the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2025
Wildfire aerosols lofted by North American pyrocumulonimbus clouds: long-range transport and aerosol-cloud-radiative effects
Abstract. Extreme wildfires threaten health, air quality, and ecosystems. Despite extensive study of meteorological links, the feedback mechanisms by which fire weather influences the long-range smoke transport remain poorly understood. This study examines the transcontinental transport of smoke aerosols emitted by intense North American wildfires in August 2024. Our analysis reveals that pyrocumulonimbus clouds (PyroCbs) formed in extreme fires exhibit strong vertical convection, injecting large amounts of smoke aerosols into upper troposphere and lower stratosphere. These lofted aerosols exhibit enhanced hydrophilicity at high altitudes, increasing the cloud condensation nuclei by a factor of 2–3. Therefore, the water cloud droplet effective radius decreases by 1/3, and the cloud fraction increases from 0.01 to 0.64, promoting the development of optically thick, high-level clouds. PyroCb efficiently lifts aerosols, prolonging their residence time and enabling long-range transport through high-altitude winds. This process significantly affects regional and global radiation, with aerosols heating downwind Europe. Smoke aerosols produced consistent effects on radiative fluxes: they reduced longwave fluxes, while increased shortwave fluxes, resulting in net anomalies of +2.84 W m⁻² over fire sources and +3.16 W m⁻² in smoke-transported areas. Conversely, aerosols over fire-source regions caused heterogeneous radiative responses, with net cooling anomalies of -2.5 W m⁻² along the US West Coast and +5.62 W m⁻² across North America. Our findings underscore the complex interplay between wildfires, smoke aerosols, and meteorology, forming a positive feedback loop that amplifies air pollution transport and radiative perturbations across continental scales.
- Preprint
(11960 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 02 Jan 2026)
-
CC1: 'Comment on egusphere-2025-5076', Michael Fromm, 16 Dec 2025
reply
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5076/egusphere-2025-5076-CC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2025-5076-CC1 -
RC1: 'Comment on egusphere-2025-5076', Anonymous Referee #1, 19 Dec 2025
reply
Review of “Wildfire aerosols lofted by North American pyrocumulonimbus clouds: long-range transport and aerosol-cloud-radiative effects” by Yan Wang et al.
This study examines the transcontinental transport of aerosols emitted by extreme wildfires in North America in August 2024, focusing on the role of pyrocumulonimbus (PyroCbs) clouds in the lofting of particles and their impact on cloud properties and TOA radiation. By integrating multi-platform satellite measurements and reanalyses data, the results show that PyroCbs promote the injection of aerosols into the upper troposphere and lower stratosphere, increasing the cloud fraction and altering the microphysics properties, which leads to significant radiative anomalies at continental and transcontinental scales.
I believe this study could be a valuable addition to the literature; the topic is interesting and the analysis deserves to be reviewed by the journal. However, the current study would benefit from additional clarifications and more detailed physical explanations; addressing these points may require major revision.
Major comments
a-Regarding Figure 4 (critical point raised by the CC1 comment):
I noted comment CC1 regarding a possible time lag or inconsistency between Figure 4 and the presence of PyroCbs. Given that this figure is central to the interpretation of long-range aerosol trajectories and their influence on clouds and TOA radiation, I encourage the authors to clarify this figure and provide additional context to ensure consistency with the presence of PyroCbs. A clear and well-documented identification of the fires responsible for the analyzed PyroCbs would strengthen confidence in the interpretation of the result.
b-Regarding the Influence of smoke aerosols on cloud formation.
The use of CERES data to study cloud microphysics is interesting, but it presents significant limitations depending on the scale and level of detail targeted. The native resolution of CERES does not allow capturing the fine gradients needed to analyze microphysical variability at the scale of individual convective clouds or PyroCb structures. Moreover, PyroCbs contain a high fraction of absorbing aerosols (BC, organic compounds), which bias the retrievals of COD and CER.
Have you considered averaging vertical profiles over the August 2024 PyroCb events (or cross validating the observed anomalies against lidar measurements) using synergistic products (e.g., EarthCARE ATLID/CPR)? Cloud property retrieval algorithms could then be applied to derive microphysical properties, thereby complementing CERES observations and improving the characterization of cloud altitude, optical thickness, and structure.
The conclusion in lines 470–475 is unclear because 4-day delay between the CCN increase (6–11 August) and the subsequent rise in cloud fraction (11–14 August) does not appear consistent with the Twomey or Albrecht effects, which are primarily microphysical processes acting on cloud-formation timescales (minutes to hours or a day). Such a multi-day lag instead suggests that the cloud response is largely governed by dynamical and meteorological processes, rather than by immediate microphysical aerosol–cloud interactions alone.
Similarly, the conclusion in lines 674–675—“the cloud response is evident from the CERES-derived ∆CTH, ∆COD, and ∆CER anomalies and is consistent with aerosol–cloud interactions and the Twomey effect”—may be somewhat strong. Relying exclusively on CERES data, without cross-validation using active remote sensing observations (e.g., lidar) to disentangle microphysical aerosol effects from dynamical forcing, makes the attribution to the Twomey effect less certain.
c- Regarding the Influence of smoke aerosols on TOA radiation
The interpretation of the SW anomalies seems not fully consistent with the figures. Some statements in the text do not appear to match what is shown in Figures 10 and 11. For example, under all-sky conditions, the manuscript reports negative SW anomalies over USW, whereas the corresponding panels seem to show positive values. Similarly, positive anomalies reported over NA, WE, and IGB are not clearly supported by Figures 10c and 11c, where negative anomalies appear to dominate in these regions. And similar with the net downward anomalies in clear sky.
Moreover, several positive and negative anomalies are visible outside the predefined study regions, which makes it difficult to attribute the reported signals uniquely to the source or transport regions discussed in the text.
Minor comments:
Lines 96-97: The sentence “These areas were selected based on the spatial distribution of cloud albedo during periods of fire activity as shown by CERES SYN1deg-Month data, in combination with fire hotspots detected by the MODIS Terra” is ambiguous. Could you clarify how cloud albedo was used to select the study areas?
Figure 2a. For clarity, it would be helpful to specify whether the meteorological variables shown in Fig. 2a were averaged over the full fire-intensive region, or only over pixels associated with active fire detections.
Lines 279-281: The local anomalies (T2M +1.45°C, winds +0.5 m/s, low RH, and local vertical uplift) clearly indicate conditions favorable for local ignition and initial fire development. However, regarding the horizontal advection (spread) of the smoke plume, the manuscript does not currently present figures or data showing fire propagation at this stage of the study. Should be included in the next chapter.
Lines 317-318 and Lines 323-324: The manuscript states that “its morphology is similar to the deep convection and upper-level ice clouds commonly observed in PyroCb events” and that “this structure is typical of PyroCb clouds during the decay phase of their transport.” For clarity, it would be helpful to cite studies or observational evidence supporting this characterization.
Figure 4c, d. Given that the CPR vertical profiles in Figure 3 suggest cloud tops reaching the stratosphere, it would be helpful if the authors could explain the rationale for selecting the relatively low altitudes (3, 5, and 7 km a.g.l.) in the NOAA HYSPLIT forward trajectories study.
Lines 377-378: It is somewhat difficult to establish a clear link between the PyroCb source over Oregon (Figure 4) and the plume analyzed in Figures 5a and 6a along the red line (further north); it would be helpful if the authors could clarify whether these plumes originate from a different PyroCb event. Additional labeling or guidance on the source points in Figure 4 could aid interpretation.”
Figure 8 and 9: Could you please clarify whether this corresponds to a geographical (spatial) average computed over the blue region in Figure 2d?
Repetition in line 475: "influence cloud microphysical characteristics" restates earlier points; it likely means "macro-physical" (e.g., cover, lifetime) to distinguish from microphysics.
Line 490: “The strongest negative anomalies occur over WE (-7.07 ± 0.22 W m⁻²) and NA (-4.48 ± 0.08 W m⁻²), consistent with heavy smoke loading in the source regions”. But WE is not a source region?
Lines 496-497: “These results demonstrate that direct aerosol absorption dominates in the source regions, while aerosol–cloud interactions modulate the SW radiative response in transported plumes.” “demonstrate” seems too strong. I would replace with suggest or indicate.
Figure 12: SW CRE and L CRE, wouldn't the plus and minus signs in the corresponding text be reversed?
Citation: https://doi.org/10.5194/egusphere-2025-5076-RC1
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 281 | 159 | 28 | 468 | 25 | 31 |
- HTML: 281
- PDF: 159
- XML: 28
- Total: 468
- BibTeX: 25
- EndNote: 31
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1