A) Overview: Federer et al. (2024): Synoptic Perspective on the Conversion and Maintenance of Local Available Potential Energy in Extratropical Cyclones

 

The authors are to be commended for preparing a well-evidenced story and well-written story on the often challenging concept of atmospheric energetics in general and available potential energy in particular. I learned something from reading this paper.

 

B) Specific comments on Federer et al. (2024): Synoptic Perspective on the Conversion and Maintenance of Local Available Potential Energy in Extratropical Cyclones

1. Line 104: Why is omega only computed at 500-hPa. Wouldn’t a lower level (e.g., 850-hPa or 700-hPabe be better for sampling and quantifying thermal advection?



2. Line 112: Is it possible that your criterion for identifying CAOs could be sensitive to your choice of the 850-hPa level for air-sea temperature differences? Could shallow cold air be a problem in some situations?



3. Line 134: ….given by the product of omega and the buoyancy……



4. General question: What is the vertical resolution of your dataset? How well are you resolving shallow Arctic air outbreaks over continents and oceans? 



5. Lines 142-148: I had trouble visualizing this somewhat convoluted text. Suggestion: Inserting a schematic diagram here that depicts the calculation of the reference state would be most helpful to the reader. 



6. Lines 150–151: Curious to learn why you chose not to compute reference states for DJF, MAM, JJA, and SON?



7. Figure 1: Could, say, 700-hPa vertical motion be added to the six figure panels so that the locations of major ascent and descent regions relative to trough and ridge positions can be better pictured?



8. Figure 1: Labels 1a, 1b, 2c, etc. need to be defined in the figure captions. Label latitude and longitude lines. 



9. Figure 1f: There appears to be a remnant cyclone near Iceland in panel 1f that is a remnant of cyclone 1b in panel 1e.



10. Figure 1: There appear to be three cold surges based on equatorward-directed trough extensions in Fig. 1. Significance? How do the three cold surges.



11. Figure 1: Cyclone 2a, and especially cyclone 3b, are not directly associated with an upper-level trough in their formative stages. Significance?



12. Lines 185-187: Please be more specific about where the APE decline is occurring.



13. Figure 2 caption: Please indicate directions at either ends of the cross-section lines (e.g., west or east, etc.) for reference purposes.



14. Lines 202-203: This sentence needs further elaboration since Fig. 2c is devoid of vertical motion.



15. Figure 3: Geography is hard to distinguish from the color shading and black contours.



16. Figure 3: SLP contours are very difficult to follow because they are so faint in Fig. 3 (and in Fig. 1)



17. More on Fig. 3. So, is it fair to say that APE generation equatorward of the jet axis acts to increase APE in lower latitudes (with a minor offset by diabatic processes in the vicinity of midlatitude cyclones)? If so, what can we conclude about these APE changes from a general circulation perspective?).



18. Line 247: Check whether AOE has been defined previously.



19. Figure 4: SLP contours are too faint to be see properly, especially in the vicinity of the trajectory swaths.



20. Figure 4b: How sensitive do you think the split between subsiding and ascending trajectories shown in Fig. 4b would be to the starting time and location of the trajectory computations? Presumably, the expected trajectory differences could be linked to the governing dynamics? 



21. Lines 264–272:  So, presumably the extent and magnitude of latent heat release in ascending trajectories would be very sensitive to the air mass in which the trajectories originate. For example, Mediterranean and Gulf of Mexico cyclones that have access to tropical and subtropical air masses would exhibit different APE changes in conjunction with ascending trajectory swarms than the higher-latitude example referenced in this paper? How should we interpret such differences?



22. Figure 5 (and accompanying text). Have you checked whether a mesoscale trailing trough behind the main trough could be altering the larger-scale QG ascent-descent dipoles? Would it be helpful to add the position of the surface cyclone to Figs. 5a,b to assist the reader in better understanding the relationship between the trajectory evolution and the surface cyclone location?



23. Lines 288-292: While I think that your conclusions mentioned here are correct to first order, it might be a good idea to allow for considerable variability the would likely be seen in a much larger sample of cases (e.g., a secondary trough deepening downstream into a pre-existing trough vs. a weakening trough that was lifting out to the northeast (among other possible trough-ridge scenarios).



24. Lines 294-311: Very interesting perspective on APE conversion that is well supported by the trajectories shown in Fig. 6 and the APE tendency plots shown in Fig. 7. An obvious question….that really cannot be addressed here….is how general this result would would be across a variety of cyclone evolutions at different times of the year and the choice of where and when to initiate the trajectories.