Reply on RC1

My biggest concern with this work is that it seems incomplete. After a description of the instrument, the data example is lacking. There is hardly any data, and all of that data is shown as time integrated plots. The integration times are very long (10-15 min -well in excess of variability in atmospheric structure) and separated considerably in time. This needs some explanation. Also the authors should really be showing time resolved plots which tend to be more revealing, allow the reader to see both the vertical and temporal structure (see, for example, the exceptional plots in figure 8 of Kokhanenko 2020, 10.5194/amt-13-1113-2020).

I don't understand why there is so little data. What is the barrier to running this instrument continuously (a very important question for an instrument paper)? The only thing we are shown is a dust layer with no observable orientation, so we have no assurance that the instrument can observe off diagonal elements. The authors could operate the instrument to observe rain, which has very strong orientation signatures (see Hayman 201410.1364. That would at least provide some coverage of the measurement space. Demonstration would not be fully complete given the intended application, but it may be asking too much to demand the authors to show polarization properties of oriented dust.

Reply:
Although we agree with the reviewer that it would be better to show measurements of rain orientation, we haven't managed to acquire them by now, due to the technical challenges these measurements entail, mainly due to the analog detection of our signals, which are saturated from overlaying clouds and/or the rain. Although this is not impossible to cope, it requires extensive experimentation, which we think it is out of the scope of this paper.
Another issue is that we had to go through repairs for the lasers (once due to laser malfunction and once due to improper operation), which delayed our field measurements.
We decided that in order to avoid confusion, we include a dust-free case to the "First measurements" section, which shows no orientation (as expected). We use these measurements to show that the instrument works as expected and provides "no orientation" flags, for dust-free atmospheres. The measurements used were acquired at viewing angle of 80 o off-zenith, to highlight the scanning capabilities of the system. Moreover, we provide the Rayleigh fit of the lidar signals, as a quality standard of our measurements.
We followed the advice of the reviewer and we present our measurements with timeresolved plots.

RC:
Given that this is an instrument paper, I would think operability is part of the design and performance criteria. Is this somehow connected to the very small set of observation examples?

Reply:
We have included some more information about the operability of the system in Section 2, which we renamed "Overview of the lidar components and operation" We added the following paragraph in line 68: " Due to the analog operation at 1064 nm, the time range of the measurements is restricted by the dark signal changes, which are mainly affected by the change of the (internal) system temperature. The investigation of the acceptable temperature changes, and corresponding acceptable time ranges during which the dark signal does not change considerably, is a work in progress, with first results to set the acceptable temperature changes to ±2 o C, which require a new dark measurement every 0.5 hour during summertime, or every 2 hours during wintertime. Also, due to the high power of the lasers there is no eye safety classification for the lidar, although the beam is expanded 5 times. This restricts the operation of the system when there are no aircrafts at the airspace of the measurements."

RC:
I am somewhat concerned that the authors seem to have decided that oriented dust is a foregone conclusion. The published work on this phenomena appears to be circumstantial (see specific comment about Line 13), so I would recommend the authors adopt a more cautious tone on the subject.

Reply:
There has been a revision on this assertion. We keep a more cautious tone throughout the manuscript and we added in the last section (8. Overview and future perspectives) the following: "Currently, the only indication of particle orientation comes from astronomical polarimetry measurements of dichroic extinction, which though cannot provide a strong proof for the phenomenon, due to their small number."

RC:
It is notable that there is no discussion of uncertainty in this work. This seems like a pretty important aspect of the instrument design.

Reply:
The quantification of the uncertainty is a work in progress, which entails e.g. an extensive investigation of the effects of the analog signal distortions.
In the revised Section 7 ("First measurements") we included a first estimation of the uncertainty level of the measured orientation flags, by providing the standard deviation of the measurements with height which is quantified to be ±2-10% for the averaged signals. Although this is not an optimum quantification of the uncertainty of the measurements (e.g. since it may include real variability), this is a preliminary estimation and more extensive testing and analysis will follow. Moreover, we included the following in the last section (8. Overview and perspectives): " Moreover, an extended analysis should be performed to characterize the analog signal distortions, which are expected to affect the quality of the signals. Specifically, we will investigate their dependence on temperature and signal strength, and their change with height range and time. A very preliminary indication of their effect is shown in the first measurements presented herein, with a standard deviation of the measured orientation flags of 2−10%."

RC:
Line 13: "Dust particles have non-spherical irregular shapes and they have been reported to present preferential orientation (Ulanowski et al., 2007)." It's worth noting that the analysis presented by Ulanowski is circumstantial. Dichroism from starlight was observed and those authors, lacking another explanation, assert that it must be caused by vertically oriented dust. This is not scientifically rigorous proof of oriented dust. The limits of imagination do not constitute scientific proof. (Remember when, lacking any other explanation, a neutrino traveled faster than the speed of light at CERN?).
The correct assertion is that dichroism has been observed in starlight when Saharan dust was present and that has led to the hypothesis that Saharan dust could have a preferential orientation. If the conclusions from Ulanowski 2007 et al are already deemed sufficient and correct, why build a lidar to look at this? Clearly there needs to be more, different observations.

Reply:
We changed line 18 accordingly: "Specifically, the only indication of dust orientation in the Earth's atmosphere comes from astronomical polarimetry measurements of dichroic extinction during a dust event at the Canary islands (Ulanowski et al., 2007)..." Moreover, we added in the last section the following: "Currently, the only indication of particle orientation comes from astronomical polarimetry measurements of dichroic extinction, which though cannot provide a strong proof for the phenomenon, due to their small number. "

RC:
Line 45: The authors note that they are using high power lasers. What is the eye safety classification of the lidar system and does that affect how and when often the instrument can be run?

Reply:
In line 45 we added that we use Class 4 lasers.
Moreover, we added in line 68: "Due to the high power of the lasers there is no eye safety classification for the lidar, although the beam is expanded 5 times. This restricts the operation of the system when there are no aircrafts at the airspace of the measurements."

RC:
Line 82: The description of the telescope system does not mention a field stop. What is the angular acceptance of the receiver?

Reply:
The telescope has a field stop with a 2mm diameter. The telescope has a focal length of 1000 mm. This means that the field of view is 2 mrads ((FS_diameter)/focal_length). We changed line 82 accordingly: "The telescopes are of Dall-Kirkham type, with an aperture of 200 mm, focal length of 1000mm (F#5), field stop with diameter of 2mm and a field of view of 2mrads."

RC:
Line 86: "The signals are recorded by two cooled Avalanche PhotoDiodes (APDs) at each detection unit," What mode are the APDs operating in? Analog or geiger? How are signals acquired and stored? Are photon counts converted to a histogram, and if so at what time and range resolution? Are analog signals digitized with A/Ds and at what sample rate and what is the analog bandwidth of the digitizer?

Reply:
We added in line 86: "We operate the APDs in Analog mode (not geiger). Signals from APD are pre-amplified and digitized by an 16 bit A/D with a sampling rate of 40 MHz and bandwidth of DC to 20 MHz. After digitization, the signals are stored as mVolts at the hard disk of the embedded computer."