| 23 Sep 2026
Morphologically complex ice crystal plate structures in mid-latitude cirrus: combining space-borne lidar, in-situ scattering, and physical optics simulation
Abstract. The scattering properties of ice crystals in cirrus clouds are critical for interpreting lidar observations, yet practical retrievals focus on idealised habits representing fewer than 10 % of natural cirrus, leaving the irregular and aggregated geometries that dominate real clouds poorly constrained. This study combines three independent lines of evidence to identify which habits are most consistent with mid-latitude cirrus observations: lidar and depolarisation ratios from the EarthCARE ATLID high-spectral-resolution lidar, in-situ angular scattering and stereographic imaging from the PHIPS probe during CIRRUS-HL, and physical geometric optics simulations of hexagonal plates, intersecting plate aggregates, and touching plate aggregates with surface roughness. Both datasets cover the same mid-latitude region and the in-situ data provides about 12,000 manually classified plate, side-plane, and aggregate particles across 22 flights. For each habit and roughness level, orientation- and ensemble-averaged scattering properties are simulated for 1000-particle ensembles with sizes drawn from the measured distribution. All three pristine habits produce lidar and depolarisation ratios within the observed range, but any roughening of the single plate habit drives the lidar ratio away from most observations, attributed to loss of retroreflective backscattering. Touching plate aggregates remain consistent across all roughness levels, owing to diffuse scattering from their open structures. The angular scattering function alone does not distinguish habits, but combined with the lidar constraints it identifies smooth intersecting aggregates and roughened touching aggregates as the most plausible dominant backscatterers. The extent to which these ratios can discriminate habit composition is discussed.
In this article, the authors compare in-situ measurements of the angular scattering function of particles identified as plate-like according to in-situ imagery, with retrievals from the ATLID lidar, and output from physical optical geometry simulations in combinations of plate-like crystals with roughened surfaces.
Major comments
The various elements of the paper are all of high quality. The introduction demonstrates a thorough understanding of the state of the art regarding how the microphysical properties of crystals in ice clouds drive their optical properties, with many relevant and recent references. It is by itself very enlightening. The confrontation of in-situ PHIPS data with ATLID measurements in Figure 2 provides clear observational evidence for a transition from large, plate or column-like particles at warm temperatures to small, irregular particles at cold temperature, with simultaneously little variation in the ATLID-retrieved lidar ratio and depolarisation ratio. I have never seen that documented before so clearly from observation. The simulation work described in section 3 is impressive in its scope -- creating realistic aggregates of planar particles, dividing these shapes into a fine mesh, and adding small-scale roughness -- all of this is evidence of advanced technical mastery. I have no problem with any individual step of the analysis. The discussion in Sect. 4 makes several interesting points, for instance on the origin of change in lidar ratio with surface roughness (l. 380-389), on the effect of growth-sublimation cycles on surface roughness (l. 414-420), or on how lidar retrievals by design will be dominated by particles with large backscattering (l. 435-440).
It is therefore all the more disappointing that combining all these elements does not lead to any firm conclusion. The phase function comparison between PHIPS and simulation (Fig. 8) is inconclusive, as is the comparison of ATLID optical properties with outputs from simulations (Fig. 9). It is unclear what we can learn by comparing ATLID data with simulations that consider a subset of existing crystal shapes (plates and plate aggregates). The discussion of the effect of roughness on lidar ratio is interesting but could be done based on simulations alone. The approach taken is clear (comparing in-situ and lidar measurements with simulations) but the purpose is not. Why are you comparing what you are comparing? What are you attempting to understand? Which questions drive your efforts? Which hypotheses are you trying to prove? All these points are not addressed in either the introduction or the conclusion.
This by itself is not a reason for rejection -- the paper makes many interesting points that deserve publication, and should be commended for pushing the discussion about ice microphysics forward in several directions. But it would make for a much more impactful paper if the authors could clarify what they are attempting to do, and better explain what their (advanced) work brings to our understanding of ice crystal microphysics. The paper begins by deploring that four decades of research were not able to make a dent in the ECS uncertainties related to crystal properties in ice clouds, and ends stating that this study presents a promising new avenue for combining HSRL retrieval with in-situ measurements. The author should help us understand how their approach will help make progress decreasing ECS uncertainties related to crystal properties in ice clouds, or at least link the two somehow. This is very good work that deserves to be better justified.
Below are additional minor points that I think should be addressed before publication.
Minor comments
Technical comments