the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2026
The Dissipation of Shallow Cumulus Clouds: Dynamics, Microphysics, and Environmental Signatures
Abstract. The dissipation stage of shallow cumulus clouds remains less understood than their growth and mature stages, despite occupying a large fraction of the cloud lifetime. Here, we investigate the dissipation of isolated shallow cumulus clouds using high-resolution simulations across a range of initial cloud condensation nuclei (CCN) concentrations. We show that, while the growth stage is characterized by relatively smooth and monotonic evolution of bulk cloud properties, dissipation is marked by an overall decline accompanied by pronounced oscillations.
Dynamically, dissipation is characterized by weakening of the coherent large-scale circulations and an increasing relative contribution of small-scale turbulence. In non-precipitating polluted clouds, continued upward transport of moist air from cloud base allows lower-cloud updrafts to persist even as the upper cloud decays. In contrast, in precipitating clean clouds, rain formation strengthens downdrafts. The microphysical evolution also depends strongly on aerosol loading: clean precipitating clouds exhibit rapid depletion of small droplets and growth of large drops, whereas polluted non-precipitating clouds undergo weaker temporal changes. Dissipation also leaves a clear imprint on the cloud's surrounding environment. Enstrophy increases outside the cloud, indicating enhanced vorticity near the cloud–environment interface that extends outward through detrainment. A humid halo develops and expands around the cloud, exhibiting oscillatory behavior in both precipitating and non-precipitating cases.
Overall, our results show that shallow-cumulus dissipation is a distinct and prolonged stage involving coupled changes in cloud dynamics, microphysics, turbulence, and the near-cloud environment. Therefore, dissipation should be explicitly considered when interpreting instantaneous observations and representing shallow convection in models.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3677', Anonymous Referee #1, 16 Sep 2026
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RC2: 'Comment on egusphere-2026-3677', Anonymous Referee #2, 28 Sep 2026
Review of "The Dissipation of Shallow Cumulus Clouds: Dynamics, Microphysics, and Environmental Signatures"
This manuscript investigates the dissipation stage of isolated shallow cumulus clouds using high-resolution SAM–SBM simulations under BOMEX conditions, spanning five CCN concentrations (25–2000 cm⁻³). The key findings are that dissipation is a prolonged, oscillatory, spatially non-uniform stage; that its dynamics and microphysics differ fundamentally depending on aerosol loading and precipitation; and that dissipating clouds leave detectable vorticity and humidity signatures in their surroundings. The manuscript addresses an important and relatively underexplored topic, and aims to improve process-level understanding for convection parameterization and the interpretation of satellite snapshot observations. However, several issues require revisions to address as I mentioned below:
- All simulations track a single, thermally triggered cloud. The authors briefly acknowledge this limitation in the Conclusions but do not quantify how the results depend on domain size, cloud size, or the absence of cloud–cloud interactions. Given that the paper's stated motivation includes improving parameterizations and interpreting satellite snapshots of cloud fields, a discussion on how multi-cloud interactions might modify the dissipation signatures is warranted.
- BOMEX includes background wind shear, yet the authors set background advection to zero. The omission of shear is potentially significant: shear can strongly affect cloud tilt, entrainment and detrainment patterns, and the spatial distribution of the humid halo. The authors should explicitly justify this choice and discuss how the results might differ under realistic shear.
- The manuscript does not mention whether radiative transfer is included in the simulations. Radiative cooling at cloud top can significantly influence dissipation rates and the development of the near-cloud environment (e.g., humidity halos). The authors should clarify the radiation configuration and discuss its potential impact.
- The authors define dissipation onset as the time when cloud-mean buoyancy turns negative and cloud-top height ceases to rise. However, the paper itself demonstrates that lower cloud layers can remain positively buoyant with active pulse-like updrafts long after upper-level decay begins, meaning different vertical layers occupy different lifecycle stages simultaneously. The work does not quantitatively compare this definition to established alternative frameworks (e.g., the normalized total-water "cloud clock" of Witte et al., 2014; the percentile-based buoyancy/velocity classification of Katzwinkel et al., 2014) or assess how sensitive reported dissipation durations and characteristics are to the chosen threshold. How to demonstrate the advantages of the concept of dissipation as defined in this article?
- Line 89: "simpale" → "simple."
- Line 92: "accuratly" → "accurately."
- Line 102: "reported bySchmeissner" — missing space before "Schmeissner."
- Figure 2 caption: Panels (d), (e), and (f) are all labeled "(d)." Please correct to (d), (e), (f).
- Figure 9 caption: States "Results are shown for the simulation initialized with 500 cm⁻³ CCN," but the text (Section 3.2.3) clearly indicates this figure corresponds to the 50 cm⁻³ (precipitating) case.
Citation: https://doi.org/10.5194/egusphere-2026-3677-RC2
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- 1
General assessment
This manuscript investigates the dissipation stage of isolated shallow trade cumulus clouds using high-resolution simulations with spectral-bin microphysics and varying CCN concentrations. The authors analyze the evolution of cloud dynamics, turbulence, microphysics, and the near-cloud environment during cloud decay, and document features such as the emergence of a humid halo, enhanced near-cloud vorticity, and changing turbulent versus coherent kinetic energy contributions. The topic is scientifically interesting, and the manuscript is generally well written. The dissipation stage of shallow cumulus clouds has received considerably less attention than the growth and mature stages, and this study contributes useful process-level insights into the structure and evolution of dissipating clouds.
I believe this manuscript has the potential to make a valuable contribution to ACP. However, I also believe that substantial revisions are necessary before publication. My primary concerns relate to (1) the articulation of the scientific advances associated with the CCN sensitivity experiments, (2) ambiguity regarding the treatment and interpretation of cloud versus precipitation hydrometeors throughout the analysis, and (3) insufficient methodological detail to ensure reproducibility. I therefore recommend major revisions.
Major comments
My main concern is that the conclusions primarily focus on the overall evolution of the simulations rather than on specific conclusions arising from the sensitivity to CCN concentration. Indeed, the final paragraph of the conclusions contains almost no discussion of CCN sensitivity, despite the manuscript being built around five simulations spanning a wide range of aerosol concentrations.
The primary conclusion from the sensitivity study is summarized as: "The microphysical evolution during dissipation depends strongly on aerosol loading and precipitation formation." This statement is broadly expected from previous literature. The manuscript would benefit from a clearer articulation of what is genuinely new and quantitatively distinct about the present results.
For example, I find it noteworthy that the cloud macrophysical evolution (Figure 2) and the TKE/CKE evolution (Figure 4) of the polluted, non-precipitating clouds appear remarkably insensitive to CCN concentration. That result seems potentially important, yet it is not highlighted or discussed in depth.
Similarly, several conclusions related to the evolution of effective radius, DSD width, humid halos, enhanced turbulence, and evolving near-cloud circulations appear qualitatively consistent with prior studies cited in the introduction. The manuscript would be strengthened by explicitly discussing how the findings agree with, differ from, or extend studies such as Lim and Hoffmann (2024) and Zhang et al. (2011). The unique findings emerging from the CCN sensitivity experiments should be stated more clearly and quantified wherever possible.
More generally, I found myself asking throughout the conclusions: what have we learned specifically because five CCN concentrations were simulated?
A second major concern is the lack of a clear distinction between cloud droplets and precipitation-sized particles throughout the manuscript.
The manuscript never explicitly states how cloud droplets and raindrops are separated within the spectral-bin scheme. For example, are bins 1-16 considered cloud droplets and bins 17-33 considered rain drops (or some similar convention)? If so, this should be explicitly stated.
This ambiguity creates difficulties when interpreting multiple figures and conclusions. For example, at line 146, LWC is defined as cloud liquid water content. My initial interpretation was the conventional one: liquid water associated with cloud droplets only. As I continued reading, however, I became uncertain whether LWC instead represents the sum across all 33 hydrometeor bins, including precipitation.
I therefore found it difficult to determine whether many of the reported signatures reflect modifications to the cloud droplet population or simply the emergence of rain water.
I believe the analysis would be substantially clearer if the conventional cloud-water definition were employed throughout the manuscript. In particular, Figure 1 would be more informative if cloud liquid water (cloud droplets: bins 1-16) and precipitation water (bins 17-33) were presented separately. Several other analyses may also benefit from separating cloud and precipitation contributions.
This concern can be addressed relatively easily. At present, the model description is too brief for an independent investigator to readily reproduce the simulations.
Beyond the description of the HUCM spectral-bin framework, the SAM configuration should be documented more completely. For example:
Finally, while the BOMEX setup is standard within the shallow-convection community, it may not be familiar to all ACP readers. A brief summary of the thermodynamic initialization and forcing in either the manuscript or supplementary material would improve accessibility.
Minor comments
Typographical corrections