|Review on "Variability of the lunar semidiurnal tidal amplitudes in the ionosphere over Brazil" by Paulino et al.|
Unfortunately, the authors did not address the major concerns that I raised in the previous round. My main question was how the authors can extract 7-11 day modulation of the lunar tide while the method involves a long-time window (29 days). The authors respond to this by pointing out that the technique was previously used by Pedatella and Forbes (2010). However, the study by Pedatella and Forbes (2010) mainly focused on the seasonal and longitudinal variability of the lunar tide in TEC and they did not examine variations with periods shorter than 29 days. Based on the work by Pedatella and Forbes (2010) and Paulino et al. (2017), I agree that the technique is suitable for evaluating the lunar tide in TEC and its seasonal variability. However, this does not mean that the technique can properly resolve day-to-day variability of the lunar tide. The authors also pointed out that short-period variations around 7-11 days are statistically significant in spectral analysis. This means that the quantity derived from the preprocessing includes variations around those periods, but my question is how much of these variations are actually related to the modulation of the lunar tide. The authors still need to establish that the technique can be used to extract short-period variations of the lunar semidiurnal tide.
To assist the authors' response in the next round, I suggest that the authors should make a sensitivity test using synthetic data. That is, the authors could generate artificial data (time series) that contains signals associated with (1) the lunar tidal, 12.42h, variation and its 8-day modulation lasting a duration of 20 days or so, (2) the diurnal, 24h, and semidiurnal, 12h, variations and their 27-day modulations, and (3) random noise. Then, the authors could analyze the synthetic data as they did the actual data in the paper to see if the technique can resolve the 8-day modulation of 12.42h variation. I would also suggest to do a test with no 8-day modulation in the lunar semidiurnal (12.42h) component but in the solar semidiurnal (12.0h) component to make sure that the technique can distinguish the 8-day modulation of the solar and lunar semidiurnal variations.
In my previous comments, I also suggested that the authors should check the spectrum of the original TEC data and confirm the presence of spectral peaks corresponding to Q8DW modulation of the lunar tide. The authors, instead of showing spectral peaks for the Q8DW modulation of the lunar tide, presented a peak corresponding to the Q8DW itself. Since it appears that the authors did not understand what I meant, here I clarify my points. The lunar tide has a frequency of f1 = 1.93 (= 1/0.5175) cycle/day. On the other hand, the Q8DW has a frequency of f2 = 0.125 (=1/8) cycle/day. The Q8DW modulation of the lunar tide should generate spectral components at frequencies of the sum and difference of these frequencies, that is, f1+f2 = 2.055 cycle/day (or 11.68h in period) and f1-f2 = 1.805 cycle/day (or 13.30h). If the Q8DW modulation of the lunar tide is truly detected, the authors should be able to find spectral peaks at periods of 11.68h and/or 13.30h. However, in Figure 2 of the authors' response letter, there is no significant peak at periods around 11-13h. This means that the technique used was not able to resolve the Q8DW modulation of the lunar tide, at least for November 2013.
I would also like to point out that, if there is Q8DW modulation of the solar semidiurnal, 12h, variation, the expected spectral peaks are at 11.29h and 12.80h. Thus, it would be challenging to distinguish between the Q8DW modulations of the solar and lunar semidiurnal variations. I suspect that what the authors observed could be the result of the Q8DW modulation of the solar semidiurnal tide, rather than the Q8DW modulation of the lunar semidiurnal tide.
As a summary, the authors still need to establish that the technique used in this study can resolve the Q8DW modulation of the lunar semidiurnal tide and that they can distinguish the Q8DW modulation of the lunar semidiurnal tide from the Q8DW modulation of the solar semidiurnal tide. As such, I recommend to return the paper for major revision.