Quantifying the impact of heterogeneous particle distributions on riming in isolated mixed-phase cumulus clouds

Riming is one of the key factors influencing the control of phase partitioning and precipitation formation in mixed-phase clouds. The riming growth rate of ice particles is strongly affected by the spatial distributions of liquid and ice particles. However, models often assume particles are homogeneously distributed in a given grid box when modelling riming, and few observational studies have quantified the effect of sub-grid heterogeneous particle distributions on riming. In this study, based on airborne in situ measurements made in isolated mixed-phase cumulus clouds, the impact of heterogeneous particle distribution on riming growth rate (Rrim) is quantified by defining an impact factor Frim, which is calculated using the observed mean Rrim divided by Rrim assuming a homogeneous particle distribution. The results show that Frim varies from ~0 to 2.8, demonstrating that the heterogeneous particle distribution has a strong nonlinear impact on riming. Additionally, it is found that Frim can be parameterized using the homogeneity of liquid-ice mixing and the range of normalized riming rate, which are related to the predictable condensed water content and ice particle concentration, respectively. Moreover, based on simulations driven by observed particle size distributions and assuming a static condition, it is suggested that riming has a significant feedback on Frim. These findings will be beneficial to improve the capability of models in simulating mixed-phased clouds and deepen our understanding of sub-grid scale cloud microphysics.