the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Clouds and precipitation in the initial phase of marine cold air outbreaks as observed by airborne remote sensing
Abstract. Marine cold air outbreaks (MCAOs) strongly affect the Arctic water cycle and, thus, climate through large-scale air mass transformations. The description of air mass transformations is still challenging partly because previous observations do not resolve fine scales, particularly for the initial development of a MCAO, and lack information about cloud microphysical properties. Therefore, we focus on the crucial initial development within the first 200 km over open water of two MCAO events with different strengths observed during the HALO-(AC)3 campaign. Based on unique sampling of high-resolution airborne remote sensing and in-situ measurements, the development of the boundary layer, formation of clouds, onset of precipitation, and riming are studied. For this purpose, we establish a novel approach, solely based on radar reflectivity measurements, to detect roll circulation that forms cloud streets.
The two MCAO events observed in April 2022 just three days apart occurred under relatively similar thermodynamic conditions. However, for the first event, colder airmasses from the central Arctic led to a marine cold air outbreak index twice that high as for the second event. Thus, the two cases exhibit different properties of clouds, riming, and roll circulations though the width of the roll circulation is similar. For the stronger MCAO, cloud tops are higher, more liquid-topped clouds exist, the liquid layer at cloud top is wider, and the liquid water path, mean radar reflectivity, precipitation rate, and occurrence are increased. These parameters evolve with distance over open water, as seen by, e.g., boundary layer deepening and cloud top height rising. Generally, cloud streets form after traveling 15 km over open water. After 20 km, this formation enhances cloud cover to just below 100 %. After around 30 km, precipitation forms, though for the weaker event, the development of precipitation is shifted to larger distances. For the stronger event, we detect riming for cloud temperatures below -20 °C. The variability of rime mass has the same horizontal scales as the roll circulation implying the importance of roll circulation on precipitation. The detailed observations of the two MCAO events could serve as a valuable reference for future model intercomparison studies.
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RC1: 'Comment on egusphere-2024-850', Anonymous Referee #1, 30 May 2024
This paper describes the analysis of airborne (mostly) remote sensing (and some in-situ) of Arctic could ir outbreak events from the HALO-AC3 campaign; however mostly using the Polar 5 & 6 aircrafts that flew simultaneous with the high-altitude HALO aircraft. The study invokes an interesting and novel method to analyze two significant events in April 2022, by collocating different instruments and relying on radar reflectivity as a proxy for up- and downdrafts in the cloud street patterns, providing both information on geometry of the cloud streets and of the cloud physics going on.
There is a lot of interesting details revealed and I liked the method, however, the paper is very poorly written and is quite hard to read. As a reviewer I get in a less than favorable mood when PhD advisors don’t do their job in helping the students with learning scientific writing. It is really not my job to do this and everything would be so easier if as a reviewer I could focus on the science, rather than language, style and lack of narrative.
First of all, the manuscript is much too long and contains too much detail not necessary for the reader to know. Much of this could just go all together, while some could be put in supplementary information. Second, the discussion is sometimes very confusing, jumping from conditions in one of the cases to the other and back, and it is very easy to loose track on which case is actually discussed. It would be much better if at least initially, with all the background conditions, the two cases are discussed entirely separately. Moreover, there is too much details in the figures, that are also much to small, and the figure captions are very disorganized. Figure captions should be very succinct and organized and – in contrast to the text – not have a narrative; just the bare bone facts about what is in the figure. Third, the author seems to work hard to make the case that the cases are really different, while what strikes me is how similar they are. Sure, magnitudes are very different but looking at – for example – trends along the fetch, I find the underlying structure surprisingly similar. Finally, the conclusions and the answers to the research questions, given initially, are very trivial and reveal nothing new, that couldn’t have been guessed given just the basic information.
I really encourage the author to rewrite the whole manuscript and to take help; the work is not done until the results are communicated to the community, and this is not done well in this otherwise interesting work.
Minor comments:
Line 35-36: Unclear; I don’t understand why ice cannot grow at the expense of the liquid just because the humidity is above saturation w.r.t. ice.
Line 45: “flow divergence” how and where?
Line 51: Why – and where - does the fluxes increase with a sharper MIZ?
Line 64-65: While the open water distance passed by an air parcel increases with open leans and open water over the MIZ, it is unclear what this means in terms of fluxes; this depends on the water temperature which to some extent will be controlled by the neaby ice.
Line 66: What cloud-street charateristics are variable and how?
Line 74: Unprecise; the spatial resolution of CloudSat along the track is quite good; however, the swats are far apart and nearby swats occur sparsely and swats are generally not aligned with CAOs.
Line 128-129: Unclear; I have a hard time seeing the convergence line discussed here.
Line 130: Maybe “different” instead of “varying”; the latter would be changes within each case while the former changes between the cases.
Figure 2: An interesting feature in this figure is that the reflectivity seems not to be symmetric; the maximum seems to be shifted to the right (in the figure) and is not centered in between yhe downdrafts. This is not even discussed, and I have no idea if this was just spurious or is a systematic feture.
Line 160: Unclear what you mean by “below 30 g m-2”. Do you mean the accuracy is “better than 30 g m-2” or that there is something special with the accuracy “at values below 30 g m-2”? Moreover, 0.5 30 g m-2 is also “below 30 g m-2”, so if the first is true, it doesn’t say anything.
Line 164: I thing you mean “… non-constant manner.” or maybe “… non-constant way.”
Line 187: What does it mean that you use 850 hPa (rather than some lower value)? Both cases are rather shallow at the distances you study, so why not use e.g. 925 hPa.
Section 3.2: For something as basic as a trajectory calculation, this section is exceeeeedingly long. It can be shortened to a third of its current length.
Line 204-209: This paragraph is very confusing and it is unclear what the argument made really is. Moreover, the assumption that the cumulative flux is independent of wind speed, since length and time are interchangeable, is based on an assumption that the temperature difference between air and sea surface is constant, which is almost certainly not the case.
Line 213: I don’t know what and integrated time means.
Line 215-217: I beg to differ; the IFS has significant biases in almost all boundary-layer parameters and especially in the turbulent fluxes. This is borne out by the quoted 200 m error in BL growth (line 217) which to mee in these conditions is not a small error.
Line 218: I don’t understand the argument of this resampling, which provides trajectories at an almost ridiculous resolution, far exceeding the resolution in the input data from ERA5.
Line 231: How do you handle the SST in the MIZ?
Section 3.2: This is one of the most interesting introductory sections, but it is also much too long and detailed. Describe the principles and leav out the details, or move them to an appendix or supplementary information.
Line 244: Why 0.7? Are the results sensitive to this choice? Why not a constant distance into the cloud? Entrainment does not necessarily penetrate deeper into deeper clouds.
Line 241-242: Does D constitute a cloud depth? Conidering boundary-layer scaling, why not use the surface as a lower limit instead? Are the results sensitive to the -5 dBZ threshold to distinguish cloud from precipitation? What about when signals reach into the surface clutter.
Line 255: This is not a retrieval; “Extract“ is better.
Line 259 & 261: Number of samples are usually integers; there is no such thing as 2.9 samples.
Line 281: “particle concentration” is better; in reality I guess it has more to do with the cross-section area…
Line 285-287: All these precipitation or size against reflectivity relationships are a bit arbitrary; an expert on radar meteorology once told me that estimating ice concentration from radar reflectivity is uncertain to about an order of magnitude. So maybe phase all this in a less distinct way.
Line 296-306: From where is the information in this paragraph about ascending and descending air coming? Much of this appears speculative to me; besides drop “mass” - “air mass is a different thing.
Line 309-311: This is a lot of words to say its colder on one day than the other; the temperature at 2 km has little impact on the boundary layer.
Line 321: What do you mean by “weakens less”? Compared to what? No other weakening inversion is mentioned in the text.
Figure 5. In fact, my interpretation of the temperature and moisture profiles is that the last profile for 4 April shows a deeper boundary layer, at almost 1 km albeit less distinct, than on 1 April that has a slightly more distinct top at maybe 600 m.
Line 327: “directional shear”, “northerly” & “westerly”.
Line 328: I disagree; I can’t see that the shear is systematically stronger for any of these heights
Line 329-330: Again, very many words to say there is a low-level jet at 200 m.
Line 332: I can’t see any Ekman spiral here.
Line 334-335: Explain the discussion on the different angles and the convective instability.
Line 335-336: How does this statement agree with the previous angles discussed?
Line 341: It is not the profiles that precipitate!
Line 351: “weaker advection” is better
Figure 7: How is the normalized width calculated; mirroring in the center? See my question earlier about the center (or the maximum updraft) appearing to be closer to one edge than the other. Also, why no LWP for 4 April; there are LWP values in the fetch plot.
Line 392-393: I suggest ”… a factor of two or more …”
Line 393: I have a hard time seeing any stagnation in this plot
Line 427-433: Suggest to drop this as you show no observations beyond 160 km.
Line 434-454: This can be dropped; if you decide to keep it shorten it and move to discussion or conclusions; it has no place here.
Section 5. I don’t really see the value of this section here. Reading this after the enormously detailed previous sections, this feels incredibly thin.
Section 6.: The whole Synthesis can be shortened significantly
Line 534-550: Is this what you learned from this study; it seems exceedingly trivial and I could have told you all of this, saving you a lot of time. There has to be more than this!
Citation: https://doi.org/10.5194/egusphere-2024-850-RC1 -
RC2: 'Comment on egusphere-2024-850', Anonymous Referee #2, 30 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-850/egusphere-2024-850-RC2-supplement.pdf
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