Overall i think this paper present great qualities . The survival ice crystals fraction parametrization seems is really impoved. However i have some concerned about the CFD presented that may affect this parametrization. Therfore i do not recommand the publication at this stage since the paper will clearly benefit from a sensitivity analysis to there computationnal domain, mesh and initial condition. More details are given in the supplement.

General comments

This manuscript presents a numerical study of ice crystal formation behind hydrogen-powered aircraft using a 3D large-eddy simulation (LES) framework coupled with a Lagrangian microphysical model. The focus lies on the vortex phase of contrail evolution, with a comparative analysis between hydrogen- and kerosene-fueled aircraft. The study explores the sensitivity of contrail properties to ambient humidity, temperature, initial ice crystal properties, and aircraft type. A new parameterization for the survival fraction of ice crystals under hydrogen combustion scenarios is proposed and tested against the LES results.

This is a timely and relevant contribution, particularly given the increasing interest in low-emission aviation technologies. The manuscript is well written and the results are clearly presented. I believe this study is of high quality and should be published after addressing a few key issues. In particular, the mesh resolution near the vortex core appears coarse when compared to prior studies (e.g., http://dx.doi.org/10.1063/1.4807063, https://doi.org/10.1063/1.4934350), which used at least 10 grid points across the vortex core radius. I would encourage the authors to comment on this, and ideally to provide a resolution sensitivity test (e.g., for both A350 and A320 cases), particularly regarding contrail properties such as vertical profiles of ice mass and ice crystal survival fractions.

Specific comments

1. Section 2.1 (second paragraph): The Lagrangian particle framework is introduced, but it is not stated whether drag forces acting on the particles are considered. Given that the ice crystals exceed 1 μm in radius, could the authors clarify whether drag is included in the particle dynamics?
2. Line 124: You mention the simulation starts “several seconds” after emission. Can you specify the exact time corresponding to the plume age? This is important for comparison with prior studies (e.g., https://doi.org/10.5194/acp-15-7369-2015).
3. Line 121: Please clarify what is meant by "rigid boundary conditions". Do you refer to the vertical boundaries being reflective, fixed-value, or another type?
4. Line 170: About the initial crystal distribution, I understand the lack of direct measurements of crystal properties (e.g., radius and density), but more explanation on how the initial mean radius and density were selected would be appreciated. Was this based on previous simulations (e.g., https://doi.org/10.5194/acp-24-2319-2024), theoretical estimates, or adjusted to match a specific ice number emission index?

Technical corrections

1. In lines 309, should be “A320 aircraft” not “A320aircraft”.
2. In lines 309, should be “with a wing span” not “with wing span”.