Lagrangian studies of ice nucleation and growth in mixed-phase stratiform clouds

The persistent presence of ice in long-lived, supercooled stratiform clouds is the result of a delicate balance between environmental conditions, ice nucleus abundance, and microphysical properties (Morrison et al. 2012, Westbrook and Illingworth 2013). The role of ice nucleation, including the origin of the ice nuclei themselves as well as the modes in which they are active, is of special interest because properties of these long-lived clouds are extremely sensitive to the amount of ice (Ovchinnikov et al. 2011). Two interesting questions are 1) where the ice particles come from and 2) why they exist for such a long time (and therefore grow to large size). Considering the continuous formation of ice particles based on classical statistical ice nucleation theory and the balance between cloud updraft velocity and particle terminal velocity, a simple theoretical 1D model predicts that the ice mass concentration is proportional to the ice number concentration to the 2.5 power (Yang et al. 2013). This nonlinear relationship also emerges from detailed large-eddy simulations (LES) of these clouds. In order to investigate why this relationship from a simplified 1D model appears in a fully 3D field, we continuously seed ice particles in the cloud region and perform Lagrangian tracking of each ice particle in both 3D velocity field from a detailed large eddy simulation and an idealized 2D velocity field. Results show that the 2.5 power slope emerges when seeding crystals uniformly in the cloud, while it disappears when seeding from cloud top or cloud base.

In addition, we find that some ice particles can recycle several times before reaching the ground or totally evaporating. Nearly ten percent of initially seeded crystals can still survive after 1.5 hours in both 3D and 2D fields. These lucky ice particles result from a combination of microphysics and dynamics. Results show that stronger circulation increases the number and lifetime of recycling particles. We also derive analytical solutions of trajectories of particles with fixed terminal velocity in simple fluid fields. Results can explain when ice particles can be recycled and when they will fall out in these idealized fluid fields.

References

Morrison, H, G de Boer, G Feingold, J Harrington, MD Shupe, and K Sulia. 2012. “Resilience of persistent Arctic mixed-phase clouds.” Nature Geosciences 5: 11–17.

Ovchinnikov, M, A Korolev, and J Fan. 2011. “Effects of ice number concentration on dynamics of a shallow mixed-phase stratiform cloud.” Journal of Geophysical Research 116: doi:10.1029/2011JD015888.

Westbrook, CD, and AJ Illingworth. 2013. “The formation of ice in a long-lived supercooled layer cloud.”Quarterly Journal of the Royal Meteorological Society 139: 2209-2221.

Yang, F, M Ovchinnikov, and RA Shaw. 2013. “Minimalist model of ice microphysics in mixed-phase stratiform clouds.” Geophysical Research Letters 40: 3756-3760.