2003) Baseline urban dispersion model evaluated with Salt Lake City and Los Angeles data.Atmospheric Environment 37(36):5069-5082 DOI: 10.1016/j.atmosenv.2003.08.014
Line 114: Do you have a reference for the 30% lower threshold value for the number of data points available? I find the value rather low.
Line 117: what is the spread of the 21 values of the mixing height? Did you consider the effect of clouds and rain?
Line 152 How was the threshold values for the stability classes obtained?
Line 162. Same as in l152 but for the stability based on the mixing height.
Line 191 Check Eq. (1). Units on left hand side is m/s and on the right hand side s m2/s2. Additionally I am not sure that V has been defined.
Citation: https://doi.org/10.5194/egusphere-2024-937-RC2 -
AC2: 'Reply on RC2', Natalie Theeuwes, 15 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-937/egusphere-2024-937-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Natalie Theeuwes, 15 Sep 2024
-
EC1: 'Comment on egusphere-2024-937', Klara Jurcakova, 05 Jun 2024
Dear authors,
Your manuscript has been reviewed by two experts. Both have agreed that the manuscript is well written and the content is sound. They have raised questions and have comments. Please answer the questions and revise your manuscript accordingly.Citation: https://doi.org/10.5194/egusphere-2024-937-EC1
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