| 21 Jul 2026
Aerosol loading delays droplet activation and suppresses drizzle mode Ka-band radar signatures in LES–LCM simulations of shallow cumulus clouds
Abstract. This article investigates how variations in aerosol loading affect droplet activation and the production of warm rain in shallow cumulus clouds. It is also examined whether the resulting differences can be identified and quantified using 35 GHz Ka-band radar signatures and how these relationships depend on the cloud life-cycle stage. Three idealized large-eddy simulations, T13 (clean, aerosol number concentration Nₐ = 100 cm−³), T53 (intermediate, Nₐ = 1000 cm−³), and T73 (polluted, Nₐ = 5000 cm−³), coupled to a Lagrangian cloud model (LES–LCM), are conducted with a fixed aerosol size-distribution shape under identical thermodynamic conditions based on the Barbados Oceanographic and Meteorological Experiment (BOMEX). Persistent particle tracking is used to reconstruct Lagrangian histories of super-droplets. Analysis of droplet activation and growth statistics shows that clusters of relative humidity (RH) values distinguish core-like and entrained-shell-like activation pathways. The simulation results further show that increased aerosol loading reduces the temporal window available for droplet activation, spectral broadening, and collision–coalescence. Accordingly, approximately 20.1 %, 15.3 %, and 15.0 % of trajectories reach drizzle size (r ≥ 40 µm) in the clean, intermediate, and polluted cases, respectively. Mixing diagnostics shift toward more deactivation-dominated behavior with increasing aerosol loading. These findings suggest that aerosol-dependent microphysical pathways remain detectable in Ka-band Doppler radar signatures. Therefore, Ka-band radar observations provide an observationally testable signature of delayed activation and suppressed warm-rain production, while it shall be noted that knowledge about the evolution state of the cloud system is essential for drawing conclusions about aerosol effects.
Review of “Aerosol loading delays droplet activation and suppresses drizzle mode Ka-band radar signatures in LES–LCM simulations of shallow cumulus clouds” by Lee et al. The manuscript presents an analysis of large-eddy simulations of shallow cumulus using Lagrangian super-droplet microphysics to investigate the impact of aerosol loading on cloud-droplet activation and subsequent growth into warm rain. The authors further employ a radar forward simulator to investigate whether aerosol-induced differences precipitation properties produce signatures observable at Ka-band.
This manuscript provides an interesting Lagrangian perspective on aerosol–cloud interactions that is not widely represented in the peer-reviewed literature. In particular, the aerosol-dependent differences in activation timing relative to cloud lifetime are potentially important and warrant consideration of the manuscript for publication. However, this contribution is challenging to access given the overall length of the manuscript, which also includes several other parallel analyses. I estimate the manuscript to be approximately 39 publishing units (~11,500 words, 10 figures, and 6 tables), which substantially affects its readability and makes the central scientific contribution difficult to identify.
I have four principal concerns. First, the manuscript would benefit from substantial refocusing and shortening around the scientific questions for which the Lagrangian particle tracking model provides genuinely new insight. Second, the representation of warm-rain development needs to be more clearly established to support the extensive interpretation of mature precipitation, particularly given continuing uncertainties regarding the processes controlling warm-rain initiation and growth, particularly in turbulent clouds. Third, some trajectory analyses are conditioned on particles present at the end of the simulation, and the sensitivity of the central activation-timing result to this population selection needs to be established. Finally, while the radar forward simulations demonstrate that the modeled aerosol perturbation propagates into observation space, it is not yet clear that the resulting signatures provide a practical observational constraint on aerosol effects since differences in cloud life cycle, dynamics, thermodynamic environment, and other factors could produce similar radar responses in nature.
Please refer to the attachment for my complete review.