the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Oct 2026
Viability of mixed-phase cloud thinning with prognostic mineral dust in CESM2
Abstract. Mixed-phase cloud thinning (MCT) has been proposed as a regional climate intervention method to offset Arctic warming by reducing the longwave cloud radiative effect of high-latitude winter clouds. While initial studies suggested that MCT could achieve meaningful Arctic cooling using idealized MCT scenarios with prescribed ice-nucleating particle (INP) concentrations, the radiative impact of a more realistic, prognostic seeding aerosol has remained largely unexplored so far. Here we evaluate the viability of MCT using prognostic mineral dust in a global climate model. We conducted a set of 16 sensitivity experiments in the Arctic during the boreal winter (Nov–Feb), varying the dust injection altitude and the seeding rate relative to a control simulation. Mineral dust seeding triggers glaciation of mixed-phase regime clouds, producing a net negative top-of-atmosphere radiative effect, that strengthens with seeding rate, driven by a competition between a positive longwave effect arising from aerosol-radiation interactions (LW ARI), a negative shortwave effect arising from aerosol-radiation interactions (SW ARI), and effects from aerosol-cloud interactions (ACI). Lower seeding altitudes retain more of the seeded dust within the target region and activate a larger fraction of it into cloud droplets, yet produce weaker cloud glaciation and thinning than high-altitude injections. Overall, these results indicate that MCT can produce a meaningful negative top-of-atmosphere radiative forcing over the target region, consistent with the surface-cooling potential suggested by earlier studies, but that both injection altitude and rate influence its efficacy and its side effects.
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Status: open (until 19 Nov 2026)
- RC1: 'Comment on egusphere-2026-5617', Anonymous Referee #1, 27 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5617', Anonymous Referee #2, 29 Oct 2026
reply
REVIEW for manuscript #egusphere-2026-5617: “Viability of mixed-phase cloud thinning with prognostic mineral dust in CESM2” by Farron et al.
Overview
The authors investigate the viability of mixed-phase cloud thinning (MCT) using CESM2 with prognostic mineral dust, conducting sensitivity experiments that vary injection altitude and seasonal seeding mass. The main contribution is to move beyond prescribed INP concentrations and examine the competing cloud-mediated, shortwave, and longwave radiative effects of aerosol subject to transport and removal. The simulations produce negative TOA radiative forcing over the target region, with negative cloud-mediated and shortwave aerosol–radiation effects outweighing positive longwave aerosol–radiation effects. The magnitude and balance of these contributions depend on injection altitude and seeding rate.
Overall, the manuscript is clearly organized and well written. However, the interpretation may be sensitive to a known ice-number limiter, the attribution of the residual forcing to cloud-mediated effects requires clarification, and the uncertainty assessment is based on only three winter seasons. My specific comments are listed as follows.
Major comments
- Lines 93-102 and Section 3.1: The authors acknowledge that the default nimax limiter restricts dust-induced ice formation and may underestimate overseeding. This limitation directly affects the process being investigated. Please diagnose how frequently and where the limiter is active, and quantify the difference between potential ice nucleation and the ice-number tendency actually applied across seeding rates and injection heights. Relate these diagnostics to ice number, particle size, precipitation, and the apparent monotonic response to seeding. A brief evaluation of CTRL cloud phase and water paths against observations, or a documented evaluation of the same model configuration, would also help establish whether the simulated cloud reservoir susceptible to seeding is credible. If the limiter substantially controls the response, a small set of matched CTRL and seeding experiments using a physically justified alternative treatment would be valuable. Simply disabling the limiter would not resolve the concern about excessive ice numbers; a complete rerun of all 16 experiments is not necessarily required.
- Sections 2.1-2.2: Because aerosol number and size influence both ice nucleation and radiation, seasonal mass alone does not fully specify the seeding intervention. Please report the emitted size distribution, geometric standard deviation, particle density, and conversion from injected mass to number, distinguishing source properties from the default or evolving MAM4 modal diameters. Explain how injection is distributed among grid cells and model layers: does “homogeneous” refer to equal flux per unit horizontal area, equal mixing-ratio tendency, or another prescription? Please also specify the vertical allocation in the Combined experiments and confirm that the integrated source equals that of the corresponding single-layer experiment. These details are necessary for reproducibility and interpretation of the altitude dependence.
- Three winter seasons provide a limited basis for estimating interannual variability. Please clarify whether the reported variability is calculated from paired SEED–CTRL seasonal differences under the same meteorological conditions, and explain how the uncertainty estimates are constructed.
- Lines 250-272: The statement that increased droplet number and optical thickness would produce cooling “via the LWCRE” requires correction or explicit supporting evidence. As described in the introduction, longwave cloud effects generally warm, whereas shortwave cloud reflection generally cools; temperature inversions may provide exceptions. A negative net residual co-located with increasing LWP therefore does not, by itself, demonstrate a negative longwave cloud effect. Please separate the shortwave and longwave contributions in the affected southern region and clarify whether the residual represents cloud-mediated effects alone. Relating these contributions to droplet number, effective radius, cloud occurrence, precipitation, and relevant temperatures would help distinguish the proposed mechanisms. Available model-process diagnostics could be used to test, rather than assume, the hypothesized explanation.
- The reported −1.85 to −2.22 W m−2 represents winter TOA forcing over the target region, rather than demonstrated surface cooling or an annual or global climate benefit. Because substantial dust is transported outside the target region, please use existing output to assess radiative effects beyond the region and seeding season. Reporting forcing per unit mass across injection rates would also help assess seeding efficiency. The abstract and conclusions should distinguish the simulated regional radiative response from broader claims of climate benefit or practical viability.
- Several interpretations rely on qualitative comparisons. Please quantify the principal changes in cloud properties and radiative components across injection altitudes and seeding rates, including absolute or relative changes and associated uncertainty where appropriate. In particular, statements that responses are “similar,” “stronger,” or “weaker” should be supported by numerical comparisons.
Minor comments
- Figure 2: define “activated INP” precisely.
- Line 168: “airborne and cloud-borne dust”, please clarify the distinction between “airborne” and “cloud-borne” dust. Does “airborne” refer to interstitial aerosol, including aerosol within cloudy grid cells, rather than dust exclusively in clear air?
- Figure 3: the vertical-axis label reads “Share of total injected mass [%],” whereas the caption and text define shares of the atmospheric burden anomaly.
- Line 233: “shows” -> “show”
- Figure 7(d, h): the caption gives water-path units as g m−3, while the plotted axes use g m−2.
- Lines 262-263: “enhancing low-level moisture convergence temperature inversions”: please clarify whether this refers to enhanced low-level moisture convergence, stronger temperature inversions, or both.
- Lines 266-272: Figure A3 plots droplet number path, not liquid water path. The reference can support the droplet-number statement but does not demonstrate that the LWP anomaly is strongest in Low.
- Section 3.3: the Greenland inversion explanation is plausible but should remain qualified unless supported by the relevant vertical temperature and radiative diagnostics.
Citation: https://doi.org/10.5194/egusphere-2026-5617-RC2
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Review of "Viability of mixed-phase cloud thinning with prognostic mineral dust in CESM2" by Paul Farron et al.
This manuscript is a description of model simulations of perturbing Arctic clouds in the mixed phase regime by adding accumulation mode dust to the aerosol scheme. The manuscript is generally well written. It could use some clarifications as I note below, but it is a stratightforward treatment of sensitivity experiments with a single model that advances the literature on Arctic mixed phase cloud pertubations. It should be publishable with minor revisions.
Page 4, L101: The statement : “overseeding…. Is likely understimated” is awkward. Can you rephrase this? This is because the ice crystal number is limited to be too low?
Page 4, L104: this is Figure 1 right?
Page 5, L136: eq 4. Is the delta ERF total just the difference in TOA flux? How do you know this is all ACI? Wouldn’t other random fluctuations in cloud properties, or semi-direct effects of aerosols then be aliased into ACI? Wouldn’t it be better to then (or in addition) call ACI the change in cloud radiative effects?
Page 5, L140: wouldn’t a MPRC have to have some significant liquid AND ice present?
Page 6, L162: you should add maybe 2 more panels to figure 2 showing maybe at least one of the Low, mid, high experiments (whichever has the largest effect).
Page 13, L265: could the semi direct effects impact the other ACI results? See comment above.
Page 13, L280: could you show another INP picture that demonstrates this? Or is Fig 1b enough? What about showing the actual process rates from the CAM6 diagnostics to understand what processes are changing. You imply processes, but don’t show them explicitly.
Page 14, L298: describe the process of ‘overseeding’. Does this mean making too many ice crystals so they are small and do not sediment?