the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Assessing the Ducting Phenomenon and its Impact on GNSS Radio Occultation Refractivity Retrievals over the Northeast Pacific Ocean using Radiosondes and Global Reanalysis
Thomas E. Winning
Feiqin Xie
Kevin J. Nelson
Abstract. In this study, high-resolution radiosondes from the MAGIC field campaign and ERA5 global reanalysis data are used to assess the elevated ducting layer characteristics along the transect over the northeastern Pacific Ocean from Los Angeles, California to Honolulu, Hawaii. The height of the planetary boundary layer (PBLH) increases as the strength of the refractivity gradient and resultant ducting decrease from east to west across the analysis transect. The thickness of the ducting layer remains remarkably consistent (∼110 m) in the radiosonde data. On the other hand, the ERA5 generally resolves the ducting features well but underestimates the ducting height and strength especially over the trade cumulus region near Hawaii. A simple two-step end-to-end simulation is used to evaluate the impact of the elevated ducting layer on RO refractivity retrievals. A systematic negative refractivity bias (N-bias) below the ducting layer is observed throughout the transect, peaking approximately 70 meters below the PBL height (−5.42 %), and gradually decreasing towards the surface (−0.5 %). Further, the underestimation of the N-bias in the ERA5 data increases in magnitude westward and while the correlation of the N-bias with the minimum gradient and sharpness are all strong; there is no evidence of zonal dependence.
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Thomas E. Winning et al.
Status: open (until 05 Oct 2023)
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RC1: 'Comment on amt-2023-150', Anonymous Referee #1, 12 Sep 2023
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The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-150/amt-2023-150-RC1-supplement.pdf
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RC2: 'Review of amt-2023-150', Anonymous Referee #2, 14 Sep 2023
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General comments:
Using high-resolution radiosondes from the MAGIC field campaign, the planetary boundary layer height (PBLH) along the transect between California and Hawaii are derived and discussed. In particular it is investigated how radio occultation data retrieval would underestimate the true refractivity below the PBLH, given perfect measurements (if radio occultation data had the very high vertical resolution of the radiosondes), when there is ducting. Results are compared to similar results obtained by using ERA5 data. In both cases (radiosondes and ERA5) data are forward-modelled and inverted in end-to-end simulations to obtain the underestimated refractivity profiles.
I find the study interesting and worthy publication, but it is a bit unclear to me what is new and what has been found before. Has it been found before that the PBLH increases along the MAGIC transect from west to east? Or is this a new result? I suppose it is new to perform end-to-end simulations to study the ducting and PBLH variations along this transect and how it would underestimate radio occultation retrievals in this area. However, it is only in principle, because in practice the radio occultation vertical resolution is somewhat coarser than the resolution of the MAGIC radiosondes. I think this needs to be mentioned.
For the same reason as above, and because the study does not actually present radio occultation data, I suggest to insert 'Potential' in front of 'Impact' in the title.
Throughout the manuscript the authors refer to their results being a 'climatology'. I think that is a misuse of the term 'climatology'. There is only one year of data which is averaged in longitude bins along the transect without taking seasonal variations into account. I wouldn't consider that a climatology. I suggest not to call it 'ducting climatology', or 'N-bias climatology', etc. Most of the results are statistics based on that one year of data, so in most places 'climatology' could be replaced by 'statistics', or just removed.
I think the paper could be shortened by taking out some paragraphs (see specifics below), and perhaps even by taking out all of section 3.3.3 that describes some rather obvious correlations. They seem to be of little importance. In my opinion, the paper would be better (and the rest of the study is sufficient) without section 3.3.3.
I find that parts of the manuscript are badly written, and there are several mistakes in the figure captions (see specifics below). In some parts of the manuscript, almost every sentence needs revision.
Specific comments:lines 137-139: It is not clear how the root mean square error of the refractivity gradient profile is calculated. Is it really an 'error'? Compared to what? Over which vertical interval? Please clarify. Please also write in the text what each of the symbols in eq. 2 stand for. Why are you writing X when it is N (I think)?
Fig. 2: Please provide information about latitude, date, and time, for this example.
Text before Fig. 2: Please provide information on how the gradients in Fig. 2 were calculated. In the ERA5 refractivity gradient there are sawtooth-like features just above the minimum gradient (and elsewhere in the profile to a lesser degree), and levels seem very unevenly spaced. Are these features real (in the model) or an artifact of how the gradients are calculated?
line 241-243: "This results in a sharper refractivity gradient caused by the frequent residual layer (below 1 km) as compared to the actual PBLH near 2 km.". How do you know which one is the 'actual PBLH' when there are such residual layers with apparently sharper gradients? What do you consider to be the definition of the PBL in this study? Please discuss this in the text.
Fig. 5b: How did you calculate such narrow ducting thicknesses for ERA5, in particular in the western part of the transect? The median thicknesses are between 50 and 100 m in the western part, while the ducting height is within 1-2 km where there are only 8 levels in ERA5 (noted in line 115). Please provide more detailed information on the calculation of the ducting thickness.
line 297: Why 'median' here? It is individual cases in Fig. 6, right?
line 320-326: I think the discussion about the small difference between the PBLH and the maximum N-bias is a bit academic. The exact size probably depends on the particular method of calculating the PBLH and the end-to-end simulations, including the smoothing that is involved. Is it important? The differences are seen later in Fig. 8, which seems sufficient.
line 327-331: It seems that this is discussing results shown later in Fig. 8. I suggest to move this text to section 3.3.2.
Table 1: The median numbers here are a bit off from the numbers discussed in the text (end of section 3.3.1). Please revise either the text or the numbers in the table.
line 348-349: I suppose you are discussing the ERA5 data here, but it should be made more clear.
line 374-379: I didn't quite understand this paragraph. For example, I don't understand that a "much higher ducting height and larger variation leads to smoothed and much smaller median N-gradient values". Why are you using the word 'leads'? Does the former cause the latter? I think you are trying to say that without the normalization the N-bias would be smaller than it is with normalization, but it is not clear. Please clarify and revise this paragraph.
line 422-426: I don't understand the sentence: "... it is interesting to note that the difference in the correlation of the radiosonde (−0.83) and the ERA5 (−0.84) does not lie in the observations with the larger magnitude peak N- bias, but in those closer to zero as the radiosonde data clearly centers below the regression line and trends above while the ERA5 with peak N-bias less than 5% are centered around the regression line.". Are you talking about the very small difference between 0.83 and 0.84? I can't see how you can conclude that this difference comes from the data with small maximum N-bias. In any case I don't think it is important. Is the sentence necessary? Please clarify if it is.
line 434: I suggest to replace 'climatology and the impact of' with 'and the'. The study did not investigate the ducting climatology since there was only one year of data. The study did not investigate the impact of the biases (impact on what?). The sentence also needs to clarify that it is in relation to radio occultation retrievals that there would be biases. Please revise.
line 438: I don't understand 'at a well-defined PBL throughout the transect' in this sentence? Could it be removed?
line 458-459: I don't think this is correct: "While this segment of the transect also coincides with a better sampling rate for the ERA5 data (~40 m vertical resolution)". Isn't the resolution of ERA5 the same throughout the transect? Maybe you mean that because the PBLH is lower in the eastern part, the ERA5 vertical resolution around the height of the ducting layer is higher in the eastern part, but it is not clear. Please clarify.
line 462-464: I don't think you can conclude that the differences that you see between the radiosondes and ERA5 are due to the 'limited number of model levels in ERA5 near 2 km'. There is no investigation of the impact of the lower resolution in this study. In principle, ERA5 could be underestimating the heights for other reasons. Please be more moderate in the conclusions.
line 465-468: I think this 'future study' paragraph should be removed. It does not belong in a conclusions section, and there is no need for it.
Technical corrections:line 21-22: I think either the 'and' in line 21 should be replaced with a comma, or the ';' in line 22 should be a comma. Maybe correlation should be plural. Please revise sentence.
line 32-36: I don't think you need 'etc' in line 36 when you have 'such as' in line 32.
line 111-112: Maybe it should be 'reanalysis', not 'Reanalysis' in line 111. I think there is no need for 'reanalysis' in line 112, as it is already part of the ERA5 acronym.
line 126: I don't understand "the minimum refractivity describes the largest magnitude value." Please revise the sentence.
line 159 and Fig. 2: I think it should be 10 x N-units (not 1/10). Like with m and km, if you plot something as a function of height/1000, where height is in m, the axis unit becomes km (1000 x m).
Fig. 2: I suppose T is in degree Celsius here (not kelvin).
line 167-168: I think it should be 'a residual layer' instead of 'the residual layer'. There has been no mention of this layer earlier in the text. Something is not right with line 168, maybe an 'and' is missing. Please revise.
line 180: '1-dimensional' instead of '1-dimentional'
line 184: I think it should be 'increases' instead of 'decreases' (if it is 'with height' as written).
line 194-195: The word 'respectively' is used here to describe what is in Fig. 3a and 3e, but it is used wrongly. What is seen in the two figures are refractivity profiles from the radiosonde and the ERA5 data, respectively. It is not the input refractivity profile and corresponding Abel refractivity retrieval, respectively. Please revise.
Fig 3 caption: I think "10 km" should be "4 km" and "minimum gradient" should be "refractivity gradient". The last sentence could be revised to be more precise, for example: "The same is shown in panels e-h for the co-located ERA5 profile".
line 211: "Out of a total of 583 ..., quality control has been implemented ...". I think I understand what you want to say, but literally it makes little sense. Please revise the sentence.
Fig. 4 caption: I believe b) and c) should be interchanged (also in lines 232-233). It seems that the MAD error bars are dotted for both radiosondes and ERA5, whereas it is the lines connecting the points that are dashed or dot-dashed. Please revise.
Reference to figures: Often references to figures are made in parentheses, in particular for Fig. 5 in Section 3.2, but also elsewhere. Probably references to figures should be made in text (without parentheses) at least the first time around (I am not sure what the AMT guidelines say).
Fig. 5b: This panel has a different x-axis coverage than the other tree panels. Please adjust.
Fig. 5 caption: I suppose it should not be 'error bars' in '(median in blue diamond and dot-dashed error bars)'. Please revise.
line 315: Should the 'e.g.' be 'i.e.'?
line 318: Should 'between' be 'of'?
line 327-331: I don't understand what is meant by 'favors the radiosonde data' here. Could it be written differently? There are ending parentheses without beginning parentheses in this paragraph. Please revise.
line 344-346: This sentence does not make sense to me: "The radiosonde N-bias variation shows a minimum magnitude of near the center of the transect and two of the largest magnitude difference values of as the bookends while the ERA5 N-bias values have a larger range but peak values (−5.41% to −6.23%) in the three bins closest to California". Could it be written differently?
line 440: I suggest to use 'California', 'Hawaii' and 'refractivity' throughout the abstract instead of 'CA', 'HI', and 'N'.
line 450-453: Correlation between the PBLH and the height of the maximum N-bias is mentioned twice. Please revise.
line 454: Past tense is used here, whereas the next sentence is in present tense. Please be consistent.
line 455: I suggest to say 'opposite' instead of 'reverse'.
line 538: doi.org/10.1175/HTECH-D-19-0206.1 is wrong. It should be 'JTECH'.
Citation: https://doi.org/10.5194/amt-2023-150-RC2
Thomas E. Winning et al.
Thomas E. Winning et al.
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