Ice crystal number concentration sensitivity to dynamical forcing and ice nuclei concentration: A global model study

This work presents the sensitivity of the global distribution of ice crystal concentration to the insoluble aerosol (dust and black carbon) freezing fraction (i.e., the freezing spectrum), cloud-scale updraft velocity, and the aerosol spatial distribution. For this, a physically-based, analytical, ice microphysics parameterization is implemented into the NASA Global Modeling Initiative (GMI) chemical and transport model. The ice cloud formation parameterization computes the ice crystal number, size distribution, and maximum supersaturation, from the freezing of soluble and insoluble aerosol, considering variations in aerosol size distribution and chemical composition. It allows competition between homogeneous and heterogeneous freezing during cloud formation and reproduces the prediction of a detailed numerical parcel model within 10%. When the parameterization is run using a diverse set of published heterogeneous freezing spectra (derived from field campaign data and theoretical considerations), the resulting global mean ice crystal concentration predicted by GMI varies within a factor of five. The sensitivity of ice crystal concentration to heterogeneous ice nuclei concentrations is however strongly dependent on the insoluble aerosol spatial distribution and decreases when the sub-grid variability in updraft velocity is considered.