| 18 Jun 2026
EarthCARE reveals details on the role of rain in closed-to-open cell transitions
Abstract. The mesoscale organisation of marine stratocumulus clouds into closed and open cells strongly affects cloud albedo and thus their cooling effect on climate, yet the processes governing transitions between these regimes remain incompletely understood. The EarthCARE satellite provides collocated observations of cloud mesoscale structure from the Multi-Spectral Imager (MSI) together with vertically resolved cloud and precipitation measurements from the Atmospheric Lidar (ATLID) and Cloud Profiling Radar (CPR), enabling detailed characterization of stratocumulus cloud microphysics. We apply a convolutional neural network to MSI scenes to identify closed and open cells and relate these classifications to EarthCARE microphysical retrievals from the synergy of ATLID, CPR, and MSI. Open cells exhibit substantially lower droplet number concentrations (Nd), greater variability in liquid water path (LWP) and droplet sizes (re), and more frequent and heavier precipitation, although light drizzle is also common in closed cells. To investigate closed-to-open cell transitions, we combine EarthCARE overpasses with GOES/ABI geostationary imagery and ERA5-driven trajectories to track cloud scenes and determine transition timing. This combined approach allows us to reconstruct the temporal evolution of cloud properties around transitions. We find that LWP and rain amounts increase in closed cells up to ~25 hours before transitions, followed by decreasing Nd and increasing re, while cloud vertical structure remains largely unchanged. These findings support a precipitation-linked transition pathway, potentially triggered by enhanced boundary-layer moisture and amplified by aerosol scavenging–rain feedback. This new observational evidence advances our understanding of stratocumulus breakup with implications for the cooling effect of these clouds.
This is a review of the manuscript titled “EarthCARE reveals details on the role of rain in closed-to-open cell transitions” submitted to ACP by Mayer et al.
This manuscript describes an investigation of transitions from closed-cell stratocumulus to open-cell stratocumulus cloud fields. The paper shows convincingly how microphysical properties change over a long period before transitions occur, support a precipitation-linked transition pathway. The manuscript is well-written. It makes excellent use of EarthCARE data as well as other data and several analysis methods. I recommend acceptance of the paper after considering a few minor comments listed below. In general, the authors may consider trimming the paper a bit by removing some figures and maybe some text.
Minor comments:
Line 68: This part of the sentence is unclear to me: “and improved identification of light precipitation occurrence”. Should this be something like ““and CPR has an improved identification of light precipitation occurrence compared to Cloudsat”. Please rephrase the sentence to make it more clear.
Line 163: By filtering for scenes with BT<273 K also super-cooled liquid clouds are removed, not only cirrus. I guess this is not a limitation here.
Line 170: Please give a reference or explanation for the “F1 score”
Line 255: The authors might want to consider to look if larger effective radii are also found by the MSI bi-spectral retrievals, comparable to MODIS.
Line 257: The Nakajima-King (bi-spectral) retrievals are also affected by cloud inhomogeneity, 3D-radiative transfer effects and influence of rain. I suggest adding that to the discussion, possibly referring to Zhang et al. (2012) and/or other literature. In addition, the passive retrievals get effective radius at cloud top, whereas the ACM-CAP effective radii are averaged over the cloud column, which in an adiabatic case should lead to larger values for the bi-spectral retrievals.
Zhang, Z., A. S. Ackerman, G. Feingold, S. Platnick, R. Pincus, and H. Xue (2012), Effects of cloud horizontal inhomogeneity and drizzle on remote sensing of cloud droplet effective radius: Case studies based on large-eddy simulations, J. Geophys. Res., 117, D19208, doi:10.1029/2012JD017655.
Figure 7: I suggest to make it clearer what the different panels show.
Line 265: Could the variation in effective radius also be caused by variations in cloud physical thickness, which clouds at the edges having slightly higher tops? With cloud size adiabatically increasing with height, a higher top would lead to larger (vertically-averaged) effective radius.
Line 295 and Figure 8: I find Figure 8 rather interesting and informative, but I am surprised that there is only one single sentence on it in the paper. Could the figure be described and discussed more, or if not, maybe the authors should consider removing it.
Figure 10: This figure does not add much compared to figure 7. Maybe consider removing it.
Lines 393 and further: This section on Sentinel-1 observations does not add much in my view. The picture in figure 15 is not very clear and, frankly, I do not know what I am looking at/for. I think this part can be removed or reduced to a sentence or two.
Line 414: This discussion on stratocumulus to cumulus transitions is a bit out of place in my view. It does not add much and the mentioned role of the development of open cells in this transition is speculative. If the authors want to keep such an discussion in, I suggest moving it to the introduction or conclusions and possibly reducing it.
Appendix B and C: Please point to the figure numbers that are related to these appendices.
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