Sea Ice Freeboard in the Ross Sea from Airborne Altimetry IcePod 2016–2017 and a Comparison with IceBridge 2013 and ICESat 2003–2008

Round 1

Reviewer 1 Report

Review of Tain et al. Remote Sensing

 

This paper focused on using elevation measurements of sea ice from ICESat, IceBridge and ICEPOD lidars in the Ross Sea. Since these measurements were all taken at different times this discussion was focused around the different interannual variability on these ICEPOD tracks. They claimed that the sea ice near the coasts were more deformed and thus thicker than the thin flat ice that was produced by coastal polynyas and katabatic winds off of the ice shelf. I think this study could be improved significantly from some of my suggestions below and that afterwards it might be suitable for publication.

 

General comment: I think just the freeboards should be compared here and the ice thickness should not be relevant in this paper. There are too many uncertainties and issues involved with determining the snow thickness on the sea ice, especially in areas of high deformation and this linear method that you are using is not telling us any new information from just what the freeboard estimates are telling us.

 

General Comment: I think that more research should be put into looking at the size, and potential sea ice production from the coastal polynyas to back up why the changes in freeboard from 1 year to the next might be causing these changes. Perhaps wind speed and direction could be used to contrast the years along with sea ice concentration data to determine the average size of the polynya from one year to the next. This would help improve the scientific importance of this paper.

 

Introduction: I would like to see more information here as to why we care about the interannual variability of the sea ice thickness in the Ross sea and more discussion of the Ross sea polynya, etc. The introduction is lacking in the appropriate background information and explanation of the importance of sea ice thickness and variability.

 

Line 41: It might be good in this paragraph to explain or at least list the instrumentation on the ICEPod flights, since you do this for OIB.

 

Line 69: Should be ‘that extend’…

 

Line 70: “Track” or ‘track’ please pick one and make it consistent throughout the paper.

 

Line 70 onwards: Please reword these sentences describing the sea ice conditions along the flight tracks. It is a bit muddled and is confusing.

 

Figure 1 might benefit from including the OIB flight tracks as well as the location of the ICESat RGTs used in this study.

 

Paragraph on line 121: I am confused, if you are using the DTU mean SSH data for the ICEPod data, then why are you also using the lowest 2% of elevations from ICESat for sea surface height in the IcePod calculations. Please explain.

 

Line 132: Please spell out ‘NB’. Also I would begin the sentence with something like this: “In order to derive the sea ice thickness from our freeboard measurements, we use the following empherical…..”. Otherwise this sentence and paragraph kind of come out of nowhere. Why not just compare freeboards between the 3 different products instead of changing it into ice thickness since this is just a linear relationship? There is a lot of uncertainty in the snow thickness on Antarctic sea ice, and I am not sure that a linear relationship to go from freeboard to ice thickness is accurate enough or appropriate.

 

Line 150 and subsequent paragraphs: A figure showing the distributions or histograms of the thicknesses of each track for each year would be beneficial to go along with the text here. --- it appears that you have a figure to address these distributions.. perhaps reference figure 4 in these paragraphs.. thanks.

 

Figure 3: What are the y-axis ice freeboard/thickness values for track 1 and 3?

 

Line 185: Here you begin with ‘IceBridge’ and before you have used “OIB”. Please pick one and be consistent.

 

Discussion beginning on line 221: It would be good to compare and contrast polynya activity in the ross sea in the ICESat, OIB and ICEPod years, and perhaps average wind speeds/directions to back up these claims and to add more scientific interest to this study.

 

Line 266: Sea surface height should be ‘SSH’.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper is well organized and easy to follow. The results are well presented.

 

However, there are two major concerns from the reviewer.

 

First, I think more details about how the data were processed are required in the methodology. Illustrations about the transferring of datum or the influence from the atmospheric parameters might be useful for the readers to better understand the methods.

 

Second, more evaluations about the uncertainty of each data source (e.g. IcePod data collected in different missions and the ICESat data) would be useful to know the magnitude of the ice thickness change.

Author Response

Comments and Suggestions for Authors

The paper is well organized and easy to follow. The results are well presented.

 

However, there are two major concerns from the reviewer.

 

First, I think more details about how the data were processed are required in the methodology. Illustrations about the transferring of datum or the influence from the atmospheric parameters might be useful for the readers to better understand the methods.

Response:

Thanks. Added. ( Lines 127-139)

“3.1 Preprocessing of OIB and IcePod Dataset

To make the results comparable among the three datasets, we resample the IcePod lidar data into 70 m pixels (by averaging 70 m width by 70 m along track window), since it is comparable to the footprint size of OIB ATM L2 (80 m width by ~60 m along track) and ICESat data resolution (~70 m). Laser altimetry detects the sea surface height (SSH). Both OIB and IcePod lidar data are providing SSH in meters above the WGS1984 ellipsoid. SSH is influenced by geoid, tidal forces, atmospheric pressure, and ocean dynamic topography [29].  The DTU15 Mean Sea Surface (MSS) released from DTU (Technical University of Denmark) is a global, high resolution MSS with a resolution of 1 minute by 1 minute [30]. The DTU15 MSS comprises geoid undulation and ocean mean dynamic topography, which are the largest variations of SSH. The preprocessing of OIB and IcePod dataset are to remove those MSS influences from SSH. We get Sea Surface Height Anomalies (SSHAs) by using SSHs minus the DTU15 MSS. SSHAs are used to calculate freeboard detailed below.”

 

Second, more evaluations about the uncertainty of each data source (e.g. IcePod data collected in different missions and the ICESat data) would be useful to know the magnitude of the ice thickness change.

Response:

Thanks. Added.( Lines 260-269)

“Because of the lack of field measurements and the complexity of snow depth in the Ross Sea, it is hard to quantitate the uncertainties. The uncertainties of total freeboard are mainly caused by the different instruments used among ICESat, OIB, and IcePod and the retrieval method used in this study: 2% lowest elevations method to derive local sea level. ICESat’s single shot elevation precision over smooth flat surfaces is <3 cm and over ocean elevation has 10.0 ± 1.0 cm bias [39]. OIB ATM system has 6.6 cm vertical accuracy and 3 cm vertical precision [40]. IcePod has 25 mm range accuracy. To reduce the discrepancy caused by the three instruments, we resample the IcePod lidar data into 70 m pixels, which it is comparable to the footprint size of OIB ATM L2 (80 m width by ~60 m along track) and ICESat data resolution (~70 m).”

Reviewer 3 Report

The paper represents a study about the change of thinness and freeboard of some sea ice areas in the Antarctic using data of three LiDAR instruments: ICESat/ATM (2003-2008), IceBridge (2013) and IcePOD (2016-2017). The paper is well written, as the context of the research has been given, and the approach and results have been detailed and well explained.

However, it is not obvious to draw a general conclusion from the output of this study because the dataset is relatively small and other important factors such as surface roughness and uncertainty of measurements/instruments have not been considered.

Other comments:

 

18 and 19: ..25km(50km), please explain

26: please give a number (ex: percentage of change)

90: 1064nm or ATM/532nm?

95: Please give the dates/period of the LiDAR data not just years

111: It will be nice to give a figure showing the footprint and track of these three LiDAR instruments side by side

128: data/approach to determine the local sea level?

129: please cite your reference for the relationship between local sea-level, freeboard and elevation

Figure 2: As thickness is linearly proportional to freeboard, I think you can plot only one variable (tackiness or freebraod) but with keeping two y-axis  

 

I do recommend the publication of this paper

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

All the concerns from the reviewers have been addressed.