| 23 Apr 2026
Direct Lagrangian tracking simulation of droplet growth in vertically-developing turbulent cloud
Abstract. We developed a new explicit cloud microphysical model, based on direct numerical simulation (DNS) with Lagrangian particle tracking. The model employs a vertically-elongated quasi-1D computational domain extending from the ground to the cloud top to explicitly capture the vertical structure of clouds. This allows us to simulate the all warm-cloud microphysical processes, including activation, condensation growth, collision-coalescence growth, and sedimentation. A homogeneous isotropic turbulence field is incorporated into this domain to explicitly resolve the turbulent wind fluctuations. Cloud microphysics simulations with and without turbulent wind fluctuations were performed to clarify the impact of turbulence on droplet growth. We obtained new insights into the altitude- and time-dependent microphysical statistics, which cannot be obtained through conventional DNS researches for a cubic box domain with periodic boundaries. The comparisons have shown that turbulence promoted the collision-coalescence growth of droplets. During the early developing stage, where the updraft was present, turbulence promoted the collisions between droplets with similar sizes (autoconversions) in the middle layer of the cloud. In later stage, relatively large droplets produced by autoconversions actively collected smaller droplets (accretions) in the middle and lower layers. The onset of precipitation at the ground occurred earlier and the first raindrop at the ground was larger in turbulence case than that in non-turbulence case.
Review for Manuscript: egusphere-2026-2143 (“Direct Lagrangian tracking simulation of droplet growth in vertically-developing turbulent cloud”, Iwashima and Onishi)
Recommendation: Major Revision
The quasi-one-dimensional direct Lagrangian approach presented in this manuscript offers a valuable extension of previous work and has the potential to provide useful insights into turbulence effects in vertically developing warm clouds. However, the lack of engagement with the eddy hopping literature and the insufficient contextualization and comparison with observations and other modelling studies in the Discussion section are major weaknesses that must be addressed before the manuscript can be considered for publication in Atmospheric Chemistry and Physics. I look forward to reviewing the revised version.
General Comment:
The manuscript extends the vertically extended quasi-one-dimensional Lagrangian framework of Kunishima and Onishi (2018) by introducing turbulence through vertical stacking of HIT snapshots. It examines the effects of turbulence on warm cloud microphysical processes, with a particular focus on collision-coalescence. While the technical approach has merit in addressing vertical development and sedimentation, the manuscript suffers from significant weaknesses in scientific contextualization, particularly in the Discussion section.
Major Comments
The manuscript makes no reference to “eddy hopping”, a key mechanism for turbulence-induced broadening of cloud droplet size distributions (DSDs). Foundational studies such as Grabowski and Abade (2017), Abade et al. (2018), and Saito et al. (2021) are highly relevant to the present vertically extended quasi-1D configuration, yet they are neither cited nor discussed. This represents a major gap in both the Introduction and Discussion sections.
The “Results and Discussion” section is dominated by detailed interpretation of the authors’ own simulations (e.g., enhanced autoconversion in the middle layer, earlier precipitation onset by ~270 s, and larger initial raindrops in the turbulent case). However, it provides very limited comparison with existing literature and observations.
Specifically, the manuscript lacks:
Furthermore, the inherent limitations of the kinematic driver approach (KiD warm-1) — including the absence of entrainment and mixing, simplification of large-scale updraft structure, lack of time evolution in the HIT snapshots, and artificial vertical repetition — are not adequately discussed in terms of their implications for the robustness and generalizability of the results to real clouds.
As a result, it is difficult for readers to assess how the present findings fit into the broader body of knowledge on turbulence–cloud microphysics interactions. A substantial expansion of the Discussion section is required to include a thorough, balanced comparison with relevant observations and modelling studies, along with a clear assessment of the strengths and limitations of the current framework.
Minor Comments
L234: “corrposonding” -> “corresponding”
References