the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The climatology and nature of warm-season convective cells in cold-frontal environments over Germany
George Pacey
Stephan Pfahl
Lisa Schielicke
Kathrin Wapler
Abstract. Cold fronts provide an environment particularly favourable for convective initiation in the mid-latitudes and can also be associated with convective hazards such as wind, rain and hail. We build a climatology of cold-frontal convective cells between 2007–2016 for April–September in a cell-front distance framework by combining a radar-based cell detection and tracking dataset and automatic front detection methods applied to reanalysis data. We find that on average around twice as many cells develop on cold-frontal cell days compared to non-cold-frontal cell days. Using the 700 hPa level as a reference point we show the maximum cell frequency is 350–400 km ahead of the 700 hPa front which is marginally ahead of the mean surface front location. The 700 hPa front location marks the minimum cell frequency and a clear shift in regime between cells with a weakened diurnal cycle on the warm-side of the 700 hPa cold front and strongly diurnally driven cells on the cold-side of the 700 hPa front. High cell frequencies are found several hundreds of kilometres ahead of the surface front and cells in this region are most likely to be associated with mesocyclones, intense convective cores and lightning. These results are an important step towards a better understanding of cold-frontal convection climatology and links between cold fronts and convective hazards.
George Pacey et al.
Status: final response (author comments only)
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RC1: 'Comment on nhess-2023-39', Anonymous Referee #1, 04 Apr 2023
General comments
The submitted manuscript contains a climatological study of convective cells and mesoscale cyclones and their nature relative to a set of automatically detected near-surface cold fronts. This combination of established front detection, which provides information on large-scale flow, and a radar-based algorithm for detecting and tracking convective cells is unique and novel.
The paper is generally well written, and the illustrations are of high quality. Most parts of the paper are descriptive without going into much detail of the individual forcing mechanisms. Something the author may reserve for the future, but the manuscript could be expanded in this regard as well, for example, by adding more information on surface slope (e.g., orographic lifting in regions along the Black Forest, Harz, Rhön, for example).
The authors derive some interesting properties from their methods, such as cell speed and cell lifetime versus distance to the next cold front. The authors might consider adding more information about the relationship between these properties and front strength or front orientation (some are more zonal some are more meridionally oriented).
Specific comments:
- Despite the uplift due to the sloping isentropes of the front, there is little discussion of forcing by the upper levels. Presumably, the front is accompanied by a trough, and a simple geopotential contour or vertical motion could be useful for discussing the results.
- It is unclear why a characteristic such as cell lifetime or lightening has a local minimum at the location of the 700-hPa front. The authors are encouraged to comment more on this observation, which is clearly seen in Fig. 10 and only briefly discussed in l. 215. It seems to be a fortunate coincidence that the largest downward motion should occur at the location of the 700-hPa front, but the author should support their hypothesis for example, by plotting vertical motion, surface moisture, and CAPE/CIN.
- I think a map of the topography of Germany in steps of hundredths somewhere in the paper might help in understanding some of the local features, especially for the non-frontal cases.
- Are the post-front cases (Fig. 7a) in the northwest related to land-sea circulations?
- Would be possible to add some information on the frontal strength in terms of temperature or humidity gradients alongside the characteristics of the convective cells (e.g., lifetime)? It looks like the 700-hPa front line is behind a strong gradient in humidity reminiscent of squall lines (Fig. 2a).
- To justify the 750 km, the author could also perform a test against randomization. The number of additional cells attributed to a front when the distance is increased in 50-km increments can be plotted against the same but using radar data from a randomly chosen date. At the radius where both changes in the additional number of cells become equal, the increase in additional cells attributed to the front can no longer be distinguished from noise. Below this threshold, however, the increase is more than noise and is therefore physical.
Citation: https://doi.org/10.5194/nhess-2023-39-RC1 - RC2: 'Comment on nhess-2023-39', Anonymous Referee #2, 18 Apr 2023
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RC3: 'Comment on nhess-2023-39', Anonymous Referee #3, 05 May 2023
General comments:
This study analyses the location and characteristics of deep moist convection associated with cold fronts over central Europe. Many novel insights are gained and nicely embedded in the existing literature. Overall, the methods, structure, and the figures are of a high quality. I don’t have any reasons for rejection and my comments can mostly be seen as suggestions, although I agree with some of the concerns of reviewer #2 (see major comment 3 below).
Major comments:
1) line 132: The bias in Fig. 1 looks more than “slight”. If it were a weak bias, shouldn't more fronts be expected towards the Atlantic where strong lows are originating from? Or is the theta gradient increasing over land? I think some more discussion for why the dataset is still useful for your purpose seems warranted, e.g., that you are mostly interested in fronts with convection impacting central Europe.
2) 159 Do you think any biases resulted from only counting the first cell detection? I think the approach is good enough, but I could imagine that long-lived cells change their location relative to the front over time?
3) Perhaps my main point of criticism (or the aspect of the study which could be improved the most) is that although you discuss lift mechanisms ahead and at the front, not much analysis is done to locate these features relative to the 700hPa front location. I realize that this is not easy and changes a lot from case to case, but since the study claims to describe the “nature of cold-frontal convection”, a deeper analysis seems warranted. For instance, is it possible to add the locations of the surface front and pre-frontal convergence relative to the 700hPa front in Figs. 4 and 5 (or rather the range of locations in your dataset, e.g., as a box and whisker)? Could this be estimated from the ERA5 data you used? An alternative would be to go through some cases manually and indicate these boundary positions for each case in the plots.
4) 295 Looking at the number of cells might be misleading because larger storm systems (MCS) result in less cells counted even though they have a larger impact. You don't need to change the analysis but this should be made clear again to the reader. In general, strengths and weaknesses of the KONRAD dataset are not discussed much.
Another example would be that you speculate that the pre-frontal diurnal cycle is broader because of more MCS. Wouldn’t that also mean that less individual cells were detected there (because one MCS has larger but less cells)? The opposite is seen in Fig. 10c. Is that because of flaws in the detection algorithm?Minor comments and technical corrections:
24-25 An alternative to this explanation would be varying DMC ingredients (e.g., CAPE) in different regions along the front.
26-38 There is a lot of good content here, but the text seems a bit unstructured. For instance, pre-frontal convergence lines are also a result of an ageostrophic circulation (Dahl and Fischer 2016), not only the lift at the front. Also, you could make a bit clearer that you start discussing mechanisms ahead of the front, then at the front and then behind, for instance by ending the first sentence (l. 27) with “… location relative to the front.” And then starting the next sentence with “Ahead of the front, …”.
41-42 I’m not quite sure if I understand the point. If this holds true for a surface observer, why is it not true? Are you referring to the fact that the convective cloud is not necessarily in the same location as the heaviest precipitation?
97-98 If I understand correctly, “higher” would only be true for cold fronts, because for warm fronts v_f would be negative. Did you mean to say the magnitude of v_f is higher with stronger advection?
101-104 Remains unclear how L was determined. Is it a continuous length of points where the other criteria were met? What about brief gaps in the boundary detection?
141 “At the start of this study,” (comma in English after such introductory words for a sentence”
section 2.2 in general: Just a suggestion, but I would bring in the hail, lightning, and mesocyclone detection methods later when they are needed. Jumping back and forth between the different dBZ thresholds was a bit confusing here.
155 Make clear what “such” is referring to. I’m assuming you mean the hail, lightning and mesocyclone detections?
163 comma behind “2.2”
164 I like this numbering of criteria. Makes it really clear.
185 Comma after “September”
187 and 262 One convective cell is a fairly low threshold. Days with >1 and >100 days are weighted equally with this method, correct? Perhaps discuss this caveat of considering cell days by e.g., showing a histogram of the number of cells per cell day to make clear that most cell days were really days with much convective activity?
189 Associated “with”
239-240 There is a clear secondary maximum around 750 km ahead of the front in Fig. 5, which is interesting. Do you speculate that this is just free convection in the warm sector or pre-frontal convergence serving as another (weak) trigger? By the way, I really like this Figure. Is it necessary to have non-linear color scheme. It may look nicer but I think it makes it harder to interpret the numbers. If it’s necessary, it should at least be mentioned in the caption.
248-255 How do supercells fit into this picture? Their long lifetime could also lead to a broader diurnal cycle. You only mention Wapler (2016) briefly and without context.
259 First time reading this, I was confused what you mean by “vary”. It might just be me, but if you refer to the spatial distribution, it's clearer to say something like: "The frequency of convective events varies in different parts of Europe."
268-270 Sounds like you believe this is due to a general increase towards the south / the mountains. Your resolution is fairly coarse, but the spatial distribution you observe would also be consistent with mesoscale enhanced convective activity in local parts of Germany, e.g., South of Stuttgart (Piper 2017 Fig. 3, Kunz 2010).
295 Here and elsewhere: “Colorbar”
303 This is consistent with enhanced activity in the Erzgebirge region (Piper Kunz 2017).
322 Such a pattern is also often associated with advection of an elevated mixed layer from Southwest Europe and pre-frontal convergence lines (Dahl and Fischer 2016).
378-385 This last paragraph was a nice finish and the results are plausible. The conclusion section is also nice and precise.
Fig. 8 I also liked this Figure and analysis. Could you add the average number of cells per event over whole domain in the top of each subplot? Eeven though some clusters might be rare, their impact/number of cells might be large, so giving the reader information about the typical number of cells could be useful.
Fig. 10 Also very informative plot. Titles for each subplot would be helpful to avoid having to jump between caption and plot.
Citation: https://doi.org/10.5194/nhess-2023-39-RC3
George Pacey et al.
George Pacey et al.
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