Review of “Distribution and morphology of non-persistent and persistent contrail formation areas in ERA5” by Wolf et al., egusphere-2023-3086

This paper describes an analysis of the characteristics and morphology of contrail formation regions from the IAGOS aircraft data and the ERA5 reanalysis. The focus is on the North Atlantic flight track around cruise altitudes. Characterising regions of persistent contrail formation is important and very relevant for the potential mitigation of the climate impact of aircraft flights by rerouting. There are a number of interesting results from the analysis, with an overall conclusion that some persistent contrail regions will be difficult to avoid by rerouting aircraft because of their large vertical and horizontal extents and their frequent colocation with the jet stream.

The paper is well written and logically presented and is appropriate for ACP. I have one main comment and several minor corrections that need to be addressed.

MAIN COMMENT

Should the crossing length for ERA5 in Figure 1 and Table 2 have values less than the grid length of 14km - 24km (i.e. values in Fig 1 for ERA5 go down to less than 1km - the native IAGOS resolution - and Table 2 has a 10th percentile for ERA5 PC of 9km)? Line 211 says “short crossing lengths occur less frequently and cannot by construction be smaller than grid-box size”. I assume the reason for small lengths is because the flight track can cut the corner of a grid cell, but then the above sentence is not correct and the reason should be made clear in the text. Whether this makes sense is another matter, as the ERA5 data is not providing useful information less than the grid length (and probably several grid lengths). Would a different methodology (e.g. a flight track at the resolution of the grid) lead to a significant difference in the results/conclusions?

MINOR CORRECTIONS

Section 2.2

Regarding interpolation of the ERA5 data, it should be noted here that there is already some interpolation from the native grid of ERA5 (reduced gaussian TL639 grid, ~31km approx equally spaced grid, and 137 hybrid sigma-pressure coordinate levels) to the lat/lon 0.25 degree grid and fixed pressure levels.

e.g. https://confluence.ecmwf.int/display/CKB/ERA5%3A+What+is+the+spatial+reference

Line 138

Duplication of "fixed grid resolution"

Line 147

bellow->below

Line 156

Given the “reservoir (R)” conditions are mentioned, it is worth just mentioning briefly in one sentence what this refers to, as it may not be obvious to the reader and shouldn’t require looking up in a different paper.

Line 340

relative -> relatively

Line 344

“Figure 5b….indicates that PC regions are generally small…”

This part of the sentence is a bit vague and the size of PC regions is already quantified in Fig 4b, so I suggest removing it.

Line 426

“…to the 2D binary arrays of PC occurrence.” I suggest adding “in the ERA5 dataset.” to this sentence for clarity.

Lines 451 and 452

extend -> extent

Line 455

“isohumes (lines of constant moisture)” should be “(lines of constant relative humidity)” for clarity.

Fig 4.

Top right x-axis says “Major axis length” but the caption says “maximum dimension” which is 2x the major axis length.

Fig 4.

caption 4th line 

“are are”

Fig 5.

Left axis label says “Major axis length” but the caption says “maximum dimension” which is 2x the major axis length.