Review of “Evaluation of the ESA CCI+ ESMR v1.1 sea ice concentration product” by Stefan Kern.

Evaluation of sea ice concentration (SIC) is very important for product development and assessment of product quality. There are a number of good reasons to extend the SIC data records back in time, before 1978, and evaluation of these records using independent SIC estimates is particularly important because there is little overlap between SIC products in the 1970s. Stefan has been developing a valuable tool for evaluating SIC products using high resolution optical satellite imagery from Landsat and this has been used for more contemporary SIC records and published (Kern et al. 2022). Here the NIMBUS 5 ESMR v1.1 SIC has been evaluated.  I have some suggestions which I think could improve the MS and then some specific comments below.

50 Landsat-1 images from the Arctic have been manually classified. The images are mostly from March and April 1974 and obviously the ESMR SIC quality changes seasonally. Could the scenes in Fig. 2c be color coded with time of year so that the reader could get an idea of the seasonal (and geographical) distribution?

One of the updates to ESMR SIC v1.0 to v1.1 was a treatment of the ice types in the SIC processing. The v1.0 SIC was underestimation multiyear ice SIC and overestimating first-year ice SIC. Evaluating this update is important. However, most of the Landsat scenes are over first-year ice and therefore a full evaluation is not possible. Please include a discussion of this limitation in the MS.

Overlap between the contemporary SIC records and NIMBUS 5 ESMR is not possible. However, the Kern et al. (2022) article presents a similar evaluation of Landsat imagery and a number of SIC products. The coverage in Hudson Bay and Baffin Bay is comparable to the selection of Landsat in the ESMR evaluation. How does the ESMR evaluation compare with the Kern et al. (2022) evaluation in these two regions?

Specific comments:

In the abstract: Please include the time of year for Landsat scenes and which ice type is covered by the Landsat scenes.

L 14 - 15: include some references to the statement.

L 15: suggest using “changes” instead of “developments”

L 16: add “satellite” before “climate”

L 17: delete “first data of the”, there was a SMMR on Seasat.

L 18: add “on NIMBUS 7” after “(SMMR)” and “NIMBUS 7” before “SMMR”

L 18: the sentence starting with “Prior…” rewrite “Prior to the NIMBUS 7 SMMR sensor there were other satellites carrying microwave radiometers, e.g. the NIMBUS 5….”

L 31: after “brightness temperatures” add “due to instrument and geophysical noise…”

L33: before “EASE” add “a predefined 25 x 25 km2 EASE grid”

L37: In Kolbe et al (2024) a threshold of 30% was selected because of the open water noise level. Why 10%? When 15% is more common for contemporary records.

L40: It is worth mentioning that the coverage in v1.1 is much better than in v1.0 (the list of missing files in v1.1 is much shorter in v1.1).

L43: what is meant with “reasonable”?

L45: It is not so clear that it is the SIC thresholds that are compared. Please reformulate.

L51: Is this criterion comparable to the evaluation criterion in Kern et al. (2022)? And if not, could this be aligned?

L85: Is it not better consistently to use 4, 5 and 7 and not just sometimes?

L95: add “v9.1” after “SNAP” and delete next sentence.

L105: Would it not be better to remap the Landsat data onto the Pol. Ster. Grid when evaluating the NSIDC ESMR SIC? So that this process is comparable to remapping the Landsat data to the EASE2 grid when evaluating the ESA ESMER SIC.

L131: are these regional differences comparable to Kern et al. (2022)?

L152: add “NIMBUS 7” before “SMMR”

L157: The reasoning about the Landsat SIC systematic uncertainty could perhaps be used for quantifying the Landsat SIC uncertainty including the pixel resolution and SIC dependent uncertainty, albedo variations, instrument noise, atmospheric noise?

L185: last sentence: “will reveal the full bias”?

This manuscript investigates two sea ice concentration data sets based on observations of ESMR, the first microwave radiometer in space. As the ESMR data set (1972 – 1977) expands our sea ice data record by 6 years into the past, this quality assessment and comparison is a substantial contribution to our knowledge about features and quality of those data sets.

 It is a pity that the comparison with Landsat imagery could not be performed in the Antarctic. It would add more weight to the comparison covering both hemispheres. However, the data at hand in themselves already are worth being published. At least, in the conclusions the extension to the Southern Hemisphere should be mentioned among the useful next steps.

The whole manuscript is written in the first-person style. To the revier's impression, this overly emphasizes the role of the author. It appears less appropriate to the reviewer at the many places, where just scientifically agreed standard work is described.

I suggest accepting the manuscript after minor revisions.

General: ‚sqkm‘ for square km is an unconventional notation, better adhere the SI system with km².

L(ine) 40 ‚I did not compute monthly means for months with daily   data from 12 or fewer days.’: Mention that ESMR only gave data each second day.

L45 ‘’I observe differences between the monthly SIE values of up to 2 million sqkm in the Northern Hemisphere and of up   to almost 4 million sqkm in the Southern Hemisphere.’: Do you mean between the 10% and 30% threshold values? In that case, the explanation 2 lines later would not hold: it would simply represent the area of SIC between 10% and 30%. Otherwise, specify differences between which quantitites.

L 157 equalling -> equaling

L 164 ‚The second one‘ -> The second deficiency