How spatial resolution of in situ observations affects the glaciation and evolution of mixed-phase clouds

Mixed-phase clouds (MPCs) are the major source for precipitation over continental mid- and high latitudes. The co-existence of cloud droplets and ice crystals makes MPCs thermodynamically unstable, allowing rapid glaciation through the Wegener–Bergeron–Findeisen (WBF) process and the efficient formation of precipitation-sized hydrometeors. Yet this simultaneous presence of both phases also enables pronounced cloud-phase heterogeneity and intermittency, such that glaciation often does not occur uniformly. Consequently, the apparent efficiency and timescale of glaciation, and its role in MPC evolution, depend on the spatial scale at which phase heterogeneity is represented and resolved.

In this study, we investigate the spatial scales at which phase heterogeneity occurs in MPCs and further assess how unresolved fine-scale variability in cloud phase affects the efficiency and timescale of glaciation. We use in situ observations from 19 targeted glaciogenic cloud seeding experiments conducted during the CLOUDLAB project and compare them with a generalized theory for MPC glaciation times based on the WBF process (Pinsky et al., 2024).

We show that the glaciation time is strongly linked to the spatial resolution at which cloud properties are sampled. Comparing observations averaged on 5, 50, and 250 m spatial scales shows that coarser resolution blurs phase intermittency. Our inferred glaciation times are systematically longer than theoretical predictions, with deviations increasing at coarser resolution. Additionally, our high-resolution in situ measurements show that phase heterogeneity in MPCs extends down to scales of at least one meter and that sub-meter resolution may be required to fully capture the intrinsic microphysical processes.

These results demonstrate that unresolved small-scale phase heterogeneity can systematically bias inferred glaciation times. This bias has direct implications for the evolution and lifetime of mixed-phase clouds and ultimately for the efficiency and timing of precipitation formation.

 

Pinsky, M., Khain A., and Korolev A., 2014: Analytical investigation of glaciation time in mixed-phase adiabatic cloud volumes. J. Atmos. Sci., 71, 4143–4157, https://doi.org/10.1175/JAS-D-13-0359.1