Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow &ndash; an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems

Gasch, Philipp; Kasic, James; Maas, Oliver; Wang, Zhien

doi:https://doi.org/10.5194/egusphere-2023-548

Preprints

https://doi.org/10.5194/egusphere-2023-548

Preprints

28 Mar 2023

| 28 Mar 2023

Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow – an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems

Philipp Gasch, James Kasic, Oliver Maas, and Zhien Wang

Abstract. There is a need for improved wind measurements inside the planetary boundary layer (PBL), including the capability to sample turbulent flow. Airborne Doppler lidar (ADL) provides unique capabilities for spatially resolved and targeted wind measurements in the PBL. However, ADL wind profiling in the PBL is challenging, as turbulence violates the flow homogeneity assumption used in wind profile retrieval and thereby introduces error in the retrieved wind profiles. As turbulence is a dominant source of error it is necessary to investigate and optimize ADL wind profiling capabilities in turbulent PBL flow.

This study investigates the potential of a novel multiple fixed-beam ADL system design to provide improved wind information in turbulent PBL flow, compared to traditional single scanning-beam ADL systems. To achieve this, an LES-based airborne Doppler lidar simulator presented in Gasch et al. (2020) is employed and extended in this study.

Results show that a multiple fixed-beam system with settings comparable to those of commonly used single scanning-beam systems offers distinct advantages. Advantages include overall reduced wind profile retrieval error due to turbulence and improved spatial representation alongside higher wind profile availability. The study also offers insight into the dependence of the retrieval error on system setup parameters and retrieval parameters for both fixed-beam and scanning-beam systems. When using a fixed-beam system, an order of magnitude higher wind profile resolution appears possible, compared to traditional scanning systems at comparable retrieval accuracy. Thus, using multiple fixed-beam systems opens the door towards better sampling of turbulent PBL flow.

Overall, the simulator provides a cost-effective tool to investigate and optimize wind profile error characteristics due to turbulence, and to optimize system setup and retrieval strategies for ADL wind profiling in turbulent flow.

Received: 23 Mar 2023 – Discussion started: 28 Mar 2023

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Journal article(s) based on this preprint

16 Nov 2023

Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow – an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems

Philipp Gasch, James Kasic, Oliver Maas, and Zhien Wang

Atmos. Meas. Tech., 16, 5495–5523, https://doi.org/10.5194/amt-16-5495-2023,https://doi.org/10.5194/amt-16-5495-2023, 2023

Short summary

Philipp Gasch et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-548', Benjamin Witschas, 04 Apr 2023

Publisher’s note: a supplement was added to this comment on 5 April 2023.
Please find my review in the attached .pdf file.

Citation: https://doi.org/10.5194/egusphere-2023-548-RC1
- AC3: 'Reply on RC1', Philipp Gasch, 14 Jul 2023
  
  Dear Dr. Witschas,
  thank you for the time and effort invested in reviewing our manuscript. Please find our replies in the attached document.
  Kind regards
  Philipp Gasch and co-authors
  
  Citation: https://doi.org/10.5194/egusphere-2023-548-AC3
RC2:
'Comment on egusphere-2023-548', George Emmitt, 11 May 2023
Review comments
Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow-an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems
Gasch, et al. 2023

Readability: Very well written; well organized and thorough.
Significance: Although the multiple fixed beam (each with its own lidar) is not novel for a proposed and simulated space based DWL (e.g. JEMCDL), the investment in another DWL tool for airborne atmospheric and ocean surface research is highly merited. Some advantages of a 5 FIXED beam (each continuous transmitting and receiving) compared with a scanning ADWL sampling in a cycloidal pattern using just a single transmitter/receiver for a certain subset of observational/research goals are obvious.
Methodology: Use of an LES-based airborne Doppler lidar simulation test bed is ideal for isolating sampling related errors of representing the “true” profile of the wind within a target volume in the presence of wind shear (both speed and directional) and turbulence. The attributions of “error” to turbulence or representativeness is very useful in operating and configuring an ADWL as well as processing the LOS retrievals to obtain estimates of the vertical profile of u and v within a dynamically non-homogeneous target volume, in this case the middle layers of an unstable PBL.
I am recommending that this paper be published after minor revisions. The revision I suggest is to acknowledge that SCA1 concept does not represent ADWL configurations currently in use by NASA, NOAA and ONR. SCA1 represents a simple continuous scanning mode suitable for this initial study.
Exceptions taken: Following is a discussion of exceptions taken to the experimental setup which raises issues with all subsequent conclusions.
The evaluation metrics in this paper are wind vector product centric. While this does not invalidate this papers investigation of the merits of FIX5 vs SCA1, a major utility of the ADWL observations is numerical model validation and numerical model Data Assimilation, both of which prefer LOS ADWL retrievals, leaving the full vector wind profile to second tier processing.

If LOS observation density and distribution in terms of along track and cross track directions were used for the basic comparisons, different conclusions could be reached as to which sampling technique serves the modeling community best.
The scanning system (SCA1) does not represent the standard (traditional?) sampling pattern used with the ADWLs used in the studies referenced (Bucci, De Wekker) nor the ADWL used on the NASA DC8 (Turk, Kavaya). The coherent ADWLs being used in the USA in large field campaigns over that last 2 decades use two types of scanning:
Fixed elevation with azimuthal scanning in a step-stare mode using 2 -13 azimuthal programmable stops (NASA’s DAWN and the new AWP which includes a nadir staring option)

ONRs (also used by NOAA) cylindrical side mounted scanner allowing programmable beam pointing routines within a large azimuth/elevation bounded target volume.

The use of a continuous scanning approach (e.g. at 20 degrees/sec) has been replaced with a “step and stare” strategy for many years and for several reasons (lag angle and the desire to eliminate the angular spread in lidar shots being integrated before preforming a spectral analysis).

A better (and more relevant) “reference” SCA1 for this sampling centric study would be the following based upon more than 1000 flight hours of observations using the cylindrical scanner:
Elevation angle from the horizontal: 60 degrees

12 azimuthal stares for 1 second with 30 degree azimuth increments

Slew rate between stares (30 degrees/second)

1 nadir stare (five-10 seconds) in the middle of the 12 stare VAD.

50m range resolution

50 -100m along track averaging.

Had the SCA1 sampling pattern described in 3. above been used for quantifying the advantages (and disadvantages) of the FIX5 system, the following conclusions and expectations might be reversed or at least quantitatively changed.
Line 35: The Goodness of Fit (GOF) value for a 12 look step stare solution for u,v provides a very useful measure of the non-uniform distribution of winds in the retrieval volume. This GOF is used to generate a confidence metric for representativeness. By performing triple pass processing a reasonable description of the non-uniformity can be made…not assumed except for the first pass.

Line 54: It is not clear why the simulation was not performed for an aircraft flying 500-1000 meters above the PBL top since that may be the preferred perspective on the PBL. For the reasons stated elsewhere in the paper, the “saftest” portion of the PBL to use for analysis is the 100-1000meter layer (middle of the PBL). That is understandable, but the horizontal data coverage from 3000m will be different than from 1500m.

Line 94: Are there any disadvantages of the FIX5 vs the SCA1 for PBL research? Would any of the stated advantages of the FIX5 ADWL change if the more relevant scanning ADWL configuration were used?

Line 175: Step and Stare scanning greatly reduces the lag angle losses. This is not an issue for the reported study, though.

Lines 330-365: Throughout this paper there are frequent references to the issue of alignment of the FIX5 scanning telescopes with the aircraft ground track (crabbing) and the wind direction. With the 12 look scanning ADWL (let’s call it SCA2), there are numerous subsets of azimuth look angles that can be used for sector wind vector retrievals, for example, quadrant retrievals. Based upon the 4 wind profiles thus obtained, horizontal gradients and other estimates of non-uniform flow can be deduced and quantified. The presence of PBL jets and directional shear layers does not impact (degrade) the accuracy of the SCA2 profile retrievals as much as might be the case for the FIX5. This point raises issues with all subsequent conclusions.

Lines N/A: This paper does not discuss explicitly the ability to measure vertical velocity. The primary interest is in the impact that very local vertical motions associated with organized structures such as OLEs will have on the calculation of horizontal wind components. However, the DC offset of the 12 point solution along with the 5 – 10 second vertical stare provide insight to the scales of vertical “contamination” of the horizontal wind retrieval. This, at the least, provides a means of attaching “quality” flags to each profile. Without this “over sampling” compared to only 2-4 perspectives, there is much less basis for judging representativeness in actual applications.

In spite of the exceptions and concerns expressed in 5. above, this paper is well written and answers the questions (Lines 94-97) based upon the assumed SCA1 configuration. However, to be relevant to how ADWLs have been scanned for that last 20 years (like ONR’s TODWL and NRL/NOAAs P3DWL with very flexible pointing options) and are being designed for the next generation of high energy ADWL (like NASA’s AWP with a vertical stare option), there is the need to simulate a SCA2 type instrument and ask the questions a slightly different way. 1.) How do the FIX5 and SCA2 serve the modeling community vs the atmospheric processes community? (i.e. LOS as primary product vs wind vector profile?) 2.) Do fewer but more accurate bi and quad perspective profiles over smaller foot prints trump more LOS samples from more (say, 12) perspectives, especially in complex flows?

Regardless of any follow-on simulations, the development of the FIX5 instrument has merit and will not only provide collocated (for no crabbing) bi-perspective simultaneous LOS measures of the winds, but will also use SOTA fiber laser technology for each telescope and , hopefully, a less expensive and thus more available means of making airborne wind profiles for both academic research as well as applications. The increased number of vector profiles per km along a track is certainly attractive.
Citation: https://doi.org/10.5194/egusphere-2023-548-RC2
- AC2: 'Reply on RC2', Philipp Gasch, 14 Jul 2023
  
  Dear Dr. Emmitt,
  thank you for the time and effort invested in reviewing our manuscript. Please find our replies in the attached document.
  Kind regards
  Philipp Gasch and co-authors
  
  Citation: https://doi.org/10.5194/egusphere-2023-548-AC2
AC1: 'Comment on egusphere-2023-548', Philipp Gasch, 14 Jul 2023

Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2023-548-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-548', Benjamin Witschas, 04 Apr 2023

Publisher’s note: a supplement was added to this comment on 5 April 2023.
Please find my review in the attached .pdf file.

Citation: https://doi.org/10.5194/egusphere-2023-548-RC1
- AC3: 'Reply on RC1', Philipp Gasch, 14 Jul 2023
  
  Dear Dr. Witschas,
  thank you for the time and effort invested in reviewing our manuscript. Please find our replies in the attached document.
  Kind regards
  Philipp Gasch and co-authors
  
  Citation: https://doi.org/10.5194/egusphere-2023-548-AC3
RC2:
'Comment on egusphere-2023-548', George Emmitt, 11 May 2023
Review comments
Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow-an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems
Gasch, et al. 2023

Readability: Very well written; well organized and thorough.
Significance: Although the multiple fixed beam (each with its own lidar) is not novel for a proposed and simulated space based DWL (e.g. JEMCDL), the investment in another DWL tool for airborne atmospheric and ocean surface research is highly merited. Some advantages of a 5 FIXED beam (each continuous transmitting and receiving) compared with a scanning ADWL sampling in a cycloidal pattern using just a single transmitter/receiver for a certain subset of observational/research goals are obvious.
Methodology: Use of an LES-based airborne Doppler lidar simulation test bed is ideal for isolating sampling related errors of representing the “true” profile of the wind within a target volume in the presence of wind shear (both speed and directional) and turbulence. The attributions of “error” to turbulence or representativeness is very useful in operating and configuring an ADWL as well as processing the LOS retrievals to obtain estimates of the vertical profile of u and v within a dynamically non-homogeneous target volume, in this case the middle layers of an unstable PBL.
I am recommending that this paper be published after minor revisions. The revision I suggest is to acknowledge that SCA1 concept does not represent ADWL configurations currently in use by NASA, NOAA and ONR. SCA1 represents a simple continuous scanning mode suitable for this initial study.
Exceptions taken: Following is a discussion of exceptions taken to the experimental setup which raises issues with all subsequent conclusions.
The evaluation metrics in this paper are wind vector product centric. While this does not invalidate this papers investigation of the merits of FIX5 vs SCA1, a major utility of the ADWL observations is numerical model validation and numerical model Data Assimilation, both of which prefer LOS ADWL retrievals, leaving the full vector wind profile to second tier processing.

If LOS observation density and distribution in terms of along track and cross track directions were used for the basic comparisons, different conclusions could be reached as to which sampling technique serves the modeling community best.
The scanning system (SCA1) does not represent the standard (traditional?) sampling pattern used with the ADWLs used in the studies referenced (Bucci, De Wekker) nor the ADWL used on the NASA DC8 (Turk, Kavaya). The coherent ADWLs being used in the USA in large field campaigns over that last 2 decades use two types of scanning:
Fixed elevation with azimuthal scanning in a step-stare mode using 2 -13 azimuthal programmable stops (NASA’s DAWN and the new AWP which includes a nadir staring option)

ONRs (also used by NOAA) cylindrical side mounted scanner allowing programmable beam pointing routines within a large azimuth/elevation bounded target volume.

The use of a continuous scanning approach (e.g. at 20 degrees/sec) has been replaced with a “step and stare” strategy for many years and for several reasons (lag angle and the desire to eliminate the angular spread in lidar shots being integrated before preforming a spectral analysis).

A better (and more relevant) “reference” SCA1 for this sampling centric study would be the following based upon more than 1000 flight hours of observations using the cylindrical scanner:
Elevation angle from the horizontal: 60 degrees

12 azimuthal stares for 1 second with 30 degree azimuth increments

Slew rate between stares (30 degrees/second)

1 nadir stare (five-10 seconds) in the middle of the 12 stare VAD.

50m range resolution

50 -100m along track averaging.

Had the SCA1 sampling pattern described in 3. above been used for quantifying the advantages (and disadvantages) of the FIX5 system, the following conclusions and expectations might be reversed or at least quantitatively changed.
Line 35: The Goodness of Fit (GOF) value for a 12 look step stare solution for u,v provides a very useful measure of the non-uniform distribution of winds in the retrieval volume. This GOF is used to generate a confidence metric for representativeness. By performing triple pass processing a reasonable description of the non-uniformity can be made…not assumed except for the first pass.

Line 54: It is not clear why the simulation was not performed for an aircraft flying 500-1000 meters above the PBL top since that may be the preferred perspective on the PBL. For the reasons stated elsewhere in the paper, the “saftest” portion of the PBL to use for analysis is the 100-1000meter layer (middle of the PBL). That is understandable, but the horizontal data coverage from 3000m will be different than from 1500m.

Line 94: Are there any disadvantages of the FIX5 vs the SCA1 for PBL research? Would any of the stated advantages of the FIX5 ADWL change if the more relevant scanning ADWL configuration were used?

Line 175: Step and Stare scanning greatly reduces the lag angle losses. This is not an issue for the reported study, though.

Lines 330-365: Throughout this paper there are frequent references to the issue of alignment of the FIX5 scanning telescopes with the aircraft ground track (crabbing) and the wind direction. With the 12 look scanning ADWL (let’s call it SCA2), there are numerous subsets of azimuth look angles that can be used for sector wind vector retrievals, for example, quadrant retrievals. Based upon the 4 wind profiles thus obtained, horizontal gradients and other estimates of non-uniform flow can be deduced and quantified. The presence of PBL jets and directional shear layers does not impact (degrade) the accuracy of the SCA2 profile retrievals as much as might be the case for the FIX5. This point raises issues with all subsequent conclusions.

Lines N/A: This paper does not discuss explicitly the ability to measure vertical velocity. The primary interest is in the impact that very local vertical motions associated with organized structures such as OLEs will have on the calculation of horizontal wind components. However, the DC offset of the 12 point solution along with the 5 – 10 second vertical stare provide insight to the scales of vertical “contamination” of the horizontal wind retrieval. This, at the least, provides a means of attaching “quality” flags to each profile. Without this “over sampling” compared to only 2-4 perspectives, there is much less basis for judging representativeness in actual applications.

In spite of the exceptions and concerns expressed in 5. above, this paper is well written and answers the questions (Lines 94-97) based upon the assumed SCA1 configuration. However, to be relevant to how ADWLs have been scanned for that last 20 years (like ONR’s TODWL and NRL/NOAAs P3DWL with very flexible pointing options) and are being designed for the next generation of high energy ADWL (like NASA’s AWP with a vertical stare option), there is the need to simulate a SCA2 type instrument and ask the questions a slightly different way. 1.) How do the FIX5 and SCA2 serve the modeling community vs the atmospheric processes community? (i.e. LOS as primary product vs wind vector profile?) 2.) Do fewer but more accurate bi and quad perspective profiles over smaller foot prints trump more LOS samples from more (say, 12) perspectives, especially in complex flows?

Regardless of any follow-on simulations, the development of the FIX5 instrument has merit and will not only provide collocated (for no crabbing) bi-perspective simultaneous LOS measures of the winds, but will also use SOTA fiber laser technology for each telescope and , hopefully, a less expensive and thus more available means of making airborne wind profiles for both academic research as well as applications. The increased number of vector profiles per km along a track is certainly attractive.
Citation: https://doi.org/10.5194/egusphere-2023-548-RC2
- AC2: 'Reply on RC2', Philipp Gasch, 14 Jul 2023
  
  Dear Dr. Emmitt,
  thank you for the time and effort invested in reviewing our manuscript. Please find our replies in the attached document.
  Kind regards
  Philipp Gasch and co-authors
  
  Citation: https://doi.org/10.5194/egusphere-2023-548-AC2
AC1: 'Comment on egusphere-2023-548', Philipp Gasch, 14 Jul 2023

Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2023-548-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Philipp Gasch on behalf of the Authors (14 Jul 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Jul 2023) by Ad Stoffelen

RR by Benjamin Witschas (03 Aug 2023)

RR by Anonymous Referee #2 (29 Aug 2023)

Suggestions for revision or reasons for rejection

Comments from a second review of:
Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow-an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems
Gasch, et al. 2023

The authors responded to my comments and have made a better case for a FIX5 ADWL being added to our airborne options for remote sensing of the atmospheric PBL. As with any reading of a manuscript multiple times there still remain a few questions. Since this paper is, in large part, lidar technology neutral and is primarily a sampling sensitivity study, the issues of PRF, EAP and backscatter weighting in sample integration required (or desired) are set aside (Line 235). In practice, it may be neither desirable nor necessary to fly within the PBL or even just above the PBL. The trapezoidal “truth volume” is defined by the flight altitude and scanning geometry. In practice, a much more capable (EAP) lidar is required to deal with the R*R losses by flying higher and thus requiring significant hardware and optical resources to have multiple perspectives illuminated with individual lasers. This reality is further in force when considering going to space. I only mention this since I do not think these simulation results should be mistaken for a general conclusion regarding scanning a high EAP lidar vs using multiple lower EAP lidars in a fixed configuration.

That aside, I have 2 points that the authors may want to address in their final submission:
1. What was the assumed PRF for the SNS13 scan type? 10Hz as appears to be the case for the FIX5 in Table 2? If so, then the spacing between samples in the along track direction during each of the 12 stares would be ~10m flying at 100m/s. Right?
2. The quality metric discussion (lines 289 – 305) still contains several possibly confusing concepts in spite of the authors’ efforts to explain the difference between MAE, MAErep and MAEturb.
a. As I understand it, it takes the SNS13 13 seconds to complete a scan with 1 second dwells that will then be processed into a vector wind profile with 30m vertical resolution. The difference between the two computed horizontal wind components (w assumed zero and thus an error source) at each vertical level and the trapezoidal “beam truth” at those same levels contributes to the MAE expression (Eq 1). “N” is the number of complete profiles achieved while “flying” the 8 transects shown in Figure 1 which would yield N ~ 144.
b. It appears that the 1 second dwell used by SNS13 in practice is replaced (Line 245) with distance integrations between 60 and 1800m. Is this integration per LOS perspective? Or is it the distance the plane flies before generating another vector wind profile? If so, then the number and location of SNS13 radials used to represent the LES domain samples will vary while only the along track length of the FIX5 integration lines will vary.
c. I can see how the FIX5 system can be programmed to generate a complete profile every so many meters since all 5 lidars are in constant operation.
d. With the frozen turbulence assumption, how is it that a system that makes 12 one second (100m sampling lines) plus 1 sec nadir dwell, has a larger representativeness error than a system with (5)13 second lines….unless there are few to no organized circulations(e.g. OLEs or plumes) on scales of order 1km in the LES simulation. In that case, the advantage seems to be all in having 5 lidars operating simultaneously (5*13 = 65 vs. 12) or a factor of 5 used to beat down the MAE due to random turbulence on the scales of 100m and less.

Hide

ED: Publish subject to minor revisions (review by editor) (30 Aug 2023) by Ad Stoffelen

AR by Philipp Gasch on behalf of the Authors (09 Sep 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (17 Sep 2023) by Ad Stoffelen

AR by Philipp Gasch on behalf of the Authors (05 Oct 2023)

Journal article(s) based on this preprint

16 Nov 2023

Advancing airborne Doppler lidar wind profiling in turbulent boundary layer flow – an LES-based optimization of traditional scanning-beam versus novel fixed-beam measurement systems

Philipp Gasch, James Kasic, Oliver Maas, and Zhien Wang

Atmos. Meas. Tech., 16, 5495–5523, https://doi.org/10.5194/amt-16-5495-2023,https://doi.org/10.5194/amt-16-5495-2023, 2023

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Philipp Gasch et al.

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This manuscript rethinks airborne wind measurements and investigates a new design for airborne Doppler lidar systems. Recent advances in lidar technology allow the use of multiple lidar systems with fixed viewing directions instead of a single lidar attached to a scanner. Our simulation results show that the proposed new design offers great potential for both higher accuracy and higher resolution airborne wind measurements.


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