On the ice nucleation spectrum Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
24 Apr 2012
I.M. Systems Group Inc., Rockville, Maryland, USA
Received: 05 October 2011 – Published in Atmos. Chem. Phys. Discuss.: 02 November 2011 Abstract. This work presents a novel formulation of the ice nucleation spectrum, i.e.
the function relating the ice crystal concentration to cloud formation
conditions and aerosol properties. The new formulation is physically-based and explicitly accounts for
the dependency of the ice crystal concentration on temperature,
supersaturation, cooling rate, and particle size, surface area and
composition. This is achieved by introducing the concepts of ice nucleation
coefficient (the number of ice germs present in a particle) and nucleation
probability dispersion function (the distribution of ice nucleation
coefficients within the aerosol population). The new formulation is used to generate ice nucleation
parameterizations for the homogeneous freezing of cloud droplets and the
heterogeneous deposition ice nucleation on dust and soot ice nuclei. For
homogeneous freezing, it was found that by increasing the dispersion in the
droplet volume distribution the fraction of supercooled droplets in the
population increases. For heterogeneous ice nucleation the new formulation consistently
describes singular and stochastic behavior within a single framework.
Using a fundamentally stochastic approach, both cooling rate independence and constancy
of the ice nucleation fraction over time, features typically associated with singular
behavior, were reproduced. Analysis of the temporal dependency of the ice nucleation
spectrum suggested that experimental methods that measure the ice nucleation fraction
over few seconds would tend to underestimate the ice nuclei concentration. It is shown that inferring
the aerosol heterogeneous ice nucleation properties from measurements of the
onset supersaturation and temperature may carry significant error as the
variability in ice nucleation properties within the aerosol population is not
accounted for. This work provides a simple and rigorous ice nucleation framework
where theoretical predictions, laboratory
measurements and field campaign data can be reconciled, and that is suitable for application in
atmospheric modeling studies.
Revised: 16 March 2012 – Accepted: 04 April 2012 – Published: 24 April 2012
Citation: Barahona, D.: On the ice nucleation spectrum, Atmos. Chem. Phys., 12, 3733-3752, doi:10.5194/acp-12-3733-2012, 2012.