the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Apr 2025
Unexpected characteristics of convective downdrafts in the upper-levels of tropical deep convective clouds
Abstract. This study investigates the thermodynamical and microphysical links of in-cloud downdrafts of tropical deep convective clouds using aircraft measurements from ACRIDICON-CHUVA field campaign focusing on the upper-levels (10–14 km). The Cloud Water Content (CWC) does not show a discernible trend with altitude or vertical velocity. This opposes the concept of condensate loading, enhancing the downdraft strength. Furthermore, the CWC in up- and downdrafts is found to be similar. The mean draft diameters exhibit a broadening trend with altitude in updrafts and downdrafts, while the mean air mass flux decreases with altitude. In the upper-levels, strong negative vertical velocities (w < -2 m s-1) are observed in the supersaturated region (RHice > 110 %), contradicting the general idea that downdrafts are driven/maintained by latent cooling. The spread in cloud particle number concentration was found to be similar in downdraft and updraft regions with a weak linear trend for |w| > 1 m s-1 in transition (90 ≤ RHice ≤ 110 %) and supersaturated regions. The mean particle size distributions (PSDs) indicate an increase in maximum particle size with altitude. Higher particle concentrations are observed in stronger drafts for particles with Dp< 100 m. Furthermore, the number concentration of larger particles (Dp > 100 m) increases faster in stronger drafts as altitude increases. Particle number concentrations in downdrafts are comparable to those in updrafts of similar strength at the same altitude. We speculate that large eddies that allow mixing between updrafts and downdrafts have an influence on the modulation of PSDs.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 17 Jun 2025)
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RC1: 'Comment on egusphere-2025-1637', Anonymous Referee #1, 11 May 2025
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This paper analyzes flight campaign measurements in deep convective clouds over the Amazon basin, focusing on vertical velocity and examining correlations with cloud water content (CWC), relative humidity, and cloud microphysical properties. The observational dataset, collected at upper levels, is quite valuable and offers a rare look at high-altitude in-cloud dynamics and microphysics. I see potential for meaningful contributions from this dataset. However, the current framing and interpretation raise significant concerns, and in its present form, the manuscript does not provide sufficient support for many of the conclusions drawn.
Several central arguments rely on oversimplified interpretations of complex processes and on assumptions that are not well supported by the data or existing theory. As a result, the conclusions overreach and, at times, appear inconsistent with the established understanding of convective dynamics. To be publishable, the manuscript would require a fundamental reframing and a more rigorous analysis.
Therefore, I recommend rejection in its current form. That said, I encourage the authors to consider resubmitting a revised study that focuses more clearly on the descriptive characterization of upper-level cloud properties in anvil regions, supported by a more physically grounded interpretation.
Major Comments:
The paper claims that a lack of correlation between CWC and downdraft intensity contradicts the role of condensate loading. This conclusion is not justified. Vertical velocity is influenced by multiple, competing factors, including pressure perturbations, phase changes, mixing, etc, and may not exhibit straightforward relationships with single microphysical variables. Grant et al. (2022) is cited in the manuscript to justify this hypothesis; however, the cited paper refers to the relationship between vertical velocity and the rate of condensate production, not directly to CWC, and this relationship was only shown for updrafts. Adiabatic compression during descent decreases a parcel's supersaturation, eventually leading to partial or total evaporation or sublimation of the condensate. This feedback can cause CWC to decrease as vertical velocity becomes more negative, making the expectation of a simple positive correlation between CWC and |w| in downdrafts, as stated in the manuscript, at least questionable.
The interpretation that negative vertical velocities in ice-supersaturated air masses contradict the effects of sublimation/evaporation is an oversimplification. Supersaturation at the time of observation does not preclude earlier evaporation/sublimation that may have initiated the downdraft. Vertical velocity at a point reflects accumulated forcing along a parcel’s trajectory, not only the local, instantaneous forcing. Without trajectory or time-resolved data, causal conclusions about downdraft drivers are not warranted. The authors acknowledge the possibility that the downdraft was driven by evaporation or sublimation prior to the measurement (lines 263-264), but still arrive at the opposite conclusion.
The suggestion that "large eddies" explain the similarity in particle size distributions between strong up- and downdrafts is vague and potentially inconsistent with the data. If the strongest drafts are most often relatively narrow, as indicated in the manuscript, then the mixing eddies that connect them should be small as well. In fact, larger eddies would typically imply longer times spent within either updrafts or downdrafts, thus enhancing, rather than reducing, differences in particle characteristics compared to smaller eddies with shorter updraft and downdraft segments. The manuscript would benefit from a clearer definition of eddy scale and a more explicit explanation of the proposed mixing mechanism.
While the title and conclusions emphasize downdrafts, the figures and analyses give comparable attention to updrafts. The manuscript might be better positioned as a study of upper-level cloud properties, particularly of anvil regions, focusing on microphysical structure and variability, rather than attempting to infer cloud dynamics from limited information.
The manuscript would benefit from thorough professional proofreading. There are frequent issues with article usage, and sentence structure overall, as well as redundant phrasing, which at times reduce the clarity of the scientific argument.
Minor Comments:
The introduction is overly long and lacks a clear narrative structure. It moves back and forth between studies without establishing a coherent line of reasoning. In several places, relatively recent studies are cited to explain long-established mechanisms, which can be misleading. A more concise and focused literature review is recommended.
Figures 1 and 2: It is unclear whether these show one-dimensional PDFs at each height or joint PDFs as a function of height and draft diameter/mass flux. If they are 1D slices, please show the number of observations per height bin. Confidence intervals or error bars should also be added to the mean and percentile curves in panels b.
Figure 5: Consider using a heatmap or 2D histogram to show point density more clearly. The current scatterplots suffer from significant overlap of data points, making it difficult to interpret the underlying distribution.
Figure 7: The green and blue shades are hard to distinguish, please choose more contrasting colors. Also, how do varying sample sizes in each vertical velocity and height bin affect the calculated distributions? Are the results statistically robust across all w-z bins? Some quantification of uncertainty or sampling error would be helpful.
Citation: https://doi.org/10.5194/egusphere-2025-1637-RC1
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