|Re-review of “Space-Time Variability of UTLS Chemical Distribution in the Asian Summer Monsoon Viewed by Limb and Nadir Satellite Sensors” by J. Luo et al.|
In general, the authors have done a good job of responding to referee comments. The manuscript has been substantially revised; in fact, in some places it has been almost entirely re-written. The writing and structural flow of the manuscript have been improved. In the process of editing to address reviewer comments, the authors have articulated a new emphasis on “process-based retrieval evaluation”, whereby, unlike in more traditional validation approaches, satellite measurements are assessed qualitatively through their dynamical consistency with meteorological fields (e.g., winds, GPH). The authors stress the value of the nadir sounders’ dense horizontal sampling, but in my view what is probably the most important result from this paper --- the observational evidence supporting the conceptual model of the preferred ASM vertical transport pathway --- was enabled more by the ability of IASI to discriminate variability in the upper troposphere from that in the lower/middle troposphere than by its horizontal resolution.
Because of the extensive nature of the revisions, I have more comments on this draft than is typical for a re-review. However, the vast majority of them should be straightforward to address.
I still object to the way the vertical information content of the nadir sounder measurements is characterized. IASI data may be reported on 19 surfaces, but as George et al.  point out, the number of retrieval altitude levels is not representative of the vertical resolution. The statement is made (P5, L16-19): “Averaging kernels for layers centred at 13.5, 14.5, and 15.5 km are highlighted in the figure, because these layers are relevant to the analysis, which is focused on the 150 hPa pressure level. This level is contributed mostly by IASI CO product layer centred at 14.5 km and with a small fraction from the 13.5 km and 15.5 km layers.” I do not see the basis for this statement in Figure 1b. Layers from ~8 to ~18 km seem to contribute almost equally to the value reported at 14.5 km. That IASI cannot discriminate between 150 and 200 hPa is also clearly demonstrated in Figure 4 and accompanying discussion. Although the authors do acknowledge that IASI has maximum sensitivity in the middle troposphere (P5, L23-25), the language they employ could be misleading to many readers, especially those who do not go through the data description subsection carefully. Figure 1b shows that it is simply NOT possible to “focus on the 150 hPa level” with IASI CO data. Thus, the authors may choose to analyze the IASI measurements reported on the 14.5 km level in the data file, but those values represent an average over a broad region of the upper troposphere and cannot be labelled “IASI CO at 150 hPa”, as is done in numerous places throughout the manuscript (e.g., P6, L19; P7, L10, L13, L21, L26, L29, L32, L33; P9, L29; P10, L5, L19; P11, L28; P14, L11, L12, L15; P21, L3; P22, L2; P23, L4; P27, L1; P29, L1). A similar comment applies to IASI data “at 500 hPa” (P9, L29; P10, L7). I think it is fair to say that IASI can distinguish between the lower/middle and the upper troposphere, and that is how the data analyzed here should be referred to, not by association with a specific pressure level.
The same comment can be made for the characterization of OMI data. It is stated (P4, L35) that the “layer 18” OMI product is “comparable to the 100 hPa level”, and that the analysis focuses on “100 hPa OMI O3” data (e.g., P12, L4, L14, L33; P14, L26, L28; P32, L2, etc.). Given that the vertical resolution of OMI profiles near 100 hPa is 11-12 km [Liu et al., 2010b], such a characterization is not justified.
Although this is a significant issue, it is fundamentally a question of semantics that is easily rectified by explicitly noting that the IASI and OMI data represent averages over relatively thick (10-12 km) layers in the upper and lower/middle troposphere and appropriately labelling them as such in a consistent manner throughout the manuscript, rather than referring to them as 500, 150, or 100 hPa values.
Specific substantive comments:
* P3, L23: MLS O3 is not mentioned in this sentence, but the Livesey et al.  validation paper cited for MLS CO also covered UTLS O3.
* P4, L1-2: In the MLS Data Quality Document cited here, the accuracies of the v4 CO and O3 data are given as the RSS or the sum of the ppbv/ppmv and percentage uncertainties, respectively. That is, the systematic uncertainty for 147 hPa CO should be quoted as the RSS of 26 ppbv and 30%, and for 100 hPa O3 it should be quoted as the sum of 5 ppbv + 7%. The multiplicative terms are probably not negligible even for O3, and certainly not for CO in the ASM anticyclone. In addition, it would be good to add “single-profile” in front of “precision” in these lines, since the precision is substantially improved when profiles are averaged together.
* P6, L26-31: The issues with IASI data over elevated terrain, discussed later in the manuscript, should probably be mentioned here as well as a possible factor in explaining some of the IASI/MLS differences. In addition, it might be good to clarify that the “missing data in IASI” (L27) arise because of cloud contamination.
* P7, L33-34: “… the IASI 150 hPa product is significantly contributed by the atmosphere at lower UT levels, including the 200 hPa range”. While this is no doubt true, based on Figure 1b it seems possible that some of the features seen in the IASI upper tropospheric seasonal map could have come from even lower in the atmosphere. It would be good to show the IASI JJA map for the lower/middle troposphere in Figure 4 as well. This is done for a daily map in Figure 7, but since the presence of a systematic vertical tilt in the chemical structure of the anticyclone is being argued on the basis of these seasonal maps, a plot representing the lower layer average might be illuminating.
* P8, L30: It is stated (here and in the caption to Figure 6) that daily means are calculated over the longitude range 0-220E. But the study domain was previously characterized as extending over 0-180E (P5, L6), and the Hovmoller plots only cover that hemisphere as well, so it raises the question of why the daily means are calculated over a broader region.
* P9, L12-14: A reference (Pan et al. ?) would be appropriate for this mention of the modeling analysis.
* P9, L32-33: “… effective test whether the retrieval sensitivity is sufficient to resolve independent CO variability”. This statement should be qualified. The comparison represents an effective test of whether IASI’s sensitivity is sufficient to resolve independent upper and lower/middle tropospheric variability.
* P11, L9-11: The weaker CO enhancement over the Tibetan Plateau is attributed to the impact of the elevated terrain on the retrieval. However, the enhancement in Fig. 9a is weak only over part of the Plateau area -- it is relatively strong around 29-30N.
* P11, L13: “the enhanced layers are centered near 150 hPa and vertically extended between 100 and 200 hPa”. Again, IASI cannot really discriminate between these levels. Given the “smoothing error” in the retrieval, it is not possible to quantify the true vertical extent of this feature.
* P11, L12-15: I am a little confused by the discussion of CO enhancement over the Western Pacific High being associated with strong easterlies in the case of Fig. 9b (though not in the case of Fig. 9d). Here and previously (P8), this feature is attributed to eastward eddy shedding, which seems inconsistent with the presence of strong easterlies. Some elaboration of this point would be helpful.
* P12, L8: The previous work of Park et al. [2007, Fig. 9] is described as analyzing MLS 100 hPa CO, but it would be more appropriate here (in a paragraph about O3) to say that Park et al. analyzed the MLS 100 hPa CO-O3 relationship.
* P12, L10: In addition to its relatively coarse vertical resolution, it might be appropriate to mention the potential impacts of clouds and terrain on the OMI data, as discussed on P13.
* P12, L23-24: The weaker signature of in-mixing of stratospheric air in the OMI data is attributed to averaging of variable fine-scale structure in the seasonal map. But it seems to me that it is probably more related to the coarse vertical resolution of the OMI measurements, which effectively smears out the signature of this relatively shallow layer.
* P14, L9-10: The weaker correlation between IASI CO and GPH is largely attributed to the effects of elevated terrain, specifically the Tibetan Plateau, on the retrieval. But I do not believe that the relative contribution of all possible factors was quantified in this work. Couldn’t the coarser vertical resolution and cloud effects (missing data) also have played a role?
* P14, L12-13: “the IASI 150 hPa data include contributions from the level below”. Again, this wording gives the impression that a much narrower layer is influencing the IASI measurements at 150 hPa than is actually the case.
* P14, L32: It is stated that the analysis shows that IASI data have sufficient information content “to resolve upper tropospheric CO variability”. I think it would be clearer and more accurate to employ wording similar to that in the abstract (P1, L24-25). I suggest something along the lines of: “to discriminate upper tropospheric CO variability from that in the lower to middle troposphere”.
* P15, L5-6: It is stated that “Although the retrieval has fewer degrees of freedom for each profile …”. I think it would be clearer and more accurate to say “Although the retrieval has limited vertical resolution and is degraded over elevated terrain, …”.
* P26, Figure 7: Are GFS winds at 150 hPa overlaid on both the MLS and IASI upper tropospheric CO maps? I expected the wind vectors to be the same in both panels, but close examination reveals small differences. Are the meteorological fields also gridded differently (as the satellite data are)?
* P29, Figure 10: Similarly, I expected the 150 hPa GPH fields (colored contours) to be identical between Figures 6 and 10, but there are small differences between them.
Minor points of clarification and suggested wording / figure changes (leaving most typos and grammar points to the journal copy-editors):
* P2, L1: I think that “demonstrate the value of” would work better here than “advocate for the use of”
* P2, L10: “composition … displays”
* P3, L18: “variabilities” --> “variability”
* P3, L21: “weak” does not seem like the right word for “resolution”; I suggest “poor” or “coarse” here instead
* P3, L27: “for using … diagnosis” --> “to use … diagnostic”
*P4, L17: delete “works of”
* P5, L9: “no IASI retrieval product once the cloud is greater than 25% in the pixel” is somewhat awkward. I think that something like “no IASI products are available if the cloud fraction in the pixel exceeds 25%” would sound better.
* P6, L20: “rational” --> “rationale”
* P7, L35: delete “are”
* P9, L23: “is almost always referred” --> “almost always refers”
* P10, L14: I think “degraded” would work better here than “weakened”
* P10, L21: “maps” --> “values”
* P10, L34: “over the Iranian mode” --> “over the Iranian Plateau”
* P11, L9-11: “weakening” --> “degrading”
* P12, L15-16: “anticyclonic flow over the ASM” would be better as “anticyclonic flow over the ASM region” or “anticyclonic flow associated with the ASM”
* P12, L17: “interception” should be “intersection”. The same comment applies to P13, L4 and P30, L5
* P13, L5: The text states that Gaussian smoothing is applied to the 1 x 1 degree maps (i.e., OMI), but the figure caption states that smoothing is applied to all maps
* P13, L14: Just to be clear, it would be good to add “in OMI O3” after “structure”
* P13, L16-17: “the weaker vertical resolution for this potentially shallow layer in OMI may contribute” --> “the coarser vertical resolution of OMI for resolving this shallow layer may contribute”
* P13, L32: “surface elevation on retrieval” --> “surface elevation on the OMI retrieval”
* P14, L4: “limb data” --> “higher vertical resolution limb data”
* P14, L9: “the season studied” --> “the 2008 JJA season studied”
* P14, L28-29: “model-based conceptual model” --> “conceptual model”
* P20, Figure 1a: the continent outlines are hard to see. Perhaps it would help to thicken the lines, or use a different color.
* P20, Figure 1b: It would be good to add a vertical line marking the zero line.
* P26, Figure 7: please either re-draw this figure so that panel (a) is at the top, as readers expect and is the case in all other multi-panel figures in this manuscript, or simply label the bottom panel as (c) and alter the references to this figure in the text accordingly.
* P26, L4: “terrains are” --> “terrain is”
* P28, Figure 9 caption: The various overlays, which presumably represent the westerly and easterly jets, tropopause height, theta surfaces, and wind vectors, all need to be defined.
* P32, Figure 13 caption: “tropopause pressure contour” --> “tropopause pressure contour (black)”. Also, although the Tibetan Plateau is clearly marked on other figures, it is pretty hard to see here. Perhaps a different color could be used (or perhaps pink could be used for the tropopause pressure, as in Figure 11, and then black could be used for the TP).