Summary of Paper

The authors examine changes in cirrus cloud properties during the

civil air traffic slowdown in March and April 2020 (due to COVID-19

air travel restrictions). CALIPSO vertical feature mask (VFM) data

over western Europe are compared with data from similar periods from

2014 through 2019, and mean cirrus properties including cirrus cloud

occurrence, average thickness, and particle linear depolarization

ratio (PLDR) are determined for both 2020 and the earlier

years. Cirrus cloud occurrence was found to decrease 30 percent in

2020 compared to three other years (2014, 2017, 2019) with no air

traffic reductions and similar meteorological conditions. The

calculated average cloud thickness was also less in 2020 (1.18 km)

than in the previous years (1.40 km), and the PLDR values were reduced

during air traffic slowdown. CALIPSO observations over China and USA

were also examined to confirm the impact of air traffic reductions on

cirrus cloud properties.

General Comments

Although the authors provide some solid evidence that cirrus cloud

properties changed during the COVID-19 induced air traffic slowdown,

several confusing aspects of the paper's presentation detract from the

paper's value. For example, the authors present bi-monthly (March and

April) in Figures 1 and 8, but only monthly (April) data for the remaining

figures. Are the authors conflating the March and April data as one

distinct period of air traffic activity? As far as I can tell, air

traffic volume was changing throughout March over Europe, so conditions between

the two months may not be as similar as presented in the

paper. Second, it is not clear why or how the years 2014, 2017 and

2019 are chosen as the closest analogues to 2020, Those years might be

the closest to 2020 in terms of mean 500-hPa geopotential height, but each

is noticeably different. If the bi-monthly (or monthly, it's not clear

which is used) mean meteorological properties are sufficient for

judging the similiarity of meteorological conditions for each year,

why not use the 6-year (2014 through 2019) mean instead? The 6-year

mean appears to be the best match to 2020 in Figure 1. Third, which

data source are the authors using to decide the proper analogues?

The temperature and humidity data from Figure 2 suggest some

meteorological differences between GEOS-5 and the NCEP/NCAR reanalyses

from Figure 1 (which is not surprising), so which do the authors

consider to be more reliable? Fourth, why include years 2015, 2016,

and 2018 in Table 3 but not elsewhere in the paper? I believe the

paper would be improved if the authors addressed these ambiguities,

and perhaps only presenting data from April.

Figures 3 and 4 are also unclear. The results of Figure 4 seem to

contradict Figure 3! Figure 4 implies that the cirrus thickness

distribution is skewed more toward thinner (less than 1 km) clouds in

2020 when compared to the other three years, yet Figure 3 shows that

the decrease in occurrence rates in 2020 compared to other years is

the largest for the thinner clouds. How is that possible? How can the

relative frequency of thin clouds increase in 2020 while the

occurrence of thin clouds decrease the most (compared to all other

thicknesses) in 2020?

The discussion about the PLDR data over China and USA seems to be

speculative, especially in terms of the meteorological conditions over

both regions and in comparison to Europe. Do the authors know that USA

and China have similar meteorology in 2014, 2017, and 2019 compared to

2020? If not, then Figure 8 implies that meteorology is not important

for determining PLDR! Do the authors have any air traffic data to

support the claims on lines 3-10 of page 12? Several factors are

claimed to affect PLDR in the discussion (cirrus cloud height, the

magnitude of air traffic, meteorological conditions) but these effects

at best only described vaguely in this section.

P12,L3-5: “We next focus on the results observed over USA and see

slightly larger values in 2017 for both months which may be due to the

variations of meteorological conditions in different years and is

comprehensible.” Have the authors checked this claim to be sure? Like

much of this section, the discussion here is vague and speculative.

Typographical errors and minor objections

P4,L23: What are "radiative forces"?

P7, L2-3: The research area has already been defined earlier in the

manuscript so the mention of the lat/lon box is here is superfluous.

P7,L9: Please use higher altitude rather than larger altitude

P9, L15-16: “But the decreases of the PLDR with height were only found

at altitudes larger ~10km in 2020.” Is this referring to Mar 2020

only? This seems to contradict Figure 7.

Several typographical errors were noticed in the text. Some of them

are listed below although this is not exhaustive (please proofread

the paper).

P3,L16 Constellatin

P4,L22 comsisting

P6,L8: Observering

P6,L20: propierties

P7,L9: differes

Table 2, column 2: Medain

P11,L14: quartiel

P12,L26: ocurrence

P13,L3: funciton

P13,L4: referece

P13,L5: charaterized

P13,L7: yeras