the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Variations in Boundary Layer Stability Across Antarctica: A Comparison Between Coastal and Interior Sites
Mckenzie June Dice
John Cassano
Gina Clara Jozef
Mark Seefeldt
Abstract. The range of boundary layer stability profiles, from the surface to 500 m above ground level, present in radiosonde observations from two continental interior (South Pole and Dome Concordia) and three coastal (McMurdo, Georg von Neumayer III, and Syowa) Antarctic sites, is examined using the self-organizing maps (SOMs) neural network algorithm. A wide range of potential temperature profiles is revealed, from shallow boundary layers with strong near-surface stability to deeper boundary layers with weaker or near-neutral stability, as well as profiles with weaker near-surface stability and enhanced stability aloft, above the boundary layer. Boundary layer regimes were defined based on the range of profiles revealed by the SOM analysis. Twenty boundary layer regimes were identified to account for differences in stability near the surface as well as above the boundary layer. Strong, very strong, or extremely strong stability, with vertical potential temperature gradients of 5 to in excess of 30 K (100 m)-1, occurred more than 80 % of the time at South Pole and Dome Concordia in the winter. Weaker stability was found in the winter at the coastal sites, with moderate and strong stability (vertical potential temperature gradients of 1.75 to 15 K (100 m)-1) occurring 70 % to 85 % of the time. Even in the summer, moderate and strong stability is found across all five sites, either immediately near the surface or aloft, just above the boundary layer. While the mean boundary layer height at the continental interior sites was found to be approximately 50 m, the mean boundary layer height at the costal sites was deeper, around 110 m. Further, a commonly described two stability regime system in the Arctic associated with clear or cloudy conditions was applied to the 20 boundary layer regimes identified in this study to understand if the two-regime behavior is also observed in the Antarctic. It was found that moderate and strong stability occur more often with clear than cloudy sky conditions, but weaker stability regimes occur almost equally for clear and cloudy conditions.
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Mckenzie June Dice et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1673', John King, 12 Sep 2023
General comments
In this well-written paper, high vertical resolution radiosonde observations from five Antarctic stations are used to study the static stability of the lowest few hundred metres of the atmosphere. Self-organizing maps (SOMs) are used to define stability regimes for each station. The seasonal variability in the frequency of occurrence of the regimes is studied and the regime structure is compared between the different stations. As a SOM is calculated for each station individually, it is difficult to compare SOMs across stations so SOM nodes are assigned to stability regimes based on a two-layer classification. A proxy for cloud cover is used to investigate the influence of clouds on stability. Interesting contrasts are found between the two interior stations and the three coastal stations studied and there is some discussion of the reasons for these contrasts and for similarities/contrasts with measurements made in the Arctic.
The paper is probably the most comprehensive study of boundary-layer and lower atmosphere stability in the Antarctic that has been carried out to date. Using a SOM approach to define stability regimes is novel but I would have liked to see a bit more explanation of how the SOM analysis (section 2.2.1) was used to inform the development of the stability regime classification (section 2.2.2). At the moment they are presented separately and it’s not entirely clear what additional value the SOM analysis brings to the study.
The paper has a strong climatological focus, with rather limited discussion on what factors drive the various stability regimes that are observed, apart from some rather detailed analysis on the impact of cloud cover. I realise (from statements in the conclusions section) that future papers will examine controls on stability regimes more deeply but it seems a little strange to examine one factor in detail in this first paper and not to discuss other factors, such as wind shear, that may be of equal or greater importance. Any future studies will also need to recognise that stability within and above the boundary layer are controlled by different mechanisms. Within the boundary layer the main controls on stability are surface energy balance and mechanical mixing while, above the boundary layer, radiative flux divergence and dynamical processes such as advection and subsidence may dominate. You will need to look at forcings in both of these regions to fully explain the results of the current study.
Overall, I would recommend this paper for publication in WCD after minor to moderate revision. Below, I set out the main points that I would like to see addressed in a revised version of the paper.
Specific comments
- The study is based on observations from radiosondes that are launched once or twice per day. During the Antarctic summer, there is a strong diurnal cycle of solar radiation at all of the stations studied, apart from South Pole. It is well-known that the diurnal cycle of solar radiation at Dome C during the summer strongly modulates the structure of the atmospheric boundary layer at that location (Mastrantonio et al, Meteorol. Atmos. Phys., 71, 127-132, 1999; King et al., 2006, doi: 10.1029/2005JD006130). The 1200 UTC daily sounding at Dome C takes place at around 0400 local time and is thus representative of early morning conditions, when the boundary layer is both shallower and more stably stratified than it is during the middle of the day. This is not a serious issue with the study but it should be mentioned in the methodology section and when the Dome C results are presented in section 3.2.
- Section 3.6. The absence of a strong link between cloudiness and stability at the interior stations is not surprising. South Pole and Dome C are both characterised by a much greater frequency of cloud-free conditions than is typical of coastal Antarctic stations or Arctic Ocean locations. The clouds that do occur at these high-altitude stations are often optically-thin as they contain little or no liquid water, so cloud classification based on downwelling longwave radiation may not work well for these stations and the impact of these clouds on surface energy balance (and hence on stability) is likely to be very different to that of optically-thick mixed-phase clouds over the Arctic Ocean and Antarctic coastal margins. See, e.g., Town et al., 2007, https://doi.org/10.1175/JCLI4005.1 , Ganeshan et al, 2022, doi:10.1029/2022JD036801
- Lines 659-660: “Somewhat surprisingly…”. Is this surprising? Strong near-surface stability in winter is the result of strong radiative cooling of the surface, which has to be compensated for by a large downward turbulent heat flux which drives strong stratification in the near-surface layer. During December and January at South Pole, net solar radiation almost exactly balances net longwave radiation at the surface so the turbulent heat flux (and, consequently, the near-surface temperature gradient) is small (King and Connolley, J. Climate, 1997, https://doi.org/10.1175/1520-0442(1997)010<1273:VOTSEB>2.0.CO;2)
Minor points and technical corrections
- Line 36: “coastal”
- Lines 46-47: You could also add W. Connolley, 1996, Int. J. Climatol., 16, 1333-1342 as a more recent reference here.
- Lines 201-201: Is this a subjective judgement or were any objective criteria used?
- Lines 239-241: Please include an equation that shows how you calculate the bulk Richardson number. Note that Rib is only an approximation for the ratio of buoyancy production/destruction to shear production.
- Lines 335-336: “…or in the Arctic”? Maybe you should also make it clear that you are talking about the Arctic Ocean here – very strong stability is seen in Arctic regions such as Siberia or over the Greenland ice sheet.
- Figure 2: I think that the top and bottom x-axis descriptions are swapped round in the figure caption.
- Figure7: Please label each panel as you have done in figure 5.
- Lines 601-603: Mixing in strong winds associated with coastal cyclones is probably the dominant control on stability at Syowa.
- Lines 638 and 639: “many of the SOM profiles” rather than “much of the SOMs”?
- Line 738: The “…quick descent into winter-like conditions in the transition seasons” is often referred to as the “coreless winter”.
Citation: https://doi.org/10.5194/egusphere-2023-1673-RC1 -
RC2: 'Comment on egusphere-2023-1673', Anonymous Referee #2, 21 Sep 2023
General
This paper analysis a large number of radiosoundings launched at five Antarctic stations, two continental ones and three coastal ones. The authors propose a classification of stability including near-surface conditions and conditions aloft. They find a wide range of potential temperature profiles and profiles of gradients, for which they propose 30 boundary layer regimes using the self-organizing maps neural network algorithm (SOM). They find large differences between the coastal and continental sites and finally distinguish cloudy and clear-sky regimes.
In most parts the paper is clearly written and to my knowledge a similar work comparing soundings of several Antarctic stations is not yet available. In principle, I like the work and recommend its publication but at some points I have difficulties to follow and I think that the description could be clearer in some aspects explained below.
Major Revisions
1) Previous work (e.g. Handorf et al.) has shown that the Antarctic boundary layer can be extremely shallow with tops below 25 m height (sometimes 10-20 m, see their Figure 1). It is a challenge to measure such boundary layers by radiosoundings. Note that at Neumayer, soundings are launched from the station roof at 28 m above the surface. Also for other stations, the given lowest measurements at 20 m are not really ‘near-surface’. The real ABL might be below, which has a large impact on turbulent fluxes. This should be explained.
2) Perhaps I was too fast, but it is difficult to understand that in Figures 2,4,6,8,10 the number of regimes amounts to 30 and thus differs from the number 20 in Table 3. Also, I cannot really follow why, e.g. in Figure 4 the same name SS occurs for classes 17 and 23.
3) Lines 220-224: As far as I understand the classification in Figures 2,4,6,8,10 is different for each station with different patterns. I did not understand why not a general classification is possible being valid for all stations. In the present form, an intercomparison of results for different stations becomes difficult. Also, it would become difficult to see if results of a model would fit into one of the different classes. I think this requires more explanation.
4) The present work gives the impression that the Antarctic boundary layer is always near-neutral or stably stratified. It should be stressed that near-neutral could also include convective cases. Note that, e.g. at Kohnen station a daily cycle is often observed with upward fluxes of sensible heat and thus convective conditions during daytime in a shallow boundary layer (e.g. Van As et al., 2005).
5) The authors do not consider the effect of condensation on the stability. So, what is called weakly stable might be convective (or near-neutral) when the equivalent potential temperature is considered, so that the presented findings might be missleading in some sense. I recommend that this is explained.
Minor revisions
Lines 86-88: There is an effect of clouds which has a strong impact on the shape of the potential temperature profile and thus on stability, which is not mentioned in the paper. This is the cloud radiative forcing and subsequent mixing (see, e.g. Chechin et al., 2023). Perhaps, it can be added here.
Line 48: Usually near-neutral (throughout the paper).
Line 615: replace present by presented (?)
Lines 796 and 797: When you click onto the given PANGAEA links, you can find the sentence: Always quote citation above when using the data! This means here that the correct citation (which should occur in the list of references) is: Schmithüsen, Holger (2022): Radiosonde measurements from Neumayer Station (1983-02 et seq). Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA, https://doi.org/10.1594/PANGAEA.940584
This might be similar for the other data sources.
Line 23, 62 and many other places: The present name of the station is Neumayer Station III, and before that it was called Neumayer Station. Only the first station was called Georg von Neumayer Station. One should write simply Neumayer Station when all stations are addressed.
References
Chechin, D. G., Lüpkes, C., Hartmann, J., Ehrlich, A., & Wendisch, M. (2023). Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations. Atmospheric Chemistry and Physics, 23(8), 4685-4707.
Handorf, D., Foken, T., & Kottmeier, C. (1999). The stable atmospheric boundary layer over an Antarctic ice sheet. Boundary-layer meteorology, 91, 165-189.
Van As, D., Van Den Broeke, M., & Van De Wal, R. (2005). Daily cycle of the surface layer and energy balance on the high Antarctic Plateau. Antarctic Science, 17(1), 121-133.
Citation: https://doi.org/10.5194/egusphere-2023-1673-RC2
Mckenzie June Dice et al.
Mckenzie June Dice et al.
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