Reply on RC3

AC: If we understand this question correctly, it is related to the evaluation of clouds in the GCMs, and this question has been already addressed in (Chepher et al., 2008). For the current work, we are looking for similarities/differences in scattering ratio and cloud fraction profiles between the two lidar missions, if some clouds are filtered out in our approach, they are filtered out in the same way for both lidars.

compare the fundamental differences. Therefore, here we used the same cloud detection for the two system. We agree that after having fully understood and quantify the differences due to the 2 systems (like we try to do here), the future work will include the algorithm adaptation to retrieve the same clouds and to build a long-term cloud record. We added the corresponding text in the conclusion as an interesting and exciting outlook.

Specific comments
RC: Page 1, line 22: "the ALADIN product demonstrates lower sensitivity because of lower backscatter at 355 nm": This statement is not clear. The backscatter at 355 nm is not expected to be lower than at 532nm. Please explain and revise accordingly.
AC: This is an important comment made by all three reviewers. Indeed, there was a confusing explanation regarding the particulate backscatter and we apologize for this. As we wrote in response to the Reviewer #1's question, we meant the contribution of the particles to the total (particulate + molecular) signal. Even though the total backscatter is larger at 355nm, the particulate part can be buried in molecular return because the molecular backscatter is larger at 355nm while the backscatter from cloud particles is about the same. If the signal-to-noise ratio is small, then the cross-talk correction will be noisy and the particulate signal will be retrieved with large uncertainty.
RC: Page 2, line 43: "Despite an excellent daily coverage and daytime/nighttime observation capability (Menzel et al., 2016;Stubenrauch et al., 2017), the height uncertainty of the cloud products retrieved from the observations performed by these spaceborne instruments is limited by the width of their channels' contribution functions, which is on the order of hundreds of meters, and the vertical profile of the cloud cannot be retrieved with accuracy needed for climate feedback analysis." The sentence is confusing. Consider revising to make it easier to follow. Possible suggestion: "…is limited by the width of their channels' contribution functions (which is on the order of hundreds of meters), and their uncapability to retrieve the vertical profile of the cloud with accuracy needed for climate feedback analysis.
AC: Thank you for this suggestion, we have simplified the text of this paragraph. RC: Page 2, line 47: "This drawback is eliminated by active sounders, the very nature of which is based on altitude-resolved detection of backscattered radiation, and the vertical profiles of the cloud parameters are available from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) lidar (Winker et al., 2003)and CloudSat radar (Stephens et al., 2002) since 2006, CATS (Cloud-Aerosol Transport System) lidar on-board ISS provided measurements for over 33 months starting from the beginning of 2015 (McGill et al., 2015).": Too big sentence, difficult to read. Consider revising. AC: We have simplified it, thanks.
RC: Page 4, line 106: "In Fig.1(a-c), we show the observation geometry and sampling of ALADIN's L2A product as well as three variables retrieved from its observations..": consider revising as: "…as three simulated variables that can be retrieved from its observations..".
AC: Since other Reviewers found this plot difficult to understand, we have replaced it with a 3D view of the orbits and observation geometries. Correspondingly, the description of Fig. 1 has changed. RC: Page 4, line 106: "In Fig.1(a-c), we show the observation geometry and sampling of ALADIN's L2A product as well as three variables retrieved from its observations..": consider revising as: "…as three simulated variables that can be retrieved from its observations..". AC: Thank you for the suggestion, but in the new version of the manuscript we have a different Fig. 1 with a somewhat different discussion.
RC: Page 4, line 120: "The cloud variability along the satellite's track has been estimated from the gridded EAMv1 data using the parameterization of (Boutle et al., 2014). Figure1 also serves as an illustration to theoretically achievable cloud detection agreement discussed below.": Although the cloud variability is estimated, in the plot the scene is cloud free. As the paper mainly investigates clouds, it would be interesting to have a cloudy demonstration also in addition to RC: Page 5, line 141: "Since the CALIOP is not a HSRL, the detailed information on AMB and APB is not available, and one has to compare the SR products.": One could also use the temperature and pressure profiles from NWP (provided with Aeolus & CALIPSO) to produce the particulate backscatter coefficient, and convert/compare these parameters. So this part should be revised to highlight the choice of this study and not state it as the only option.
AC: Thank you for this suggestion, that's exactly how it's done in the new version of the manuscript. There's a small correction, though -the molecular backscatter coefficient is recalculated using P/T profiles, and not the particulate one.
RC: Page 5, line 145-150: "The choice of the fitting parameter is not crucial for the purposes of the present work … collocated data.": I strongly advise the authors to follow the comment of the first reviewer regarding the wavelength conversions. Alternatively, if they decide to keep the analysis as is, then please provide a detailed discussion on the uncertainties induced from this simplified conversion.
AC: For the new version we have updated the wavelength conversions and we discuss the uncertainties associated with it.
RC: Page 6, line 167: "To avoid the risks associated with the solar contamination, we picked up only the night-time cases": As Aeolus is in dusk-dawn, still variability is expected in the PBL with the CALIPSO nighttime observations above land. Can you comment on that in the manuscript? AC: This is a valid point and, indeed, the diurnal cycle can spoil the comparison. Our answer is in our Fig. 3 (now Fig. 4), which estimates the diurnal effects along with the geometric and sampling differences. In addition, we rebuilt our new Fig. 5 (SR-height histograms) and Fig. 7 (cloud fraction profile per latitude) for the daily data without temporal difference filtering (these versions are not shown in the manuscript). In this approach, the diurnal effects are compensated because both local times are used for both instruments. Still, the SR-height histograms (Fig. 5) and cloud fraction profiles (Fig. 7) plots look about the same for this enhanced dataset as they do for a subset used in the manuscript, so one can conclude that the diurnal effects cannot explain the observed behavior.
RC: Page 6, line 172: "…we have performed a numerical experiment using the same calculated data as we used in Fig.1": Shouldn't they be stated as "simulations"?
AC: This is correct, but now we have a different Fig. 1 and a new section dedicated to the simulations, so this phrase does not exist anymore.
RC: Page 6, line 173 -180: "This time… the passive observations": It is very hard to follow the approach. A scheme/flowchart would be useful AC: We added a flowchart and we simplified the text, thanks for the suggestion.
RC: Page 6, line 182: "Overall, we considered about 1E5 pairs of pseudo-collocated data and we present the results of cloud detection in Fig.3": Please include also the region and season(s) used to produce these pseudo-collocated data, which represent the outputs of Figure 3. AC: We have updated the text of the paragraph and added a flowchart (Fig. 3). Briefly, we used 15 simulated orbits of one day in autumn equinox that cover both hemispheres and give, therefore, a representative snapshot of various atmospheric scenarios.
RC: Page 6, line 184: "or each altitude bin, the cloud detection agreement is a ratio of a number of cases when both instruments have detected a cloud (SR>5) ….": Please elaborate this choice of cloud cut off (e.g. literature) and comment on the uncertainties on the cloud detection induced from this choice for different altitudes. Could you include in results ( Figure 3) and discuss, the percentage of the clouds missed to be detected, from the 2 sensors in your simulation, with the presented methodology? AC: As for the choice of cutoff, we'd like first to refer to our answers to Reviewer #1's questions and to the two definitions of SR existing in the community. Indeed, a threshold applied to the SR defined as in Eq. 2 of present version of the manuscript should be altitude-dependent. But, as it is shown in (Chepfer et al., 2008(Chepfer et al., , 2013 a fixed threshold can be applied to a SR defined as in Eq. 3 of the manuscript to estimate the difference between the two lidars. Future work will include a more advanced cloud detection algorithm to build a long-term cloud record. But this will be a whole new study.
RC: Page 7, section 3.1. It should be stated clearly in the section that the discussion refers to the SR retrieved products used in this study from the 2 sensors. As for example, a study with the cloud statistics from the Atlid L2A and CALIPSO L2 backscatter coefficient product products may provide different results.
AC: This is true, we hope that the new title clarifies that point.
RC: Page 8, line 224: "In Appendix A, we demonstrate the correlation between individual pairs of CALIOP and ALADIN SR profiles; the conclusion of this exercise is that it justifies using Eq.1, but the uncertainties of the analysis do not allow to refine the conversion coefficients". This statement is very strong. One could refine the conversion coefficients, independently of the uncertainties of the analysis. I support that the authors should formulate this statement to correctly reflect the choices and limitations.
AC: In the new version of the manuscript, we do not use Eq. 1 and we do not want to retrieve or validate its parameters anymore, so we do not seek to rebuild this plot.
RC: Page 8, line 229: "This observation gives a hint that the instrumental part provides the backscatter information sufficient for some cloud detection up to 20km, but the detection algorithm suppresses noisy solutions." This sentence is not clear. Please improve the phrasing.