|The manuscript has improved significantly as a result of the authors responses to the referee comments. In particular the analysis has improved and details like density plots (2D histograms) in Figs 5, 10, as well as using mean difference as measure for comparison.|
Some of the modifications include major changes and editing. In the following I have several remarks, comments, and questions regarding some of the changes. Please have a look and change and improve/clarify the manuscript.
Unless otherwise noted, i refer to Fig. and Lines as they are numbered in the version2 manuscript.
RE Major 3) (Precision)
L404-405: "As we established above, the 2DVD is the least affected by motion blur and, as a result, has the most accurate area measurements, although the large
pixel size may impact the precision of those measurements."
I still beleive that "precision" is the wrong term to use here.
Larger pixels ultimately affect accuracy of determined dimensions (picel size will contribute to the error and this contribution will scale with the pixel size). Furthermore, the negative effects of pixelation will be intensified with larger pixels (with consequences on perimeter stretching, see below about "cap").
RE Major 4) Motion blur
Added horizontal blur at 10m/s?
Check new Fig 4 and discussion.
MASC 33.5um one pixel; 40us exposure
=> at 1m/s 40um blurred "streak" for any arbitrary point on particle
This blur streak is linearly (if motion along one pixel axis) on average on 40um/(33.5um/pixel)= 1.19 pixels.
This expected value is equal to that found by statistical analysis of 100000 random particles.
New L311-312: "would contribute to at least two pixels and possibly three, although our statistical analysis indicates an average 1.19 extra particle pixels are added."
Regarding contributions to three pixels, it may be worth commenting: ...three: in that case the contributions to two of these pixels is minor.
Mention that 1.19 is exactly as expected!
Corresponds to the average extra pixels you show in brackets for 4m/s and 10m/s
L315: "For a particle moving at 10 m s−1..."
You seem to mean "For a particle moving horizontally at 10 m s−1..."
Note, the 400um blurred line is true for a vertically or horizontally moving particle (for normal camera orientation), not for any other oblique angle (i.e. the most likely motion). Nevertheless, I would not change the analysis by including oblique angles (would make it unnecessarily complicated).
Is motion blur in horizontal and vertical directions treated separately for PIP (L319-321)?
L353-356: "Since the image compression only impacts vertical motion blurring, the 10 m s−1 particle motion is not included in Fig. 3, but were a snowflake
to fall at such an extreme speed, motion blurring would, on average, add 3.80 pixels after accounting for the effects of image compression."
"the 10 m s−1 particle motion" => "the horizontal 10 m s−1 particle motion"
"add 3.80 pixels after accounting for ..." => "add 3.80 pixels after accounting for ... and X pixels without compression (for horizontal motion)"
Fig 3 caption: "The darker blue shading indicates the area covered by the actual particle at the end of the image exposure period" => "... the start of the image exposure period"?
L 351, PIP: for suspended particles an average of 1.01 pixels added??
The 1.01 have not been mentioned or explained. (Maybe thsi refers to L345-346 ("the image compression can add between zero and two additional pixels to the height of a suspended particle.")
Comment: Even without pixel compression: a four pixel particle may result in a five pixel particle on picture (50% on lowest and 50% on highest pixel.
L366-372: The area error also depends on particle orientaion. A needle or column is more affected if oriented horizontally.
RE Major 5) "Cap" Major 6) gap
I think you should discuss in appropriate places the problematic definitions of the PIP ellipse and rectangle and the consequences of these: long dimension of ellipse can be far longer than maximum dimension; pixelation leads to lowered aspect ratio for elipse; many particles for which rectangle fit is not possible. These problems explain the "cap" in elipse fitted aspect ratios and the apparent gap (in rectangle-fitted aspect ratios).
L621 "PIP suffers from the issue that the shape-fitting routines do not perform well on precipitation particles..."Where was this discussed in paper?
Does it refer to the "cap"/pixelation discussion (see below)? Related to my comments on problematic definitions of PIP ellipse and rectangle (see above)? In any case, it is unclear to me why you talk about precipitaion particles here. Do you mean ice/snow?
L623: "PIP variables"?
Please explain what you mean with "cap" when you first use this expression!
New L432-434: "In terms of aspect ratios, the PIP-fitted rectangle aspect ratio (Fig. 5h) does not suffer from the artificial cap that was present with the PIP-fitted ellipse aspect ratio (Fig. 5g) and much of the reduction in the mean absolute difference (0.254 versus 0.184) is likely due to this change."
L432: Cap? Fig 5h shows rectangle which does not have the "cap", refer to Fig 5g) instead!
L 478: refer to Fig 6a): I would refer to Fig 5g) and Fig 6a).
L434: What is "this change"?
L434-435 "That said, the PIP-fitted rectangle aspect ratio still has a tendency to greatly underestimate the aspect
ratio relative to the tensor-fitted ellipse aspect ratio (Fig. 6b)"
Cannot be seen from Fig 6b (alone) => Refer to Fig 5h!
L436-438: "It should be noted, however, that the PIP-fitted rectangle aspect ratio does capture the increase in aspect ratio associated with the periods of lump graupel precipitation on 8 March around 0900 UTC and after 1400 UTC."
The text suggests that you are looking at Fig 6b. It is not obvious at first waht you are referring to exactly: the higher density at higher aspect ratios. But then you should say the same also for aspect ratios from PIP-fitted ellipses and refer to both Fig.s 6a and 6b).
L447-454 Discussion of Fig 7: example particles.
You should state that you now refer to "MASC-fitted and the tensor-fitted ellipses" as correct or reference.
Panel c and d:
L449-452 "Particles (a) and (b) are both likely some type of aggregate frozen precipitation based on their odd shapes.
450 Based on the relatively circular shapes of the remaining two particles, relatively high fall speeds (black line, Fig. 1), subfreezing near-surface temperatures (red line, Fig. 1), and the lack of an above freezing temperature layer in a nearby thermodynamic sounding (not shown), particles (c) and (d) are likely both examples of lump graupel."
L460-463: "For a particle with complicated outlines, such as particle (b), the particle perimeter is far greater than the perimeter of an ellipse or rectangle of either equal area or equal dimensions."
This is wrong: the definition of the fitted ellipse is that it has the same perimeter and area, so you cannot say that the perimeter of the particle is far greater than that of the ellipse. Further, it is unclear what "equal dimensions" refer to.
L464: "perfect ellipse"??
"The relationship between excess perimeter for a given area, relative to a perfect ellipse..."
It is the excess with respect to a circle (not "perfect ellipse".
L475-476: "this missing information is, in fact, the core issue with both the PIP ellipse and rectangle fits."
This is one issue, i.e. the definition of ellipse and rectangular fits. The other issue is pixelation.
circle should result in perimeter stretching of 1 and aspect ratio of 1, but it does not due to pixelation.
L489-490: "...very small deviations of the particle 490 edge from a perfect circle as well as by the inability to perfectly represent a circle using square pixels (i.e., pixelation effects)."
Cannot separate these effects. However, pixelation alone explains the "cap" regardless if there are any small natural dieviations (that are not resolved due to the same pixelation.
Take a circle and determine the measurements after emulating PIP images. Then check the perimetr stretching factor. You will likely get a perimeter stretching factor larger than 1 as a consequence of pixelation and determination of the perimeter of the pixelized particle image. The pixelation itself creates a variation of perimeter location from any defined centre point around the radius of the equivalent circle.
Cfr. Fig. 9.
Rectangle fit: perimeter stretching below 1.128: apparently the algorithm does a compromise and accepts a square with too long perimeter and too small area (and aspect ratio of 1).
The rectangle is less susceptible to pixelation: the "cap" therefore does not exist.
The lowest perimeter stretching factor (~1.05, see below) is below the limit where rectangular fits are possible. Therefore, no cap.
Discussion in L499-507 can be simplified, it is all due to the definition of rectangle fit.
Fig 9 shows again that rectangle fit is not well defined: for half the particles the fit is not possible and the PIP algorithm assigns an aspect ratio of 1.
There is nothing special about the aspect ratio 0.6. Instead you could look at tha maximum aspect ratio for PIP-fitted ellipse that you found (~0.65) and relate that to the minimum stretching factor found for PIP (~1.05).
Fig 7 improved in New Fig8; check discussion!
Check discussion new Fig 8 and 9 (tracked changes new L 464-507: perimeter stretching)!
I appreciate the new sentence about PIP perimeter calculation (Lines 127 – 129), but it is not clear:
1) boundary points are located at the corners of the pixels that make up the particle perimeter
2) subsampling the boundary points to produce a smoother representation of the perimeter
What does subsampling mean?? How does smoothening work?
This is crucial: the smoothening and perimeter calculation affects the perimeter stretching factor and enlarges it more or less (the pixelation effect).
L 299: "...particle moving at ... 10 m/s (in the along-particle direction)"
Is the "along-particle direction" the direction defined by the particle motion? Then that is not needed. In any way "along-particle direction" is confusing.
I think "movement speed" and "motion speed" sound somewhat odd. I would use "speed" and/or "motiion".
Further, "horizontal movement of particles" => "horizontal movement of particles" sounds better.
measurements made using
L467 duplicate "area... and area"
L601: "...although the relatively low resolution may impact the precision of those measurements (section 5.1)."
Also here, I would make clear what you mean with "resolution":
"...although the relatively low resolution (large pixel size) may impact the precision of those measurements (section 5.1)."
See also the comments on precision above.
amt-2021-427: Helms et al. A Comparative Evaluation of Snowflake Particle Size and Shape Estimation Techniques used by the Precipitation Imaging Package (PIP), Multi-Angle Snowflake Camera (MASC), and Two-Dimensional Video Disdrometer (2DVD)
In short: This study compares the different algorithms behind the measurement techniques of three digital video disdrometers: the Precipitation Imaging Package (PIP), the Multi-Angle Snowflake Camera (MASC), and the Two-Dimensional Video Disdrometer (2DVD) in observing snowflakes. The focus is on defining the uncertainties in the defined area influencing the equivalent diameter, and the aspect ratio. The authors quantify the motion blurring, in the case of PIP also the image compression, the shape-fitting measurements, and in the case of 2DVD, the estimate of the bounding box measurement when particle horizontal motion needs to be adjusted with an unskewing algorithm.
The topic is interesting and relevant for surface observations and the development of retrieval methods in global monitoring of snowfall. The study has novelty in the way it examines the measurement algorithms internal to the instruments, which typically are not transparent to the end-user of the data. The theory is clearly outlined with illustrative examples, and the conclusions are well-supported and valid. The manuscript is well written and provides a clear storyline, however, at least in my opinion, it leaves the reader questioning what is the magnitude/importance of these studied uncertainties in respect to the other uncertainties e.g. particles out of focus or only a fraction of particles observed, wind effects to particle fall velocity, miss-matching of particles, partially illuminated measurement space or limited observations of particles from only one plane projection, these are referred in several publications prior to this one. I would like to see more discussion on this topic and references to other related studies. My recommendation is to publish the paper after addressing this concern and some small remarks mentioned below.
Line 11: “… PIP or 2DVD which provide similar precision once the effects of the PIP image compression algorithm are taken into account.” This sentence is somehow unclearly connected to the previous statement in lines 10-11 and is not clear for a reader who just reads the abstract. Please rephase.
Line 41: “There are numerous examples of studies which rely heavily on either of these measures of particle size.” The statement “numerous examples” follows only by two references, leaving for example relevant fields such as the snow model or satellite retrieval development unmentioned. I would like to see a broader scan of the research field, just mentioning applications and example references would be enough.
Line 64: “separate from the snowflake size,” This is unclear. I don’t understand how these mentioned studies are concerning only aspect ratio separate from size. Please rephrase.
Line 87: Altitude of the site?
Line 89-91: I would like to see more data of this event to support the assumptions of aggregation and lump graupels, e.g., time series of temperature, PSD, and mean fall velocity.
Line 336-8: “Even with a very fast fall speed of 4 m/s, the overestimation of the equivalent diameter for very large circular particles (diameter ~ 10 mm) is approximately two orders of magnitude or smaller than the actual equivalent diameter.” Two orders of magnitude? This is not clear to me.
Line 339: “perfectly circular”. Why assumed the particle to be circular, though written in lines 68-69 “aspect ratio is frequently prescribed, often with a mean value of 0.6 assumed (e.g., Matrosov et al., 2005)” and then without quantification stated that for the oblate particles “the relative (and absolute) effects of motion blurring on the area and equivalent diameter measurements will also grow”. Please justify and elaborate.
Figure 4: Could you add the number of analyzed particles and a density plot would add information instead of a scatter plot.
Figure 6. Just to add more information about the particle habit, could the approx. fall velocity be added to the corner of the image? The colored fitted shapes, could the line be slightly thinner or the image larger, it is now hard to see the lines in respect to the shaded image, they are all on top of each other.
Lines 404-406. As PIP is only seeing a plane projection of the particle, but here the particle is referred to as an ellipsoid, it is confusing whether in this perimeter stretching factor analysis, the computations are performed in 3D with ellipsoids and is it then assumed the same axes ratio in both directions or is it performed in the 2D projection. Could you please clarify this?
Paragraphs 408 – 445: I understood that this section provides explanations why the ellipse-fit in PIP has an arbitrary upper threshold close to 0.6, and why with the rectangular fit in PIP, there is a gap in aspect ratio between 0.9 and 1.0. However, it was not always clear, which “gap” the authors were pointing at. I would suggest that you would refer in the text (when addressing for the first time) to the image, where the “gap” is shown. E.g. in lines 436-438, I assume here the authors are referring to Figure 5b?
Figure 9. Same as Figure 4. It would be nice to see the number of analyzed particles and then rather a density plot than a scatter plot.
Lines 526-527: “and, as a result, the maximum dimension and aspect ratio measurements are unreliable; however, the PIP variables other than the ellipse and rectangle dimensions appear to be reliable” I assume here it is referred that the PIP-fitted maximum dimension of an ellipse is unreliable and not that the observed maximum dimension is unreliable. Please clarify.
Lines 528-530: “As the present study has demonstrated, the PIP imagery can be reprocessed and reliable measurements of maximum dimension (the previous comment) and aspect ratio can be made via the application of an alternative ellipse-fitting algorithm, such as the MASC or tensor-based algorithms.” In the manuscript, it was described that the AVI file contains only the first 2000 frames from the 10 - minute section, and with 380 frames per second, this translates to 5.3 seconds of data. It is unclear that can an end-user reprocess the whole data volume or just the sample frames in the AVI-files? Could this be elaborated?