Abstract
Abstract. Numerical simulations were carried out in a high-resolution two-dimensional framework to increase our understanding of aerosol indirect effects in mixed-phase stratiform clouds. Aerosol characteristics explored include insoluble particle type, soluble mass fraction, influence of aerosol-induced freezing point depression and influence of aerosol number concentration. Simulations were analyzed with a focus on the processes related to liquid phase microphysics, and ice formation was limited to droplet freezing. Of the aerosol properties investigated, aerosol insoluble mass type and its associated freezing efficiency was found to be most relevant to cloud lifetime. Secondary effects from aerosol soluble mass fraction and number concentration also alter cloud characteristics and lifetime. These alterations occur via various mechanisms, including changes to the amount of nucleated ice, influence on liquid phase precipitation and ice riming rates, and changes to liquid droplet nucleation and growth rates. Alteration of the aerosol properties in simulations with identical initial and boundary conditions results in large variability in simulated cloud thickness and lifetime, ranging from rapid and complete glaciation of liquid to the production of long-lived, thick stratiform mixed-phase cloud.
Highlights
Wegener-Bergeron-Findeisen mechanism (Bergeron, 1935; Findeisen, 1938; Wegener, 1911), under which ice grows at Aerosol effects on clouds are among the largest sources of uncertainty in understanding of future climate predictions (IPCC, 2007)
Whereas previous studies have focused on the influence of ice forming nuclei (IN) concentration on mixed-phase clouds, here we investigate the influence of both parameterized aerosol type and concentration on simulated mixed-phase stratiform cloud characteristics and lifetime
Where Nfbl is the number of frozen droplets in the bth bin of the liquid hydrometeor mass spectrum, Nlb the number of unfrozen droplets, mbl is the mean mass of the bth bin, T the temperature in ◦C, T0 a reference temperature, Tf the freezing point depression due to soluble material, ρw the density of water, t the microphysics timestep, and a and Bh,i laboratory-derived constants
Summary
Wegener-Bergeron-Findeisen mechanism (Bergeron, 1935; Findeisen, 1938; Wegener, 1911), under which ice grows at Aerosol effects on clouds are among the largest sources of uncertainty in understanding of future climate predictions (IPCC, 2007). Mixed aerosol particles can result in ice formation via the condensation and immersion freezing modes. In these ice formation mechanisms, aerosol particles containing a combination of soluble and insoluble material are situated in environments that are approaching or at water saturation. The previously cited works of Harrington and Olsson (2001) and Jiang et al (2000) demonstrated the influence of nucleation via deposition/condensation freezing on cloud lifetime These results were strongly dependent upon the IN budget provided in the simulations and (in the case of Harrington and Olsson (2001)) whether IN were removed via precipitation or not. Unlike in that work, experiments are carried out utilizing twodimensional, cloud-resolving simulations of a mixed-phase stratiform layer in order to analyze the ultimate influence of aerosol properties on both cloud micro- and macrophysics
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