Review of

Contrail models lacking post-fallstreak behavior could underpredict lifetime optical depth

(egusphere-2025-278) by  Caleb Akhtar Martínez, Sebastian D. Eastham, Jerome P. Jarrett

General comments

In this paper, the authors conduct urgently needed tests of contrail simulation codes, CoCiP and APCEMM. These codes, in particular the first one, are applied by a number of groups to offer contrail avoidance services. The presented results point to serious questions about the reliability of these services.

The research strategy is sound and reasonable: first a comparison for a "standard" case is conducted, which already displays the most striking disagreement between the two models. The disagreement is explained to originate from two model differences: A contrail is treated in CoCiP as a Gaussion plume (not further resolved spatially) with bulk microphysics (whether actually a monodisperse size distribution is applied must be checked, see below), while APCEMM treats them with 2D-spatial (cross sectional) resolution and with spectral microphysics. The basic comparison is then followed by sensitivity studies, where 6 system parameters are varied one-by-one. The two models agree on the sign of the sensitivities for 5 of the parameters, but not on the sensitivity to the most important parameter, namely the relative humidity in the supersaturated layer. The lifetime-integrated optical depth, used as a proxy for climate impact, decreases with RHi in CoCiP, but increases in APCEMM. All these differences are explained by the authors with the construction of and the assumptions in the two models.

The tests have been conducted in a special background situation, which is good for a first test. The authors state the necessity to check further parameters and that much work remains before one can trust the contrail avoidance service. I agree with this. I do not agree with the final conclusion that it would be currently better to avoid all contrails instead only the ones with strongest warming. With the current ATC system this would not be possible, at least not in dense airspaces.

This paper deserves publication in ACP, but e few questions need still to be resolved and the paper can also be improved in various aspects, as detailed below.

Major issues:

1) I am not a CoCiP user, so I am not familiar with its representation of a crystal size spectrum. My question here is whether it is actually monodisperse which means that every ice crystal has the same size (variance zero), or whether it rather may have a variance which is implicit. Note that many bulk cloud physics models (1-moment or 2-moment) implicitly assume a size distribution, where the variance is a function of the mean size. Please check, how CoCiP does this, i.e. whether it is monodisperse or perhaps monomodal. 

Later, you say that CoCiP uses the ice mass and number and that the single size of all crystals is computed from these two values. There may be reasons for this, but in cirrus models, if they have 2-moment schemes (e.g. ice mass and number), one of the reasons for this is that this allows more freedom in treating size distributions implicitly. 

Figure 5 left shows that dN/dt = -N/τ, that is the crystal loss rate is not constant. Wouldn't a monodisperse distribution of crystals, falling with identical speed, lead to a constant loss rate? Furthermore, if all crystals do what the crystal in the contrail centre does, why then is there ongoing crystal loss instead of instantaneous vanishing of all crystals?

In line 200 you write "average ice particle". If the distribution is monodisperse, all ice crystals are average.

I think, your interpretation is valid for both cases, but your statements should be correct.

2) Section 2.2, Eqs 1 and 2: Why is the spatial integral only over the width of a contrail and not along its length? This quantity as used here seems to have some similarity to the "total extinction" of Unterstrasser and Gierens and a similar quantity introduced by Lewellen. These authors use them as proxies for climate impact, perhaps an instantaneous one. But in the present case, I fear this could not serve the intended comparison. The total radiative effect of a contrail should be the lifetime integral of the vertical optical thickness at every point of the contrail (width X length). For the current purpose, length should somehow increase with lifetime, and it seems that this effect is overlooked. Wouldn't the differences between CoCiP and APCEMM be even larger if the integrals would cover the complete contrail area over the complete lifetime? 

Minor issues:

The title can be improved. It is not clear what "lifetime optical depth" may be. I think, the problem is not the optical depth, but the lifetime-integrated radiation effects or the change of the radiation energy flow integrated over the contrail lifetime.

When the same expression appears in the abstract, it should be written as "lifetime-integrated optical depth". As an explanation is given (proxy), the expression is acceptable, but in the title it should be changed. At the end of the abstract you explicitly write "contrail climate impact", why not so in the title?

Abstract, line 19: "a strategy avoiding all contrail formation is still expected to yield a reduction in climate impact". I believe such a strategy does not work for practical reasons (ATC problems) and does not exist therefore. I also do not believe that such a strategy has been proposed, as written in Line 25. 

Line 29: why hypothetical? Why not as well in actually occurring situations?

Line 44: cross sectional area of ~ 100 km², what kind of cross section do you mean? Say a contrail is 1 km deep, than it must be 100 km broad in your example. 

Line 47: "The limited comparisons that have already been performed for large eddy simulations indicate disagreement in this regard", can you be more specific? As far as I remember, the cited papers weren't model comparison papers. What do you mean?

Line 50: "inconsistencies"? Probably you simply mean model differences or disagreements or contradictory results. To my view, two different models, independently developed, can neither be consistent nor inconsistent.

Lines 68-70: The two sentences "Teoh shows..." and "For this reason" are not logically connected, to my opinion. To only consider long-lasting contrails is justified without Teoh's results. Whether the latter turn out tenable can be doubted in view of your results, in particular the probably wrong sensitivity to layer supersaturation lets me doubt to which degree Teoh's results are believable.

Line 109: "Equivalent"? Is there a subtle meaning that I do not understand or do you just mean "Equal"?

Line 216: correct "observable in from satellites".

Figure 6: The figures are not entirely understandable. Partly, because they are just scatter plots and it is not clear which CoCiP point is paired to which APCEMM cross. Then, while CoCiP should indeed be represented by a single group of points for "fallstreak only", there should be 2 groups of crosses for APCEMM: "fallstreak only" and "all phases". I suggest, to have the integrated optical thickness on the y-axis, while the x-axis should be numbered 1-14, that is the number of the sensitivity experiment. Then each number would have one blue point for CoCip, and, say a red point and a red cross for APCEMM "fallstreak only" and "all phases". A similar outline would also work for the rhs panel.

Figure 6, caption: correct "unobserbavle".

Line 300: "Varying the layer RHi causes the lifetime optical depth to decrease in CoCiP and increase in APCEMM". This sentence is a bit unclear, since the word "varying" includes both decreasing and increasing. Please correct.

Line 345/6: The Schmidt-Appleman criterion says nothing on contrail persistence, so I suggest to add ice supersaturation as a condition. Avoidance of all, that is, including very short contrails, is not useful and probably worsening the climate (unnecessary fuel consumption and emissions). 

Line 385: "our results suggest that contrail avoidance strategies which focus on avoidance of all contrails will have the greatest chance of producing a real climate benefit". I would not subscribe to this conclusion. It renders contrail avoidance practically impossible, in particular for ATC reasons. The ATC sectors where no contrails form would become overcrowded if they are neighbours to sectors where contrails can form. I think, the appropriate conclusion of your test is that more work is needed to "calibrate" the models to realistic behaviour and to test whether the promised results are satisfying. Your Section 5 points to this direction and I fully agree to the statements of Sect. 5.

Insightful study. The attached document contains my review.

Despite my recommendation with only 'minor revisions', I strongly recommend that the authors critically go over the whole manuscript to improve the presentation quality.