What Controls the Microphysical Variability in Cirrus Clouds?

A limited understanding of the multiscale processes driving cirrus cloud evolution makes it difficult to represent cirrus in numerical models. Thus, our understanding of cirrus response to climate change is also limited. Moreover, some geoengineering approaches (e.g., “cirrus cloud thinning”) to mitigate global warming also rely on the response of cirrus to ice-nuclei perturbations. This study investigates different factors controlling microphysical variability in cirrus. We use a Lagrangian particle-based scheme in a set of large-eddy simulations (using the CM1-SDM model). It computes and tracks the properties of individual computational particles (“super-particle”) representing a multitude of physical cloud particles with identical properties and includes their feedback to the dynamics. This scheme was improved by implementing a new ice particle growth model based on recent controlled laboratory experiments of vapor deposition. The measurements show that ice crystal growth rates are distributed as a function of supersaturation. Using these measurements in the model allows each particle to experience a unique, but realistic, growth rate. This novel modeling framework is used to simulate a cirrus case in the central U.S. that was observed as part of the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Ice Cryo-Encapsulation by Balloon (ICE-Ball) field project. Figure 1 shows ice crystal size distributions at different altitudes. The primary microphysical sources of variability investigated here are fluctuations in ice deposition density, critical crystal size for morphological changes relative to solid ice, and variability driven by different crystal growth habits. Our analysis suggests that variability in the dynamic and thermodynamic histories of particles dominates these microphysical sources of variability in particle sizes, densities, and fall-speeds. Moreover, there is little correlation between ice crystal properties and supersaturation (e.g., as would be sampled using an aircraft or a balloon) even though particle growth is directly tied to supersaturation. This is explained by substantial variability in the growth histories of particles with trajectories that end up in near proximity (i.e., the same model grid cell) at a particular time. Variability in the deposition density and thermodynamic conditions at the time and location of nucleation is also important for determining crystal sizes and densities if the crystal habit is set at the time of nucleation.