Intercomparison of large-eddy simulations of Arctic mixed-phase clouds: Importance of ice size distribution assumptions

Large-eddy simulations of mixed-phase Arctic clouds by 11 different models are analyzed to understand the causes for diversity in predicted cloud properties and their sensitivity to ice number concentration Ni. In a case based on observations from the Indirect and Semi-Direct Aerosol Campaign (ISDAC), it is found that Ni exerts significant influence on the cloud structure and evolution. Increasing Ni leads to a substantial reduction in liquid water path (LWP) and increase in ice water path (IWP), in agreement with earlier studies. In contrast to previous intercomparison studies, all models in this study use the same ice particle properties (i.e., mass-size, mass-fall speed, and mass-capacitance relationships) and a common radiation parameterization. The constrained setup exposes the importance of ice particle size distributions (PSD) in influencing cloud evolution. A clear separation in LWP and IWP predicted by models with bin and bulk microphysical treatments is documented and attributed primarily to the assumed shape of ice PSD used in bulk schemes. Compared to the bin schemes that explicitly predict the PSD, schemes assuming exponential ice PSD underestimate ice growth by vapor deposition and overestimate mass-weighted fall speed leading to an underprediction of IWP by a factor of two in the considered case. Sensitivity tests indicate that LWP and IWP are much closer to the bin model simulations when a modified shape factor similar to that predicted by bin model simulations is used in bulk scheme. Notwithstanding the fact the best-fit value of the shape factor is case specific, the results clearly demonstrate the importance of representation of ice PSD in determining the partitioning of liquid and ice and the longevity of mixed-phase clouds.