Publisher’s note: a supplement was added to this comment on 5 April 2023.

Please find my review in the attached .pdf file. 

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.

1. 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.

1. 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:
   1. 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)
   2. ONRs (also used by NOAA) cylindrical side mounted scanner allowing programmable beam pointing routines within a large azimuth/elevation bounded target volume.
2. 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).
3. 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:
   1. Elevation angle from the horizontal: 60 degrees
   2. 12 azimuthal stares for 1 second with 30 degree azimuth increments
   3. Slew rate between stares (30 degrees/second)
   4. 1 nadir stare (five-10 seconds) in the middle of the 12 stare VAD.
   5. 50m range resolution
   6. 50 -100m along track averaging.
4. 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.
   1. 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.
   2. 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.
   3. 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?
   4. Line 175: Step and Stare scanning greatly reduces the lag angle losses. This is not an issue for the reported study, though.
   5. 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.
   6. 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.

1. 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.