|As I commented already in my last review the paper is much improved. Therefore, I had only a quick look through the paper and want to comment on a couple of small mistakes and suggest slight changes. I want to congratulate the authors on this interesting paper.|
1. Table 1:
a) The Burkhardt and Kärcher estimates were calculated for air traffic of the year 2002 for both linear contrails and contrail cirrus. Please correct the year.
b) The Chen and Gettelman estimate was revisited and corrected in the Lee et al 2020 paper From Lee et al: ‘The estimate of Chen and Gettelman (2013) was corrected by redoing the CAM simulation using a lower ice crystal radius of 7 μm and a larger contrail cross-sectional area of 0.09 km2 for the initialization of contrails at an age of about 15–20 min, in agreement with observations (Schumann et al., 2017). The resulting change in cirrus cloudiness including the adjustment in cloudiness due to the presence of contrail cirrus leads to a radiative forcing of 57 mW m− 2.’ It would be good to note this in your table.
2. Line 65: When you talk about the factors that are important for estimating the increase in contrail cirrus radiative forcing in the future you omitted ‘climate change’ as a factor. Chen and Gettelman (2016) as well as Bock and Burkhardt (2019) discuss (amongst other factors) the impact of climate change on the contrail cirrus radiative forcing for future air traffic.
3. Line 67: The impact of a reduction in initial ice crystal numbers (as resulting from a use of biofuels) on contrail properties is analyzed in detail in Burkhardt et al. (2018). They show a shortening of contrail life times and a decrease in contrail optical depth.
4. Lee et al (2020) includes a long discussion of uncertainties connected with contrail cirrus radiative forcing (see their Appendix E) discussing uncertainties in the upper tropospheric water budget, contrail cloud overlap, contrail life times and other factors.
5. Line 173: ‘These approaches’…. is a bit unclear. I suggest ‘The approaches estimating the impact of cloud overlap on contrail radiative forcing’ …
6. Line 323: ‘In several previous radiative forcing calculations in literature, clouds and contrails have been assumed to maximally overlap,‘. You have deleted the entries in table 1 which specified the overlap schemes connected with the different models, but as far as I can see all entries would have been ‘maximum random overlap’ except for the Schumann and the Spangenberg estimate. Either you state that most models use maximum random overlap or you cite exactly which models use maximum overlap.
7. Line 326: According to your table 3, two models use random overlap while four models use maximum random overlap. Therefore, most climate models assume maximum random overlap. Please correct this.
8. Line 362: Please note that Bickel et al. (2020) estimate a much stronger reaction of natural clouds to the presence of contrail cirrus.
9. Line 739: It may be of interest to compare the contrail cloud overlap in this study with the results of Bock and Burkhardt (2016) who find a large variability in the fraction of contrail coverage that leads to an increase in overall cloud coverage. On average this fraction amounts to 43% while over the Northern Atlantic and Northern Pacific this fraction is about 20%.
Chen, C.-C. and Gettelman, A.: Simulated 2050 aviation radiative forcing from contrails and aerosols, Atmos. Chem. Phys., 16, 7317–7333, https://doi.org/10.5194/acp-16-7317-2016, 2016.
Bock, L. and Burkhardt, U.: Contrail cirrus radiative forcing for future air traffic. Atmos. Chem. Phys., 19, 8163–8174, https://doi.org/10.5194/ACP-19-8163-2019.
Burkhardt, U., Bock, L., Bier, A., 2018. Mitigating the contrail cirrus climate impact by reducing aircraft soot number emissions. npj Clim. Atmos. Sci. 1, 37. https://doi.org/10.1038/s41612-018-0046-4.
Bickel, M., Ponater, M., Bock, L., Burkhardt, U., Reineke, S., 2020. Estimating the effective radiative forcing of contrail cirrus. J. Clim. 33, 1991–2005. https://doi.org/10.1175/JCLI-D-19-0467.1.