{"id":"950824f8-0b20-4137-89a2-049961d01e7a","arxiv_id":"2505.19348","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"A validated LES framework for jet and early vortex contrail formation is used to rank aircraft size, soot emission index, and fuel consumption as the main drivers of ice crystal number and radiative forcing.","lead":"This paper builds a high-resolution 3D simulation framework for the first seconds of contrail formation and tests how the predicted ice crystal count and radiative forcing respond to aircraft, engine, and atmospheric inputs. The simulations rank aircraft size as the strongest driver, followed by soot emissions and fuel flow, and show that ambient aerosol makes the soot-to-ice relation nonlinear.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Widebody case contradicts its stated bypass ratio: r_core=0.61 m and r_total=0.85 m yield BPR≈1.5, not ~8, undermining the headline 'aircraft size is the most significant sensitivity.'","rationale":"The reader's weakest assumption was the temporal jet/vortex idealization and the neglect of axial compression. That concern is real but partially mitigated by the paper's V&V against prior temporal jet simulations and by the citation of a previous temporal-vs-spatial comparison (Lewellen 2020) that showed reasonable agreement in parcel lifetime and dilution. The bypass-ratio inconsistency, by contrast, is an internal, concrete error that directly affects the headline sensitivity ranking. The widebody case is not just one point on a continuous parameter sweep; it is offered as the evidence that 'aircraft size' is the most significant factor. If the widebody simulation actually used a low-bypass geometry (BPR ≈ 1.5 instead of ≈ 8), then the comparison is not between a representative narrowbody and widebody engine, and the resulting factor-of-three difference in radiative forcing may be an artifact of the misconfigured exhaust profile rather than a genuine aircraft-size effect. The paper's own Section V.D establishes that the bypass stream has a non-negligible effect on ice crystal activation and radiative forcing, so this is not a negligible detail. The fix is straightforward: recompute the widebody case with a consistent bypass ratio and report the actual profile used. Until then, the central claim about the sensitivity ranking should be treated as conditional. This does not change the reader's verdict (CONDITIONAL), but it adds a specific required revision. The V&V in Appendices A–C and the transparency about the earlier bug in the conference version are genuine strengths; the issue is not with the general framework but with the interpretation of one of the key comparison cases.","tokens_in":22604,"tokens_out":10768,"duration_ms":75166,"concrete_test":"Re-run the widebody case with r_total chosen so that the mass-flow bypass ratio is 8 (e.g., r_total ≈ 1.51 m using the same core/bypass velocities and temperatures), keeping all other Table 4 parameters fixed. Compare ice crystal number and net radiative forcing at t = 3 s with the published widebody result and with the EIsoot = 1e15 case; if the ranking changes (e.g., widebody no longer exceeds EIsoot = 1e15), the central claim fails. Also verify the actual bypass ratio from the initialized velocity, temperature, and density profiles in the code.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section V.E/Table 4 defines the widebody (B777/A350) case as scaled from the narrowbody 'with ... bypass ratio of about 8.' The listed radii are r_core=0.61 m and r_total=0.85 m. Using the same core/bypass velocities (480.3, 311.6 m/s) and temperatures (580, 242 K) as in Table 1, the bypass-to-core area ratio is (0.85^2−0.61^2)/0.61^2 ≈ 0.94, and the mass-flow bypass ratio is approximately 0.94·(311.6/480.3)·(580/242) ≈ 1.5. The narrowbody values (0.305, 0.755) do give BPR ≈ 8.0. Thus the implemented widebody profile is not a high-bypass turbofan and is inconsistent with the paper's own description. This matters because the paper shows in Section V.D that adding a bypass stream changes nucleated ice crystals and reduces net radiative forcing by about 9%, and the widebody case is the basis for the headline ranking (Fig. 11 and the Conclusion). To obtain BPR ≈ 8 with r_core = 0.61 m, one would need r_total ≈ 1.51 m, so the reported 0.85 m appears to be an error or an undocumented change of engine cycle. If the widebody geometry is misconfigured, the 'aircraft size' sensitivity is not yet established, because the widebody result could change substantially under a realistic bypass ratio, potentially altering the ranking relative to the soot-emission-index and fuel-consumption sweeps.","agreement_with_reader":"disagree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents a numerical framework, built on the charLES solver, for large-eddy simulation of the early jet and vortex phases of contrail formation. The approach couples compressible LES with Lagrangian particle tracking and two microphysics schemes: a simple ice-deposition model and a more complete aerosol-to-ice model including activation, condensation, freezing, and deposition. Simulations are temporal, using idealized hyperbolic-tangent jet profiles and Lamb-Oseen vortices. The framework is validated against published results for stratified vortex descent (Sarpkaya 1983), ice deposition growth (Karcher et al. 1996), and the jet phase of a contrail simulation (Paoli et al. 2004), as detailed in Appendices A-C. A sensitivity analysis is then performed over modeling choices (bypass flow, latent heat, ambient aerosol, microphysics complexity) and over atmospheric, aircraft, and engine parameters (temperature, relative humidity, aircraft size, fuel flow rate, soot emission index). The headline findings are that aircraft size is the most significant sensitivity, followed by soot number emission index and fuel consumption, and that the relationship between emitted soot number and nucleated ice crystals is nonlinear when ambient aerosol is included.","tokens_in":22951,"tokens_out":9630,"duration_ms":81954,"significance":"If the framework is correct, it provides a valuable new tool for near-field contrail simulation, with a more detailed aerosol-to-ice microphysics treatment than most existing LES studies. The verification and validation effort is a genuine strength: the stratified vortex descent, ice deposition box model, and jet-phase comparison all show reasonable agreement with published data, and the authors are transparent about the idealized temporal setup. However, the paper contains internal inconsistencies that affect the headline sensitivity ranking, most notably the widebody bypass-ratio discrepancy and an erroneous-looking collision-factor equation. These issues are fixable but require substantial revision before the conclusions can be accepted.","major_comments":[{"comment":"The widebody case described in Section V.E is inconsistent with its stated bypass ratio. Taking r_core = 0.61 m and r_total = 0.85 m from Table 4 together with the core and bypass velocities (480.3 and 311.6 m/s) and temperatures (580 and 242 K) from Table 1, the bypass-to-core mass-flow ratio is approximately (0.85^2-0.61^2)/0.61^2 * (311.6/480.3) * (580/242) ≈ 1.5, not the 'about 8' stated in the text. Repeating the same calculation for the narrowbody entries (r_core = 0.305 m, r_total = 0.755 m) gives a ratio close to 8.0, so the discrepancy is specific to the widebody setup. Because the widebody case drives the headline result that aircraft size is the most significant sensitivity (Fig. 11 and the Conclusions), the ranking is not established until the widebody geometry is corrected or the deviation is explicitly justified.","section":"Section V.E (Table 4)"},{"comment":"The collision factor G in Eq. (12) is written as G = [1/(1+Kn + 4/3 Kn/α)]^{-1}, which simplifies to 1+Kn + (4/3)Kn/α. This expression increases without bound as Kn becomes large, whereas the text states that G should tend to 0 in the free-molecular limit (Kn ≥ 1). The displayed formula therefore contradicts its own stated limits and presumably has a misplaced inverse. Since G is central to the deposition growth model in Eq. (10) and to the sensitivity study of the deposition coefficient α in Section V.D, the correct functional form must be restored and the verification results in Appendix B re-examined under the corrected formula.","section":"Eq. (12)"},{"comment":"Equations (1)-(4) contain a mass source term ω_v in the continuity and water-vapor equations, but the momentum equation (2) and the energy equation (3) show no source terms, despite the text asserting that ω_m and ω_h couple momentum and energy. Later, Section V.D reports the effect of 'adding latent heat' for condensational and depositional growth; however, the single-particle equations (8) neglect heat exchange and no energy equation source is visible. The manuscript should clarify exactly how latent heat enters the carrier-phase equations, or the latent-heat sensitivity conclusion (less than 2% in radiative forcing) lacks a stated basis in the governing equations.","section":"Section II.A"},{"comment":"The 'aircraft size' comparison in Fig. 11(a) changes many parameters simultaneously: fuel mass flow (0.34 to 1.5 kg/s), wingspan (35.7 to 65 m), circulation (290 to 570 m^2/s), core radius (0.305 to 0.61 m), and total radius (0.755 to 0.85 m). The resulting differences in ice crystal number and radiative forcing are therefore not attributable to aircraft size alone. To support the stated sensitivity ranking, the authors should isolate the size effect (e.g., by scaling geometry while holding fuel flow and emission indices fixed) or reinterpret the result as a combined aircraft-engine scaling.","section":"Section V.E (Fig. 11)"}],"minor_comments":[{"comment":"The text 'B373/A320' appears to be a typo for 'B737/A320'.","section":"Section V.E"},{"comment":"In the sentence about particle number, 'we initial ensured' should read 'we initially ensured'.","section":"Section V.C"},{"comment":"The y-axis label 'Number of ice crystals [m^{-1}]' is missing the power-of-ten scale used in other panels; values are shown in logarithmic scale without a clear unit prefix.","section":"Fig. 11(c)"},{"comment":"The statement that the widebody emits '4.5 times more soot particles' does not match the fuel-flow ratio 1.5/0.34 ≈ 4.41. Please align the text and the table.","section":"Section V.E"},{"comment":"The non-dimensional time T and height Z appear in Fig. 13 but are defined only in the text; consider adding the definitions to the caption.","section":"Appendix A"},{"comment":"The sensitivity analysis in Fig. 11 is based on single realizations. Given the 1.3% variability in mean particle radius reported in Section V.C, adding error bars or ensemble information would strengthen the confidence in the ranking.","section":"Section V.C and Fig. 11"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the journal's scope and the V&V is a clear strength. However, the widebody bypass-ratio inconsistency suggests that the actual simulation setup may differ from the description; the authors should be asked to provide the exact geometry and, if necessary, rerun the case. The collision-factor equation is likely a typographical inversion, but it must be corrected and the V&V rechecked. The confounded aircraft-size comparison is a conceptual issue affecting the main claim. These problems are fixable within the manuscript's scope, so I recommend major revision rather than rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know upfront. This is a competent, transparent paper that builds a charLES-based LES contrail framework and validates it in three appendices. And the headline ranking—aircraft size most significant, then soot emission index and fuel flow—should not be taken as established, because the widebody case used for the aircraft-size comparison is internally inconsistent on bypass ratio.\n\nWhat is genuinely new: a documented end-to-end implementation in charLES (compressible LES, Lagrangian particles, water-vapor transport, Kärcher deposition plus Kappa-Köhler activation and freezing) and a sensitivity scan over temperature, relative humidity, aircraft size, fuel flow, and soot EI. The V&V in Appendices A-C is a real asset: stratified vortex descent against Sarpkaya/Shirgaonkar/Spalart, deposition against Kärcher, jet phase against Paoli. The agreement is reasonable across all three. The paper also discloses a bug in its own prior conference version and explains how it changed earlier conclusions. That is honest and useful.\n\nNow the soft spots. The stress-test note checks out. Table 4 gives r_core = 0.61 m and r_total = 0.85 m for a widebody with a stated \"bypass ratio of about 8.\" Using the same core/bypass velocities and temperatures as the narrowbody, the mass-flow bypass ratio is about 1.5; the narrowbody radii (0.305/0.755) do give roughly 8. To get BPR 8 with r_core = 0.61 m you would need r_total around 1.51 m. This is not a typo in isolation, because Section V.D shows that adding a bypass stream changes ice number and reduces net radiative forcing by about 9%, and the widebody case is the basis for the central \"aircraft size matters most\" claim. A correctly configured widebody could shift the ranking.\n\nSecond, the sensitivity analysis uses single runs with no uncertainty quantification. The aircraft-size comparison changes several parameters at once, and the fuel sweep changes soot and water vapor together. The authors are upfront about the temporal idealization (periodic domain, Lamb-Oseen vortex, no axial compression) and about the simplified optical-depth radiative forcing parameterization, but those caveats should also qualify the headline ordering.\n\nWho this is for: people building near-field contrail LES platforms and those planning alternative-fuel contrail studies. The framework and V&V are reusable, and the nonlinear soot-to-ice relation matches earlier literature without being oversold. It deserves a serious referee, not a desk reject, but the referee should require a corrected widebody geometry and a re-examination of the aircraft-size sensitivity before that result is cited.","headline":"Useful LES framework with honest V&V, but the headline aircraft-size sensitivity rests on a widebody geometry whose listed radii give a bypass ratio near 1.5, not the stated ~8.","tokens_in":23539,"tokens_out":2949,"would_cite":false,"duration_ms":28625,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Aircraft size, then soot emissions and fuel burn, control how many ice crystals a contrail forms in its first seconds.","keywords":["contrails","large-eddy simulation","ice nucleation","soot emission index","radiative forcing","jet-vortex interaction","ambient aerosol","sensitivity analysis"],"falsifier":"A spatial near-field simulation, or a flight measurement campaign, that resolves the developing plume with engine geometry, axial compression, and ambient aerosol concentration would test the temporal idealization: if the resulting ice crystal number per meter, or the ranking aircraft size greater than soot emission index greater than fuel consumption, deviates beyond the roughly 1.3 percent simulation variability the authors report, the central sensitivity claim would need revision.","tokens_in":22333,"feed_emoji":"✈️","tokens_out":6798,"duration_ms":61852,"temperature":0.7,"pith_summary":"This paper develops a three-dimensional large-eddy simulation framework for the jet and early vortex phases of contrail formation, solving the airflow on an Eulerian grid while tracking soot and aerosol particles in a Lagrangian manner. The authors show that, for a conventional-fuel contrail at typical cruise conditions, the number of nucleated ice crystals and the estimated net radiative forcing are most sensitive to aircraft size, followed by the soot number emission index and fuel consumption. They also find that adding ambient aerosol as a precursor reveals a nonlinear relation between the number of emitted soot particles and the number of nucleated ice crystals, so that in low-soot regimes ambient aerosol contributes substantially to ice formation. The work matters because contrail radiative forcing is one of the most uncertain aviation climate impacts, and the framework is intended as a precursor for studying contrails from alternative fuels.","feed_headline":"Aircraft size leads contrail ice formation drivers","feed_subtitle":"Simulations rank ice-crystal drivers: aircraft size first, soot index and fuel burn next, with an ambient-aerosol twist.","key_machinery":"The load-bearing object is the two-phase large-eddy simulation model: a compressible Navier-Stokes carrier phase with a static-coefficient eddy-viscosity subgrid model, coupled through mass and energy source terms to Lagrangian particles that can each represent many physical particles. The contrail-specific machinery is the microphysics chain, which includes saturation-with-respect-to-water activation, depositional growth through a diffusion law with a collision factor and deposition coefficient, hygroscopicity-based droplet activation, condensation with latent heat, and homogeneous freezing, combined with an idealized initialization: a hyperbolic-tangent jet for the jet phase and a Lamb-Oseen vortex for the early vortex phase, embedded in a stably stratified atmosphere with periodic axial boundaries. This machinery converts inputs such as fuel mass flow, soot emission index, aircraft size, ambient temperature, and relative humidity into particle-level ice growth and, through a Mie-based optical-depth parameterization, into net radiative forcing per meter of plume.","core_discovery":"The central claim is that a two-stage temporal large-eddy simulation, starting from an idealized turbulent jet and then adding a Lamb-Oseen vortex, reproduces the jet and early vortex phases of contrail formation, and that within this framework the number of nucleated ice crystals and the optical-depth-based net radiative forcing are governed most strongly by aircraft size, then by soot number emission index, then by fuel consumption. A widebody aircraft forms roughly 4.8 times more ice crystals from soot and about three times the net radiative forcing of the narrowbody baseline after three seconds. Varying the soot number emission index from $10^{12}$ to $10^{15}$ per kilogram of fuel produces a nearly quadratic, rather than linear, relation between emitted soot and nucleated ice crystals, because ambient aerosol particles with higher hygroscopicity activate when soot is scarce. The paper further claims that more detailed aerosol-to-ice microphysics changes the nucleation timing but not the final ice crystal number or the optical effect, and that modeling the bypass flow delays jet development and reduces the net radiative forcing.","pith_inferences":["The paper leaves implicit that a switch to low-soot alternative fuels may not cut ice crystal number proportionally; in a soot-poor regime ambient aerosol activation could dominate, so the climate benefit of a fuel switch would depend on local background aerosol concentrations.","Because larger aircraft entrain more ambient aerosol, the sensitivity ranking could shift for future widebody aircraft burning low-soot fuels, with ambient aerosol becoming a co-dominant factor alongside aircraft size.","A testable extension is to repeat the sensitivity analysis in a spatial simulation that includes axial compression and engine geometry, comparing not just mean ice radius but the full sensitivity ranking, since the paper validates jet-phase particle size rather than the ranking itself.","The radiative forcing estimates use an optical-depth parameterization at a fixed solar zenith angle and albedo, so extending to diurnal solar angles would show whether the ranking by warming potential matches the ranking by ice crystal number."],"forward_implications":["In non-threshold baseline conditions, switching from simple ice-deposition microphysics to the full aerosol-to-ice pathway changes how quickly particles become ice crystals but leaves the final ice crystal count and net radiative forcing nearly unchanged.","Adding the bypass flow delays jet decay and lowers peak relative humidity, cutting net radiative forcing by about 9 percent even though ice crystal number changes by less than 2 percent.","Aircraft size is the dominant lever: the widebody case produces about 4.8 times more soot-nucleated ice crystals and about three times the net radiative forcing of the narrowbody at three seconds.","At high soot emission indices most ice crystals form on soot, while at low indices ambient aerosol with higher hygroscopicity contributes a much larger share, making the soot-to-ice relation nonlinear.","Atmospheric temperature near the formation threshold can suppress visible contrail formation: at 225 K, less than 5 percent of the baseline ice crystals form and the 90th-percentile optical depth falls below 0.02."],"supporting_citations":[{"why":"Supplies the idealized jet profile and the jet-phase simulation used as the framework's verification target.","marker":"[11]"},{"why":"Supplies the stratified-atmosphere initialization, baseline atmospheric values, and the particle-number convergence criterion the sensitivity study relies on.","marker":"[12]"},{"why":"Supplies the particle-based cloud microphysics scheme for activation, condensation, freezing, and deposition growth.","marker":"[8]"},{"why":"Documents the nonlinear relation between emitted soot and nucleated ice crystals near the formation threshold that this study reproduces.","marker":"[4]"},{"why":"Provides the deposition growth law and the homogeneous-plume test case used to verify the ice-growth model.","marker":"[19]"},{"why":"Supplies the ambient aerosol properties and the trajectory-box modeling approach used for the low-soot and alternative-fuel context.","marker":"[9]"},{"why":"Provides the stratified vortex-pair descent experiment used to validate the vortex-phase descent.","marker":"[38]"},{"why":"Provides the box-model microphysics comparison and earlier vortex-descent simulation results used for validation.","marker":"[37]"}],"fun_headline_variants":["Aircraft size tops contrail ice sensitivity","Wing size paces contrail ice crystal counts","Contrail ice most sensitive to aircraft size","Soot-ice relation nonlinear in contrail sims"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The result rests on the premise that the periodic, temporally evolving jet-plus-vortex idealization, without the axial compression and streamwise development of a real engine plume, captures the entrainment and mixing processes that set ice crystal number; the paper itself notes that this neglects the axial compression of the jet flow.","fun_headline_variants_meta":{"raw":{"variants":["Aircraft size tops contrail ice sensitivity","Wing size paces contrail ice crystal counts","Contrail ice most sensitive to aircraft size","Soot-ice relation nonlinear in contrail sims"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00028,"raw_usage":{"total_tokens":1680,"prompt_tokens":981,"completion_tokens":699,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":597,"completion_tokens_details":{"reasoning_tokens":640}},"tokens_in":597,"tokens_out":699,"duration_ms":6155,"temperature":1.0,"reasoning_tokens":640,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T14:16:10.491673+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A spatial near-field simulation, or a flight measurement campaign, that resolves the developing plume with engine geometry, axial compression, and ambient aerosol concentration would test the temporal idealization: if the resulting ice crystal number per meter, or the ranking aircraft size greater than soot emission index greater than fuel consumption, deviates beyond the roughly 1.3 percent simulation variability the authors report, the central sensitivity claim would need revision.","supporting_citations":[{"cited_title":"Contrail formation in aircraft wakes,","cited_arxiv_id":null,"evidence_quote":"Supplies the idealized jet profile and the jet-phase simulation used as the framework's verification target."},{"cited_title":"Effects of jet/vortex interaction on contrail formation in supersaturated conditions,","cited_arxiv_id":null,"evidence_quote":"Supplies the stratified-atmosphere initialization, baseline atmospheric values, and the particle-number convergence criterion the sensitivity study relies on."},{"cited_title":"Box model trajectory studies of contrail formation using a particle-based cloud microphysics scheme,","cited_arxiv_id":null,"evidence_quote":"Supplies the particle-based cloud microphysics scheme for activation, condensation, freezing, and deposition growth."},{"cited_title":"Formation and radiative forcing of contrail cirrus,","cited_arxiv_id":null,"evidence_quote":"Documents the nonlinear relation between emitted soot and nucleated ice crystals near the formation threshold that this study reproduces."},{"cited_title":"The initial composition of jet condensation trails,","cited_arxiv_id":null,"evidence_quote":"Provides the deposition growth law and the homogeneous-plume test case used to verify the ice-growth model."},{"cited_title":"Contrail formation on ambient aerosol particles for aircraft with hydrogen combustion: a box model trajectory study,","cited_arxiv_id":null,"evidence_quote":"Supplies the ambient aerosol properties and the trajectory-box modeling approach used for the low-soot and alternative-fuel context."},{"cited_title":"Trailing vortices in homogeneous and density-stratified media,","cited_arxiv_id":null,"evidence_quote":"Provides the stratified vortex-pair descent experiment used to validate the vortex-phase descent."},{"cited_title":"Large eddy simulation of early stage aircraft contrails,","cited_arxiv_id":null,"evidence_quote":"Provides the box-model microphysics comparison and earlier vortex-descent simulation results used for validation."}],"review_version":1}