(341c) Parameterizing Ice Cloud Formation in Large-Scale Atmospheric Models

This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from the freezing of aerosol with any size distribution and chemical composition. The formulation is based on parcel model theory and includes homogeneous and heterogeneous freezing mechanisms. The diversity of heterogeneous nuclei is described through a nucleation spectrum function, which is allowed to follow any form (i.e., empirically or theoretically derived), and depends on the surface area, chemical composition and size distribution of the precursor aerosol, and on the contribution from different freezing modes. A complete analytical expression is obtained by reformulating the parcel model equations decoupling ice crystal nucleation and growth. The parameterization reproduced the predictions of a detailed numerical parcel model with an average error in ice crystal number concentration of -4 ± 20%. The parameterization was implemented into the NASA Global Modeling Initiative chemical and transport model to study the aerosol indirect effect through the modification of ice clouds. Results suggest that the temperature and updraft velocity global distributions, as well as the insoluble aerosol load and freezing efficiency, are the key factors determining the global distribution of ice crystal number concentration.