Review of Shin & Park (2019), The Relationship Between Low-Level Cloud Amount and Its Proxies over the Globe by Cloud Types

Abstract. We extend upon previous work to examine the relationship between low-level cloud amount (LCA) and various proxies for LCA – estimated low-level cloud fraction (ELF), lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI) – by low-level cloud types (CL) over the globe using individual surface and upper-air observations. Individual CL has its own distinct environmental structure, and therefore our extended analysis by CL can provide insights into the strength and weakness of various proxies and help to improve them. Overall, ELF performs better than LTS/EIS in diagnosing the variations in LCA among various CLs, indicating that a previously identified superior performance of ELF to LTS/EIS as a global proxy for LCA comes from its realistic correlations with various CLs rather than with a specific CL. However, ELF as well as LTS/EIS has a problem in diagnosing the decrease in LCA when CL0 (no low-level cloud) is reported and the increase of LCA when CL12 (cumulus) is reported over the deserts where background stratus does not exist. This incorrect diagnosis of CL0 as a cloudy condition is more clearly seen in the analysis of individual CL frequencies binned by proxy values. If CL0 is excluded, all ELF/LTS/EIS have good inter-CL correlations with the amount-when-present (AWP) of individual CLs. In future, an advanced ELF needs to be formulated to deal with the dissipation of LCA when the inversion base height is lower than the lifting condensation level, to diagnose cumulus updraft fraction as well as the amount of stratiform clouds and detrained cumulus, and to parameterize the scale height as a function of appropriate environmental variables.


The analysis is extensive but not always well-conceptualized and the presentation needs significant work. In particular, it is not clear why it would be expected that LTS/EIS would correlate well with deep convective cloud types given their main use in analyzing shallow convective clouds or why we should expect to be able to approximate both shallow and deep convective cloud fraction with a single equation. Some variables that appear to be important and linked to the ELF derivation are left undefined. Figures are over-crowded and difficult to read. The overuse of jargon in terms of the cloud type abbreviations makes the reader's job very difficult.
Overall, substantial revisions are necessary to re-focus the manuscript, ideally to tell a more compelling story in the main narrative and perhaps provide more information as supplementary information (I rarely recommend moving material to a supplement, but here that may be quite helpful).
A suitably revised manuscript could be quite helpful for the modeling community and anyone else interested in estimating low cloud fraction and understanding its meteorological controls. I will happily recommend a revised version of this work be published in ACP assuming the authors are able to justify some of the choices made (or reduce the focus to what can be justified) and improve the presentation and organization of the manuscript.

Jargon
The almost exclusive use of cloud type numbers (e.g., CL12) makes this paper extremely difficult to follow. (As a side note, "CL" is not a terribly intuitive abbreviation of cloud type either.) Table 1 is helpful but not sufficient, and does not list the combined types defined by the authors.
The authors should standardize how they describe each major cloud classification used (e.g., CL12 could be "shallow-to-moderate cumulus") and try to pair the descriptive words with the cloud type number as often as possible. Page 8, Line 29 does this very well -something like this should be done for the entire paper (including figure captions).

Treatment of LTS, EIS, and ECTEI
I am confused by the authors' treatment of LTS and EIS as low cloud "proxies" rather than as cloud-controlling factors. Clearly LTS and EIS correlate with stratiform clouds, but the strength of the boundary layer inversion is really only one relevant factor among several in explaining low cloud behavior. LTS/EIS can certainly be used as proxies for low cloud fraction, but this is not their primary/sole purpose.
This conceptual treatment leads to several statements that sound off, at least to my ears. For instance, on Page 9, Lines 4-5, is it truly "undesirable" that we can associate particularly large values of LTS/EIS with cloud clearing? This could be a useful observation to better understand potentially non-linear cloud behavior. This seems to me like a strange way to conceptualize LTS/EIS and why one would examine these variables.
The authors mention ECTEI in the abstract and (barely) define it in the introduction before noting it is similar to EIS and therefore not shown at the end of the Methods section. I would recommend having a supplement with the ECTEI results or not mentioning it at all (or only as a parenthetical). As written, the authors appear to promise an analysis they do not deliver.

Definition of "low-level" cloud and its reasonableness
While the observer-based methods define deep convection as "low-level" cloud based on the cloud base, there should be some discussion/reflection of whether this is a reasonable treatment in this analysis. LTS/EIS really are meant to explain cloud behavior in shallow boundary layers, not in deep convection. I don't particularly understand why we should expect one equation or metric to apply globally for both shallow and deep convection. If the authors do have a good explanation for this, it would be very helpful to provide it.

Missing variable in the derivation of ELF
Many times in the manuscript, the authors refer to and analyze a factor (1 -β2), but this is never defined. Please address this in the methods section. It also might be possible to reorganize the section deriving ELF to be more clear, especially with an eye toward the issues brought up in the final discussion of possible improvements for an "advanced ELF." Although the finer details of the ELF calculation addressed previously do not need to be explained in great detail, it should not be expected that all readers are familiar with PS19.

General presentation and organization of figures
The figures are far too crowded, and each subpanel much too small, to be easily interpreted by readers. In Figures 1-3, the black contours showing the climatology are nearly illegible. For Figure 1, a suggestion could be to split the figure up by cloud type (as is done for Figures 2-3) and have an added column for the climatology in its own map.
For Figures 2-3, I would also recommend subdividing further. One solution could be to have one figure include ELF and comparisons to LTS/EIS in one figure and the components of ELF in another. This could also help structure the discussion -first the differences between ELF, LTS, and EIS can be discussed, and then the contributions of the different components of ELF can be discussed.
It may also be a good idea to split up Figure 4 in a similar manner.
In Figure 5, the caption should explain that the color scheme is the same as that used in Figure 4. The open versus closed symbols also are not defined, although I assume they relate to day and night.
For the regressions in Figure 5, it would be good to address to what extent CL11 drives the regressions. Especially for subpanels b) and d), the scatter of points excluding CL11 (and CL0 and CLIM) do not appear to be very strongly correlated.
In Figure 8, the caption should make more clear that the adjustable scale height as a function of the environmental variables in g) and h) is shown as the "viridis" shading and is in units of meters.
6. Interpretation of ELF correlation with cumulus cloud fraction in Tables 2 and 3 On Page 12, Line 12, the authors write that ELF captures variations in cumulus clouds (CL12) better than LTS and EIS. Unless there is a typo in the tables, this is contradicted by the evidence provided in Tables 2 and 3. The global correlation of ELF with CL12 is ~0.03 whereas it is between -0.45 and -0.75 for LTS and EIS. Or is this sentence actually referring to CL84? In that case, the correlations are more all over the map. In any event, this is another good example of where the elimination of jargon in favor of clearly indicating which cloud type is being discussed would be helpful.

Specific issues
Page 1, Line 18: As the citation of Klein & Hartmann (1993) suggests, the efforts to quantify low cloud effects on Earth's climate long predate the last decade.
Page 2, Line 14: If you do choose to include ECTEI, its definition needs more exposition here.
Page 3, Eq. (5): It would be helpful to discuss that you then force the inversion height to lie between the LCL and the LCL plus a scale height in your analysis here. It's easy to miss as written. Also, for shallow convection, there's essentially no way for the inversion height to exceed the LCL plus scale height, right?
Page 4, Line 9: "f" does not denote the amount of water vapor, it is a function of water vapor.
Page 4, Line 25: Individual components of ELF really aren't "proxies" for low cloud fraction by themselves. It would be more straightforward to just discuss these as components of ELF.