Review of: Evaluation of methods to determine the surface mixing layer height of the atmospheric boundary layer in the central Arctic during polar night in cloudless and cloudy conditions
This paper evaluates methods for determining surface mixing layer (SML) height in the central Arctic using tethered balloon, radiosonde, and tower-based observations from the MOSAiC campaign. The SML height detection methods involve the energy dissipation rate, bulk Richardson number, and Monin-Obukhov similarity theory. The analysis is conducted considering both cloudless (with a surface-based inversion) and cloudy (with an elevated inversion) conditions.
Overall, the paper is well-written and easy to follow, and provides value to the scientific community. However, there are many points throughout the paper where I suggest the authors provide more detailed discussions, so the results can be better interpreted in context of the complex Arctic boundary layer dynamics. While I provide many specific comments below, they are all relatively minor in nature, and after consideration of these points, I would recommend this paper for publication.
General Comments:
Throughout the paper, there is some inconsistency between some methods/descriptions, and actual application, as it related to the time of year for the study period. For example, the title specifies that the study is for polar night, and throughout the intro and methods, ABL processes, and related surface energy budget are discussed in the context of what is relevant/true for polar night. However, Fig. 1 shows that some of the observations occurred during polar day. At other parts of the paper, it is explained that the study is conducted for cases both during polar night as well as during the transition to spring. This all needs to be cleared up. If the study period is really polar night AND spring, processes relevant to spring (e.g., the presence of solar radiation) need to be discussed and considered. Or if you only are using the cases during polar night and the transition to spring before the sun comes up (and not the cases during polar day that are shown in Fig. 1) this needs to be better clarified.
Specific Comments:
L16: This first sentence is a bit weak. The authors mention “intertwined mechanisms and feedbacks” and “Arctic climate parameters” without examples, so it comes across as very vague. This sentence could be strengthened by adding more descriptive terms.
L23: It is unclear what you mean by “The ABL is the atmospheric layer above the Earth’s surface whose effects are perceptible on small time scales.” Do you mean that the effects of the Earth’s surface on the ABL are variable on small time scales? If so, say that more clearly. If you mean something else, please clarify.
L34: It might be useful to add some more examples of how the Arctic ABL is unique. For example, the lack of convection most of the time, and the absence of a residual layer because there is no diurnal cycle of the sun for most of the year.
L41: Perhaps remind the reader here that you are referring to the processes during polar night. Or another option would be somewhere in the Intro to state that the processes described henceforth are characteristic of polar night (while there may be different processes at play during polar day that are not described).
L46: Clarify that this is the case with low clouds. If there are very high clouds, there can be little to no effect on the ABL or height of the inversion.
L77: A more true statement would be that Rib is a measure of the likelihood of turbulence to exist, where Rib below the critical value indicates an atmosphere that is likely to become or remain turbulent and Rib above the critical value indicates that an already laminar layer will not become turbulent.
Throughout: Sometimes you use the whole phrase ‘bulk Richardson number’ and other times you use the abbreviation ‘Rib’. Should be consistent.
L97: The statement “in different fields” is vague. Do you mean to say that the measurement included those of the atmosphere, ocean, sea ice, biogeochemistry, and ecosystem? If so, say that.
Figure 1: I assume the white background in panel c indicates the brief period when there was a full diurnal cycle including day and night? You should clarify this by adding that to the legend, or stating that in the figure caption.
L124: Specify the distance between Met City and the balloon operations. This is important, considering that the greater the distance, the more likely that the two instrument sites are sampling a different or evolved airmass.
L128: It would be good to note that the net irradiance you are referring to is the net longwave (or as you say, terrestrial) irradiance, which is a proxy for radiative energy budget at the surface only during polar night when there is no shortwave (solar) radiation. However, regardless of the presence or lack thereof of solar radiation, the net longwave irradiance can be used to differentiate between cloudless and cloudy conditions. Clarify all of this in the text.
L139: Jozef et al. (2022) showed that over 3 hours, when comparing UAS to radiosonde observations, the ABL height did not change significantly at the 5% significance level. You could add this reference to support your choice to compare coinciding balloon and radiosonde profiles, despite that atmospheric structure can change over this 3 hour time span.
L140: A cloudless ABL can be even shallower than a cloudy ABL. This should also be mentioned when discussing observational challenges.
L177: Some more description should be provided about how you examined the thermodynamic profiles and settled on the threshold value.
L185: Please clarify that values below the critical value imply turbulence.
L200: It should be noted in the text that there could be a temperature offset between the met tower and the balloon due to a variety of reasons, e.g., the airmass evolved as it was advected between the two sites, or the two sites are sampling airmasses that were differently impacted by upwind features such as leads. This is a source of uncertainty in the Rib method that is explained.
L246: How do you know that the elevated inversion base indicates cloud top in this example? You should provide some evidence to confirm this speculation, for example, from the MOSAiC ceilometer measurements. While what you say is likely true, this should be confirmed if you are going to make the definitive statement. I make this point because many studies have concluded that there is often a shallow stable layer between the ABL and the cloud, decoupling the two (e.g., Brooks et al., 2017).
Figure 4: Is the red line the average? Please specify this in the caption for with a legend.
L294: So have you shown here that the critical Richardson number does not necessarily vary based on atmospheric conditions (cloudless vs. cloudy)? Maybe state this as a conclusion, and note whether this agrees with any previous work, or is a new finding.
L307-308: What are the implications of this statement? If you are going to mention this, you should explain the potential impact on the results.
L315: Change to “… around half of the profiles contained an SML with height less than 150 m”
L317: What conditions do the tethered balloons miss? Based on previous discussion, it seems the balloon would miss the stormy conditions, where wind speeds are too high for the balloon to fly. In these cases, the SML is much deeper, likely related to the high winds and also the presence of clouds. Explain this, and the implications for the results. For example, your cloudy conditions in this paper are all those with relatively low winds. What might you expect to see when you have clouds AND high winds? Do you think that would change the critical Rib number?
L325: While different cloud characteristics can influence the SML differently, that is not the only factor at play here. Another important factor for SML depth is wind speed, which can also be highly variable. You could suggests here that the variation in SML during cloudy conditions may be affected by variations in wind speeds.
Figure 11 caption: “Further, shading refers to similar to daylight condition.” Something sounds weird there.
L355: Do you have an explanation for this atmospheric vertical structure? Perhaps advection of a warmer airmass at higher altitudes contributed to the elevated inversion?
L377: This sentence doesn’t make sense. Is there a typo?
L382: It should also be noted that for cloudy conditions, warming of the near-surface atmosphere, relative to in the absence of clouds, lessens the suppression of shear-driven turbulence by the static stability (because the longwave cooling is reduced).
L388: Rather that saying the cause is irrelevant, perhaps say something like “the cause for turbulence generation is not of concern for the current study.”
L390: This method also requires the collection of skin temperature or 2 m temperature, in addition to the radiosondes, correct? This could be seen as a drawback as well, and should at least be noted, perhaps with a suggestion of what to do if no such measurements are collected.
L395: I find it hard to believe that not a single decoupled cloud situation was observed, as I would expect such conditions to occur occasionally throughout winter and spring. But you also did not include actual cloud height observations in this study to be able to make the claim that you make. So a more correct statement might be “For example, we were not able to quantify coupled versus decoupled clouds.” I wonder if cloud coupling/decoupling could also explain some of the variability in SML height under cloudy conditions. In addition to repeating this study during the summer, you might also suggest for future work to add cloud observations so that the coupling/decoupling state can better be quantified.
L425: The sentence would read better as “… but profile measurements with radiosondes can also be useful to either…” |
