Review for Detection of Multi-Modal Doppler Spectra. Part 1: Establishing

Characteristic Signals in Radar Moment Data by Wugofski et al.

This study develops a method for using radar moment data to determine periods with multi-modal Doppler spectra signatures. This issue of detecting multi-modal spectra is difficult because the signature can be rare and is often buried in the large amount of spectra data available throughout various cases.  In their analysis of three cases from three separate sites, the authors find that multimodality appears to be associated with large average values of spectrum width compared to control layers and small standard deviations of mean Doppler velocity compared to turbulent layers.  Using these moment metrics and various other filters, the authors then create a criteria-based methodology for flagging periods of potential multi-modal spectra. Part 2 of this two-part paper will apparently use this method with a large number of cases.

This study seems like an important step in curating and understanding datasets with multi-modal spectra. I’m a little unsure whether three short time periods from three cases is enough to create appropriate criteria for detecting multi-modality. However, I suppose that the merits of this methodology will be more apparent in part 2 when the authors apply their methodology to a larger dataset where they can independently verify their results. However, I do have one major concern that I think the authors should address in part 1. Throughout the study, there is no mention of quantifying the degree of multi-modality. It seems like the authors use a relatively subjective reflectivity threshold of 5 dB between local peaks (with local peaks determined computationally?) to determine if there is or is not multi-modality in the Doppler spectra at various heights. I think that the authors really should perform a more objective and quantitative analysis here. If the moment statistics that the authors use actually are representative of multi-modal Doppler spectra in general, then I would expect that these statistics should correlate directly with the amount and degree of multi-modality present in the spectra data according to: MEAN(SW)/STD(MDV), perhaps normalized in some way. This quantity could potentially be used as a proxy for degree or strength of multi-modality or the probability of multi-modality. To me, the lack of objectivity of determining multi-modality from the data is concerning because the authors could be potentially missing periods of developing or subtle multi-modality that their methodology would not flag. If the algorithm misses a particular multi-modal event, then it would be good for the authors to illustrate why the event was missed. Also, because the authors only consider three cases here, it seems to me that it would be quite simple and fast to objectively determine the degree of multi-modality as a metric itself. For part 2, the authors will need to be very careful to identify periods/heights that are correctly labeled as either multi-modal or unimodal. In order for the authors to address these concerns, I recommend major revisions. 

Major Concerns:

• The authors should come up with a better way of quantifying multi-modal spectra. The authors could easily develop quantitative metrics for degree of multi-modality using a variety of techniques such as fitting the spectra at each height with a Gaussian Mixture model and by using the Gaussian mixture components to derive relevant statistical measures of multi-modality. My concern is that the authors are being a bit too selective about what they consider to be multi-modal and unimodal. An objective metric that is more independent of researcher biases here would be more informative and would provide a fairer representation of multi-modal/unimodality inherent in the spectra data.
• What is the expected miss rate and false positive rate of the detection algorithm? From Figure 7 it seems like there is a missed multi-modal spectra as pointed out in the main text. However, because the authors only look at three cases, it’s hard to actually evaluate the performance of the algorithm. The authors will need to be very careful in part 2 to objectively evaluate the algorithm’s performance and to properly quantify the hits, misses, false alarms, and nulls using a larger dataset. In my opinion, this really necessitates the ability for the authors to provide a much more objective set of criteria for determining multi-modality or uni-modality. 

Minor Concerns:

• The authors state in the conclusions that: “The design of the criteria and methodology is targeted at reducing false positives; it is likely that the use of these criteria may miss the detection of some multi-modal spectra (cf. Fig. 7).” This seems backwards to me. If anything, wouldn’t you want to reduce missed events? The reasoning is that you can always revisit the spectra data for flagged periods to verify multi-modality after the fact. Therefore, false positives, unless there are many, aren’t that big of a deal whereas missed multi-modal spectrums represent a serious deficiency in your algorithm as possibly suggested by Figure 7.
• I was disappointed to see a lack of complimentary data from other instruments present at these three sites. The SBU site in particular has a wealth of potentially useful ground instruments like Pluvio, Parsivel, and the MASC imager. These instruments could be helpful in part 1 of this paper to provide some hints to readers as to the actual mechanisms behind periods of multi-modal spectra. I don’t necessarily think that the authors have to include such data in their analysis (perhaps that will be in part 2?). However, I do think that the authors should at least consider including one or two figures that use these additional instruments to help provide context into the potential sources of the multi-modality for each of the cases. That is, do MASC images of the particles at the ground support hypotheses of rime splintering, ice-ice collisions, or freezing drop fragmentation near regimes of multi-modal Doppler spectra? 

Suggestions/Typos/etc.: 

• I didn’t find any typographical errors. The authors did a good job writing the preprint draft.

In this study the authors develop an algorithm that allows to detect multi-modal Doppler spectra by only considering MDV and SW, thus reducing the complexity of a Doppler spectrum analysis to “simple” moment data. As both multi-modalities and turbulence affect MDV and SW, one needs to find a way to distinguish between the two. For this, the authors manually selected mutli-modal layers in a few cases and recognised that the standard deviation of the MDV and mean SW in a time window of approx. 2 minutes is sufficient to distinguish between multi-modalities and turbulence. A threshold of mean SW>0.19m/s and standard deviation of MDV<0.1m/s are found as criteria to detect multi-modalities. In a second part of this study, the algorithm will be applied to several other cases, testing its robustness.

The manuscript is well written and easily understandable. While the idea proposed in this study is interesting and likely a valid method for determining multi-modalities in Doppler spectra by only considering a few moments, I am not convinced that the initial selection of the multi-modalities is statistically sound. Further, I do not understand the split into part 1 and 2, as I think there is not a lot more information needed in order to evaluate the proposed technique (see major comment below). I therefore suggest major revisions.

Major comment:

In my opinion the detection of a second mode and turbulence is to subjective. You are likely missing a lot of cases due to your rather arbitrary requirement of a 5dB reduction of spectral Ze and a minimum spectral Ze of -20dB for a multi-modality. Especially because your entire technique is based on those few subjective cases. Also, I am not sure if the number of cases you used to establish the MDV and SW thresholds are sufficient. It could very well be that these few cases are special, especially since you had such subjective selection criteria. To my knowledge, there are a few Doppler spectra peak detection algorithms openly available. For example, I have recently used the peako-peaktree toolset (https://amt.copernicus.org/articles/17/6547/2024/) and found it to be easily applicable to Doppler spectra of several different radars. In my opinion, detecting peaks in such an automatic, objective way would greatly benefit your study, especially since then you can use much more data to make your MDV, SW clustering and threshold definition more robust. Similar algorithms are available for turbulence detection, for example eddy dissipation rate retrievals. I would suggest to first apply a peak detection algorithm, filter for cases where multiple peaks were obtained (also perhaps were the feature is consistent enough to be caused by microphysics) and then do the clustering in sigma(MDV) mean(SW) space. If you apply a peak detection algorithm, also your validation of the study is much easier, as you do not need to manually go through a lot of cases in order to visually see if your algorithm is valid. This is also why I think you do not need 2 papers to describe the method. Using a peak detection algorithm already reduces this study to the clustering in sigma(MDV) mean(SW) space, allowing you to cover the validation of the method in the same paper.

Minor comments:

Line 82: you are saying that in order to identify microphysical processes in the radar you require additional information such as in-situ obs, or other radar obs. However, neither a radar of in-situ will ever be able to observe processes. The only thing we can observe with a radar is the effect a process has on several aspects of the observed particle distribution. Maybe rephrase that sentence to make it a less strong statement

About the introduction: I am missing a paragraph about peak detection algorithms, since you are trying map your number of peaks (single or multi-modal) to radar moments, in my opinion it is important to mention and discuss those peak detection algorithms

Table 1: you are specifying that you are only averaging the KASPR data over 1 second. However, in my experience to reduce noise in the spectra it is necessary to average over at least 3 seconds, better close to 4 seconds. Can you comment on the integration time used?

Figure 2: in the title you are using SBU for stony brook university, however, in the text you refer to it as SBRO, perhaps it would be good to change that to be consistent

Figure 3: the time display is a bit confusing (i.e. 18.8 as time [UTC] perhaps it would be better to go to HH:MM format)

Line 244: how do you know that SW>0.2m/s is enhanced?

Line 297: you say you are using 499 data points. How many uncorrelated layers of turbulence and multi-modalities are these?

Line 338: you are saying you are restricting your cases to MDV←2m/s. From your Figure 6 I am taking that you have LDR available correct? Perhaps it would be beneficial to first do a melting layer detection using LDR (LDR strongly increases at the edge of the ML, allowing a detection of the ML), and then afterwards apply your multi-modality detection algorithm to all data above the ML. Then you would also be able to use the algorithm on cases with graupel