Dear authors,

Many thanks for your revised submission, which I think is much improved. However, some issues still need to be addresssed, as you will see in the referee reports.

I strongly agree with referee #3 comment that the discussion on the relationship between Arctic lead fraction, thermodynamics and ice dynamics is to superficial (this point was also raised by referee #1's report on the initial submission). I suggest to either remove section 5.3 (possible integrating some of the text in other parts of the manuscript), or provide a more thorough analysis. In case of the latter, I might contact an additional expert in this field to comment on your analysis.

In addition to the suggestions of the referees, I ask you to make the following changes:

1. on p. 25, remove "which implies that recent sea ice status has become more vulnerable to anomalous atmospheric and oceanic conditions.": given the short time span of the observations, this is speculative.
2. in section 5.4, on novelty and limitations, justify why your present method should be used rather than the method proposed Lee et al., 2016.
3. include a discussion on the difficulties in detecting leads in the summer season, as in your response to R3 on the P5L25 comment:

"Jun. to Sep. is generally considered as the melting season. In this season, the presence of leads as well as meltpond in sea ice are dominant. It is difficult to accurately distinguish leads from sea ice due to the fact that the waveform of the meltpond is quite similar to that of leads. Since the lead detection methods for the retrieval of sea ice thickness do not work well in the melting season, the sea ice thickness in the melting season is still unavailable (Tilling et al., 2017). More advanced lead detection methods need to be developed for the summer season with careful consideration of meltpond."

I'm aware that these difficulties are inherent to all current altimetry-based methods, and it is OK to mention this.

technical correction:
rephrase ""CryoSat-2 observations less than 5 are in the 10 km grid around the coast line of Arctic" to, for example, "less than 5 observations are found in the 10 km grids in the marginal zones of the Arctic ocean"

I'm looking forward to your revised version.

Regards,
Bert Wouters - TC/TCD Editor

Dear authors,

Many thanks for submitting the revised manuscript, which satisfactorily addresses the concerns of the referees. Before publication, a number of minor textual changes need to be made to improve readability of the manuscript. Please see my comments below.

Regards,
Bert Wouters - TC/TCD Editor

- - P2, L28: "The lead detection using the proposed waveform mixture algorithm does not need to change any parameters" -> "The proposed waveform mixture algorithm does not require changes in any of the parameters"

- P6 L14: 15th from Jan. to May and from Oct. to Dec. -> 15th day of the month for Jan. to May and Oct. to Dec.

- P7 L12-13. “However, such visual analysis cannot guarantee how the waveforms are quantitatively different by month and year.”
Unclear what you mean by guarantee, do you mean "such visual analysis cannot guarantee that the waveforms are quantitatively different by month and year.”, or "such visual analysis cannot quantify how the waveforms are different by month and year.”, or "such visual analysis cannot guarantee how the waveforms are quantitatively different by month and year", or ...

- page 14, L21-24: suggestion for textual changes, please check if this doesn't change the message you want to convey:

"Changes in the Arctic Ocean circulation have contributed to the change in the state of sea ice. The lead fraction along the coast of Northwestern Greenland in Figs. 6 and 7 is low because of the convergence of sea ice by two major circulations, as shown in Kwok (2015). Kwok et al. (2013) revealed that the currents speed of Beaufort Gyre and Transpolar Drift increased from 1982 to 2009, leading to a decrease in the fraction of multi-year ice."

- page 14, L24-25: "However, the increasing lead fraction from the years of 2011 to 2016 in this study was not seen due to the high inter-annual variability of lead fraction, particularly in the spring season (Figs. 6 and 7).":
This sentence is difficult to interpret, "the increasing lead fraction from the years of 2011 to 2016 in this study" seems to imply that you observe an increase in lead fraction between 2011 and 2016, but on line 27-28 you state that you do not catch the increasing trend reported in literature. Do you mean "However, we do not find an increase in lead fraction between 2011 and 2016, likely due to the high inter-annual variability in lead fraction, particularly in the spring season (Figs. 6 and 7)."? Rephrase to make your message clear.


- page 14, line 26-26: "High sensitivities in the marginal sea ice zone might result in not catching the increasing trend of Arctic lead fraction reported in the literature. In order to properly compare the Arctic current circulations and lead fraction, long-term lead fraction data are needed.”

Sensitivities of what to what? Lead fraction to ocean circulation

- page 22, line 8-10: "The proposed waveform mixture algorithm would be very useful in an operational system than the threshold-based methods including Lee et al. (2016).”: Grammaticaly incorrect -> "These advantages make the proposed waveform mixture algorithm useful for integration in operational (sea icde forecasting?) systems"

- page 22, line 10-11 "In addition, this study showed the high inter-annual variability of Pan-Arctic lead fractions in recent years (i.e., 2011-2016)." This sentence is a bit disconnect from the rest of the paragraph. I would move this to the conclusions, p 23 line 7, to replace "The recent inter-annual variability of Arctic sea ice conditions was found."

- page 23 L5-7: "“Unlike thresholds based lead detection methods, since the waveform mixture algorithm solely uses waveforms, not beam behaviour parameters, it does not need to change any parameters when the CryoSat-2 baseline version is updated, which will be useful for future altimeter missions.”: Complex sentence, I suggest to split this up in shorter sentences:

"Thresholds based lead detection methods depend on beam behaviour parameters. The waveform mixture algorithm, on the other hand, solely uses waveforms, and parameters do not need to be changed when the CryoSat-2 baseline version is updated. This will be useful for future altimeter missions.”

But doesn't the analysis have to repeated when the CryoSat-2 baseline is updated to select new end-members? Please comment.
Also, 'useful' is very general. Do you mean that the method can be easily adapted to be applied to data from future missions?

Dear authors,

I have reviewed your revised manuscript and it's my pleasure to inform you that your work is now almost ready for publication in The Cryosphere. I would like to ask you to make one textual change in the following sentence in section 4.2:

"High grid sensitivities (referring to section 4.3) of lead fraction in the marginal sea ice zone due to the smaller number of observations than in the higher latitudes might not represent the increasing trend of Arctic lead fraction shown in the literature."

Grid sensitivities are discussed in the next section. For the flow of the paper, it would be better to incorporate this statement to section 4.3:

"The high standard deviation values around the coastline of the Arctic Ocean imply that the reliability of lead fractions was low. This might further explain why we do not observe the increase in lead fraction in marginal zones as reported in the literature. On the other hand, the relatively large number of CryoSat-2 observations around the North Pole produced low standard deviation indicating less sensitivity (Fig. 9i-l)."

Congratulations, and thank you for your cooperation throughout the review process, and most of all, thank you for your valuable contribution to our journal.

Regards,
Bert Wouters - TC/TCD Editor