This is interesting paper, which is mostly well done, and only minor revision is requested.

Comments:

Figure 3: Right and left columns in figure, its capture and body of text do not agree each other.

Lines 181-187: You describe what is in panels 3d and 3h but no results which follow from these figures are presented.

Figure 5: Why is break in 2008; visual inspection of Fig. 5 suggests rather 2006. However, I agree with you that it is not possible to determine the break point quite accurately and your interval 2008 ± 2 years includes year 2006.

Wording and significant misprints:

• Line 107: “Juliusruh PPR” should be “Juliusruh PRR”
• Line 266: “leafed length” should be “leafed season length”

Line 331: “summerl length of” should be “summer length or”

Reviewer Report on the manuscript angeo-2021-51

Long-term studies of MLT summer length definitions based on mean zonal wind features observed for more than one solar cycle at mid- and high-latitudes in the northern hemisphere

by J.Jaen, T Renkwitz, J.L.Chau, et al.

General Remarks

• 1) The paper presents an interesting data set of zonal winds obtained from several measurement stations.

• 2) The data are interpreted in terms of the “summer length” and compared to other data.

• 3) Different definitions of “summer length” are used, part of which are not convincing.

• 4) The paper is difficult to read because of the many abbreviations used.

• 5) The paper can be recommended for publication after major changes have been made.

Major Comments

• 1) The definition of the summer length in the MLT is dubious. If I understand your text correctly you determine the begin and end of summer at two different altitudes (as well at mid as at high latitudes). A justification is not given. At both times the wind reverses from westward to eastward. This is confusing, as at lower altitudes opposite reversals are used for begin and end of summer. You can, of course, analyze your data this way, however, you should not call this a “summer length”. And has it a meaning?

         By the way: The high altitudes in Fig.1 indicate a semi-annual oscillation which is

     difficult to reconcile with a summer/annual oscillation.

2)   Fig. 5a:   The data after 2008 show a clear four-year signature after the “break”. If

     there is a real break in the atmosphere one might calculate the spectra separately for

     the two intervals. The four-year oscillation has been analyzed in detail recently by

     French, Klekociuk, and Mulligan, ACP 2019. The time interval before 2008 has been

     analyzed for the summer duration by Offermann et al. 2010. These authors find a

     substantial increase contrary to what is shown in Fig. 5a.

           Fig.5 b) and c):   Please elaborate on these spectra. What are the ordinates? Are

     there no longer periods?

   3)   To read the paper is somewhat strenuous because of the many abbreviations used. I

             have counted more than a dozen of them. Please try to reduce this, at least in the text.

   4)   The title of Section 5.2 “Mid-latitudes” is misleading. The text discusses the MLT as well.

         5)   Table 1, Column 4:   Please explain “A-T”, “J-C” “Jul”

Minor Comments

   1)   The title of the paper is a bit misleading as the text always presents MLT- and M-results.

          2)   Fig.3 and 4:   The print in the text of the Figures is too small. Part of it can be read only by

                 means of a magnifying glas.

         3) Lines 167-168:   left and right columns are interchanged.

• 4) Lines 266 and 273: Do the two notions “leafed length” and “growing season” mean the

     same?

         5)   Line 280:   “definition is not sensible…”   Do you really mean “sensible”, or rather

               “sensitive”?

         6)   Line 306 and others: “from west to east”   Please change to: “from westward to eastward”