Effects of Heterogeneous Freezing on Winter Mixed-Phase Clouds and Associated Aerosol Effects

Heterogeneous freezing is extremely sensitive to both environmental and aerosol conditions and has profound effects on mixed-phase clouds. However, it still remains poorly understood, mainly due to reasons including 1) large uncertainties associated with intrinsic representations of various freezing mechanisms in numerical models, 2) lack of high spatial and temporal resolution simulations of mixed-phase cloud properties and processes coupled with realistic aerosol conditions, and 3) robust observations of mixed-phase cloud properties. Here in addition to aerosol-cloud interactions through cloud condensation nuclei (CCN), we implement the parameterization of DeMott et al. (2015) into WRF-SBM-MOSAIC (SBM-DeMott) to connect ice crystal formation with realistic aerosol conditions. In contrast with the original ice nucleation scheme Bigg (1953) (SBM-ORI), which is temperature-dependent only, this scheme specifically connects ice nucleation with aerosol particles at sizes larger than 0.5μm as well as temperature. A mixed-phase cloud case occurring over the Sierra Nevada Mountain on 7 February, 2015 during the ACAPEX field campaign are simulated. Compared with aircraft in-situ observations, we find that SBM-DeMott improved the simulated mixed-phase cloud properties compared with SBM-ORI, with ice number concentrations of about ~ two orders of magnitudes larger. We conduct process-based analysis of key cloud microphysics such as condensation-evaporation, deposition-sublimation and riming, to understand how the DeMott et al. (2015) scheme can lead to such a dramatic improvement on simulating mixed-phase cloud properties. We also conduct sensitivity tests by changing the associated aerosol loadings to examine how the mixed-phase cloud properties are sensitive to aerosols acting as ice nuclei. This study can help to improve the parameterization of heterogeneous freezing in models and our fundamental understanding of aerosol impacts on mixed-phase clouds.