Refining the Resolution of DUACS Along-Track Level-3 Sea Level Altimetry Products

Round 1

Reviewer 1 Report

This manuscript provides a valuable assessment of recent updates to altimeter processing, as applied to the DUACS product.  Updated processes that have been published elsewhere are applied both for SAR mode data (here tested on Sentinel-3) and from Low-Resolution Mode data (here tested on Jason-3).  The authors show that by moving to 5-Hz products, they are able to obtain measurements closer to the coast with lower overall noise than was obtained from older 1-Hz products.  Overall this is a valuable contribution that will be helpful to readers.

There are a few issues that the authors should address prior to publication.

1) Overall the analysis is thorough and complete, but the figures lack uncertainty estimates, even in places where this information should be readily available.  It would be helpful for readers to have access to uncertainties for Figure 5, for the stars in Figures 6 and 7, and for the currents in Figure 9.  Uncertainty estimates will make it easier for readers to quantify the differences between the cases that are examined in this study.

2) line 211.  DT-2018 needs to be defined.  The sentence is written as if to imply that DT-2018 is a tide correction, which is incorrect, and which doesn't make sense based on Table 1 and the discussion of FES2014 in the next paragraph.

3) lines 403-404.  Higher validity in coastal areas is not obvious from Figure 4.  Additional statistics would be helpful to make the case.  Is it possible to quantify the fraction of valid data within a certain distance of the coast?  In addition, the phrase "notwithstanding the high rejection rates there" is worded in a confusing way, coming on the heels of saying that the data availability is higher in the 5 Hz product in coastal areas.  There's not really a reason to think that high rejection rates in coastal areas would make the 5 Hz product worse.

4) line 605 and Figure 7 caption.  It would be helpful if the colors of the stars matched the colors of the spectra.  As written it may be unclear to readers that the line colors don't match the star colors.

5) line 612.  "SNR=1".  This could be explained more carefully.  The cutoffs appear to come from the intersections of the straight lines, but the fits aren't great, and readers may need more guidance to understand the logic behind these scales.

6) line 751. "perfectly captures":  What evidence shows that this is perfect?  It would be advisable to show uncertainties on the current estimates and to plot any available in situ observations.

7) lines 751-753.  "The core current is located between ~15 and 40km from the coast along the J3 track #222 and S3A track # 741.":  Is this information inferred from the figure or is it information previously obtained from in situ observations?

8) Typos and questions of grammar and style

line 15.  "0cean" should be "Ocean".  The leading zero will be a problem for search engines as well as confusing readers.

line 16.  "Km" --> "km"

line 17.  "aperture"  --> "Aperture"

line 19.  I would hyphenate "Low-Resolution" since it is used as an adjective before "processing techniques"

line 33.  "an"  --> "a" AND "cross-over" --> "cross-overs"

line 37.  "outputs" --> "output"

line 54.  "these evolutions"  --> "these advances" (or else "this evolution")

line 56.  I don't think you need the word "different".

line 56.  "small to sub mesoscale sea":  Specific scales would be useful here.  The wording is unclear, and "small" is particularly non-specific.

lines 59-61.  Revise to "Because altimeter measurements allow synoptic monitoring of the signal over the global ocean, including coasts, they are fundamental for both sub-mesoscale and coastal applications."

line 61.  I think that "thus" can be removed without loss of meaning.

line 62.  "being able to accurately process the signal"  --> "accurately processing the signal"

line 64.  "large swath":  I think "large" is the wrong word here, and the authors might have intended to side "wide swath".  But SWOT doesn't have a particularly wide swath, so a better word choice might be "swath".

line 72.  "and predominance"  --> "and the predominance"

line 73.  "the altimeter"  --> "altimeter"

line 141.  "introduced" --> "have been introduced"

line 147, and everywhere else "quite" is used.  The adverb "quite" can mean either "entirely" or "slightly", which are more or less opposites.  That makes "quite" effectively meaningless in written language.  (In an oral presentation, a speaker might be able to convey meaning effectively, but readers will never grasp the right inflection....)  So it's best to remove it everywhere.  Adverbs rarely strengthen writing.

line 186. "noises" --> "noise"

line 372.  "Indeed, the"  --> "The"

line 382.  "much" can't be used this way.  A better word choice might be "significantly" or possibly "greatly"

line 389.  "doppler"  --> "Doppler"

lines 394-395.  "measurements available"  --> "available measurements"

line 413.  "Norther Spanish" --> "northern Spanish".  There's no reason to capitalize cardinal directions unless they refer to specific place names.

line 434.  "the20Hz"  --> "the 20Hz"

lines 453-454.  "care when considering the data within ~5km of the
coast."  --> "care within ~5km of the coast."

line 458.  "methodology applied" --> "methodology"

line 483.  "wintertime, notwithstanding"

line 483.  "correction applied" --> "correction"

line 497.  "we are comparing" --> "we compare"

line 502. "region considered"  --> "region"

line 528.  "forbade"  --> "prevented"

line 558.  "results obtained" --> "results"

lines 566-567.  "the one representative"  --> "the slope"

line 567.  Remove "indeed"

line 573 and line 733.  Change "On the contrary" to "In contrast".  Guidance from https://www.dorisandbertie.com/goodcopybadcopy/on-the-contrary-or-in-contrast:  'So remember, “on the contrary” introduces a statement that contradicts the previous point. And it can be replaced with something like “that statement I've just written is wrong!” But “In contrast” introduces a statement that simply contrasts with or provides a juxtaposition to the previous point.'

lines 574 and 575.  Remove "reached"

line 591.  Remove "however" (in addition to removing "quite")

line 592.  "In that case" is ambiguous.  Does it mean "In winter,"?

line 662.  "additional"  --> "an additional"

line 663.  "Northern Sea"  --> North Sea"

line 687.  "1Hz" --> "the 1Hz"

line 687.  "or" --> "and"

line 717.  "region considered" --> "region"

line 721.   "rms of the differences." --> "rms differences."

line 754.  "rather between" --> "between"

line 760.  "These observations range the values" --> "These observations encompass the range of values"

line 760.  "measurements.[73]" --> "measurements. [73]"      [missing space]

line 762.  "West" --> "west"

line 763.  "South" --> "south"

line 765.  "high resolution" --> "high-resolution"

line 768.  "one" --> "intensity"

line 774.  "1Hz product" --> "the 1Hz product"

lines 774-775.  "however"  --> "albeit"

line 777.  "5Hz product" --> "the 5Hz product"

line 778.  "vein" --> "core"

line 793.  "this last season" --> "summer"

line 799.  "over the year considered" --> "over 2018"

line 822.  "Indeed, quality" --> "Quality"

line 837.  "is" --> "are"

line 838.  "North East" --> "northeast"

line 872.  "veins" --> "cores"

line 882.  "that that" --> "that"

line 904.  "contaminated)" --> "contaminated),"

line 969, Figure 6 caption.  Spell out FMA and ASO for readers who start with the figures.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

Review Comments:

This paper “Refining the resolution of DUACS along track (level 3) altimeter Sea Level products” written by Pujol  et al. describes the algorithm for improving DUACAS level3 altimeter sea level products toward fine resolution (5HZ) and higher accuracy over North Atlantic Ocean. This paper reviewed the algorithm for the current operational 1HZ level3 products and details the algorithm difference/improvements for 5HZ products, and compared the consistency with tidal gauge datasets. Finally, the capability of the new products in resolving the finer resolution coastal current (North Current) and improvements in data assimilation with high resolution models are discussed.  I think this paper is well written and organized. The high resolution products are interested to AVISO users for studying finer scale and near coast ocean processes.  This paper is well written and the logic is scientifically sound.  I recommended it to be published at Remote Sensing with minor corrections, so that the new regional high resolution Sea Level Anomalies products can be prompted to more users, and the improved algorithms can be used/referenced for other satellite missions, such as SWOT.  

Minor Comments:

Line 33: Change ‘The demonstration L3 product’ to ‘The new L3 product’ or similar phrase.

Line 46: it is more appropriate to say ‘for ocean applications, climate monitoring, geophysical and biological studies’.

Line 53:  Change ‘processing and corrections’ to ‘processing algorithms and improvements’.

Line 109, what’s the actual along-track spatial resolution for this product?   Should also mention which altimeters you use for this L3 products.

Line 116,  L3 altimeter or L3 product? High sampling or high rate sampling? Please rephrase.

Line 119, Do you really mean that the methods are the same for 1HZ and full rate processing?

Line 122, ‘in in [4]’, duplicated word ‘in’.   

Line 122, Logically I can not follow. By saying ‘review’, do  you mean ‘reviewed in this paper’ or  ’ updated’. Please revise Line 116-133 to make the paragraph more concise and clear.

Line 139, If this is for L2 upstream products, what standards are used for L3 products?

Line 144, It would be better to say ‘In order to improve … observation accuracy’.  

Line 153, ‘as is ‘ or ‘so is’.

Figure 1 The dashed lines are hard to follow what values they point to, please rearrange the top labels.

 Line 237 to 249, Acronym of ‘Hybrid Mean Profile’  is HMP not HPM, which appears several times in later context.

Line 371, what prevents the validation for other missions?

Line 378, I do not understand what the difference between this DT-2018 standard and GDR-E standards in table-I.  Given the  previous context, the readers are easily to think the 1HZ products uses GDR-E standards. Please explain.

Figure 4, the data product is 5HZ L3, but you showed comparison among 20HZ and 1HZ. Please explain.

Line 409, ‘when’ should be ‘while’.

Line 426, ‘is reaches’ should be ‘it reaches’.

Line 431 to 432, Are you sure that what you say are all about 1HZ products?  

Figure 5, how to explain the abrupt increase of variance  of S3A near 55-60km toward coast?

Line 559/561, spell out QG.  What is SQG?

Figure8， suggest to label the places mentioned in the paper, e.g. those appearing around line 688. Where are the Iberian coast and the Ligurian coast?

Line 737, Why do the authors want to use  this product: EALEVEL_EUR_PHY_L3_REP_OBSERVATIONS_008_061?  What’s the difference between this one and the 1 HZ products mentioned in the previous context?

Line 831, need  a period ‘.’ before ‘Improved SSB and HFA..’

Line 883, ‘Karin’ should be ‘KaRIn’.

In Summary, there should d be some discussion why the products are only for the area of North Atlantic region. What’s the difficulty to extend to global area or other regions?   

Author Response

Please see the attachment

Author Response File: Author Response.docx