Evolution of Synoptic Systems Associated with Lake-Effect Snow Events over Northwestern Pennsylvania

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of: “Evolution of Synoptic Systems Associated with Lake-Effect 2 Snow Events Over Northwestern Pennsylvania” by Wiley and Elcek (2024)

 

Journal: MDPI Meteorology

 This study investigates the synoptic conditions associated with lake-effect snow (LES) over northwestern Pennsylvania with a focus on classifying cases based on the tracks of cyclones.  Synoptic composites were constructed using the NARR.  Bootstrapped confidence intervals were computed for each cyclone track. Results showed that synoptic composites of all LES cases exhibited a common synoptic pattern. The study revealed that different tracks varied depending upon the form of enhanced turbulent mixing, synoptic vertical forcing, lapse rates and heat and moisture fluxes.  Minimal differences in LES impact were found between the cyclone types. 

 

Background: The synoptic conditions associated with LES events (both along- and cross-major axes of the lakes) were established 40 years ago and have been long used by operational forecasters.  The authors state that no effort has been made to assess if differences in the evolution of these synoptic regimes may have an influence on LES characteristics, thus motivating the primary objective of this study: “to perform a climatology of the large-scale conditions associated with LES with a focus on the temporal evolution of the synoptic regime.”  Other factors known to be influential in understanding LES events, such as the length of the fetch of air over the relatively warm lake waters, differential surface drag due to terrain generally sloping upward away from the shoreline, or mesoscale phenomena such as lake vortices, were not examined in this paper.  The latter factors are particularly noteworthy, since the authors define an LES event as “convective snow bands that occur in the lee of large bodies of water when relatively cold air flows over warm water.”  In other words, their focus is not on the nature of the bands while over the lakes, but only over the land (NW PA entirely).

 

This study assigned cyclones to one of four primary tracks: Alberta Clippers (ACs), Colorado Cyclones (COs), Nor’easters (NEs), and Great Lakes Low (GLs).  Their upper-level composite results showed the following:

AC events: A low 500 mb geopotential height anomaly centered over the northern Hudson Bay along with a primarily zonal structure over the Great Lakes Basin and a subtle shortwave trough located upstream that amplified during the event leading to increased PVA, plus strong cold air advection resulting in the some of the largest values of “Lake (thermodynamic) Index.”

CO events: similar in most respects to the AC events, the exception being that the initial upper-level trough was located over the Rocky Mountains region.

NE events: These situations were significantly different, in that the initial broad 500 mb trough was observed over eastern Canada and a shortwave embedded within the longwave trough was present with an associated local surface cyclone located over the southeast United States that rapidly intensified as it progressed northeastwards.  The Lake Index was the lowest of all the cyclone types, but the dynamics the strongest.

GL events: The NARR composites featured similar spatial and intensity patterns observed with the Alberta Clipper composites, which is understandable given similarities in their cyclogenesis regions, but the Lake Index was the strongest of all cyclone types.

 

In summary, the authors found the following to be supported results: “A conventional synoptic setup and evolution for the Great Lakes region existed across all observed LES cases”, namely an upper-level low trough upstream of the Great Lakes basin that propagates east and intensifies throughout its progression, resulting in more upper-level cyclonic vorticity and subsequent quasigeostrophic (QG) vertical forcing (though the latter two dynamical impacts were never shown in any quantitative way in this paper). The authors should be a little clearer about that.  These findings are by no means startling or unexpected.  Likewise, though they seem supported by the bootstrapping statistical analysis of differences between such variables as duration, snowfall, winds, etc., the magnitude of those differences is quite small and not terribly interesting.   So, the contribution to basic understanding of LES events resulting from this study are by no means transformative.

Author Response

Comment 1: In summary, the authors found the following to be supported results: “A conventional synoptic setup and evolution for the Great Lakes region existed across all observed LES cases”, namely an upper-level low trough upstream of the Great Lakes basin that propagates east and intensifies throughout its progression, resulting in more upper-level cyclonic vorticity and subsequent quasigeostrophic (QG) vertical forcing (though the latter two dynamical impacts were never shown in any quantitative way in this paper). The authors should be a little clearer about that.

Response 1: The authors appreciate the care and attention this reviewer has put into their comments about this manuscript. While the authors agree that the results of this work were not necessarily paradigm shifting or transformative, the difference in thermodynamic vs. dynamic support between the different cyclone tracks was an unexpected finding that no lake-effect research has analyzed to the author’s knowledge. While the differences were in fact minimal, we do not know if this will necessarily hold for different lakes, snow band types, etc. and the authors plan to conduct similar research to this work in the future to assess if this is the case. Regarding the QG forcing, given this was an exploratory work devoted to assessing the broad synoptic setups associated with northwestern PA lake-effect cases, the authors only performed a qualitative and rudimentary analysis of the QG dynamics. QG forcing and lake-effect has not received a lot of attention from the research community as far as quantitative analysis and has been largely empirical. A quantitative analysis would likely require further investigation (possibly including numerical weather model simulations) which was beyond the scope of this work.

Reviewer 2 Report

Comments and Suggestions for Authors

General Comments: 

This study examines the occurrence of Lake Effect Snow in the northwest Pennsylvania region. They classify the events into synoptic types from the NOAA severe storms database, which are available since 2006.  They use the North American reanalyses. They examine the relevant dynamics and thermodynamics for each type finding two favored dynamics and two thermodynamics. Then they use statistical analysis which revealed little difference between the character for the types of events archived by NOAA. 

The figures are generally neat and readable. The data and methods used are appropriate, and the findings are supported by the analysis. The figures are neat and readable. All in all a well-written and well executed study. This paper should be published after some minor revisions. 

Minor Comments: 

1. It is understandable that there is only 15 years-worth of "reliable" information, even though (supposedly) these events have been archived since 1996. The analysis ends with 2020. A sentence or two as to why events through winter 2023-2024 are not included would be appropriate. 

2. Fig. 5 - Stretch the abscissa out to make this figure easier to read. Any tendency for when "indeterminates" occur? They don't need to be included in the figure, but a brief note about them would be interesting given they are nearly one in ten cases. 

3. Table 1: Anything to add about the character of the "indeterminates"?  Again, see point two above. 

4. In the conclusions; "When comparing the LES environments between the different cyclone tracks, some differences were noted."   Expand on what was learned, including the seasonality. As this study breaks some new ground, make sure the conclusions include all that's been learned. 

Author Response

Comment 1: It is understandable that there is only 15 years-worth of "reliable" information, even though (supposedly) these events have been archived since 1996. The analysis ends with 2020. A sentence or two as to why events through winter 2023-2024 are not included would be appropriate. 

Response 1: Thank you for that suggestion as I am sure the readers would be curious to know as well. Ultimately, 2020 was the ending point for our period of record because this was the last year that all of the attributes we looked at for each case had accurate documentation. Specifically, this was the last year that accurate economic damage was available. A sentence has been added on lines 179 - 181 to clarify this point to the reader.

Comment 2: Fig. 5 - Stretch the abscissa out to make this figure easier to read. Any tendency for when "indeterminates" occur? They don't need to be included in the figure, but a brief note about them would be interesting given they are nearly one in ten cases. 

Response 2: Figure 5’s abscissa has been stretched and the legend has not been placed inside the chart so the figure could be larger and easier to read. Great suggestion! As far as the “indeterminate” cases, please see comment below

Comment 3. Table 1: Anything to add about the character of the "indeterminates"?  Again, see point two above. 

Response 3: Due to the ambiguity of the “indeterminates” in terms of classification and not wanting to analyze synoptic fields that differ from each other, we decided not to analyze all of the attributes we looked at for the other cyclone categories. The authors feel this would not be an objective comparison to the other categories as the composites would possibly be a misrepresentation of these scenarios as not all of them feature the same or even similar synoptic patterns. However, a couple of sentences have been added on lines 288 - 294 which provide rudimentary descriptions of the “indeterminate” and “other” cases. Additionally, a sentence was added to the Figure 5 description which addresses the seasonality of both of these categories. 

Comment 4: In the conclusions; "When comparing the LES environments between the different cyclone tracks, some differences were noted." Expand on what was learned, including the seasonality. As this study breaks some new ground, make sure the conclusions include all that's been learned. 

Response 4: Thank you for the feedback and we appreciate the acknowledgment of new ground being broken. The sentences following the referenced line describe the key differences. We have added a better transition to blend these sentences to ensure that the reader knows that these are the main differences in LES environments between the cyclone tracks. In terms of the seasonality, we did not explore that in the context of LES environments. This is something we hope to look at in future work with a larger domain. This is highlighted in the Discussion (lines 585 - 587).