ARM Observations Used to Evaluate New Model of Ice Formation
Low-level clouds play an important role in the Arctic surface energy budget due to their high frequency and extensive lifetimes. These “mixed-phase” clouds simultaneously contain ice crystals and liquid drops within the same cloud layer and are able to persist for long periods due to balances among complex dynamical, microphysical, and thermodynamic processes in the Arctic boundary layer. One of these complex, and not well-understood processes, is the nucleation (or formation) of new ice particles. Past studies have been unable to explain how new ice particles can continue to form for hours in Arctic mixed-phase clouds without unrealistically high aerosol concentrations.
Scientists used data from the Indirect and Semi-Direct Aerosol Campaign (ISDAC), a field campaign conducted by the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility near Barrow, Alaska, to evaluate a new model of ice nucleation. The model accounts for time-dependent changes in ice nucleation by considering that aerosol particles that are most efficient at forming ice will be the first to nucleate, and thus the properties of aerosol and nucleation rates will change over time. Evaluation of the new model with the ARM observations illustrates that the model can produce a reasonable representation of the ice water path and ice crystal size distributions in the observed mixed-phase clouds. The study also finds that the production of new ice crystals in the upper part of the cloud is controlled mostly by the competition among radiative cooling (resulting in more aerosol particles becoming efficient ice nuclei as the temperature decreases), cloud-top entrainment (bringing fresh particles into the cloud), and nucleation scavenging (ice-forming aerosol particles removed as the ice crystals fall out of the cloud). The relative contribution of each process is mostly determined by the cloud-top temperature and entrainment rates. These results suggest that modeling the time evolution of the aerosol population’s ability to form ice is required to accurately model Arctic mixed-phase cloud processes.
Savre, J., and A. M. L. Ekman. 2015. “Large-Eddy Simulation of Three Mixed-Phase Cloud Events During ISDAC: Conditions for Persistent Heterogeneous Ice Formation,” Journal of Geophysical Research: Atmospheres, DOI: 10.1002/ 2014JD023006.