A warm-bin-cold-bulk hybrid cloud microphysical model

This study describes a newly developed bin-bulk hybrid cloud microphysical model named MSSG-Bin, which has been implemented in the Multi-Scale Simulator for the Geoenvironment (MSSG). In the hybrid approach, a spectral bin scheme is used for liquid droplets, while a bulk scheme is used for solid particles. That is, the expensive but more reliable spectral bin scheme treats the relatively well-understood physics of the liquid phase, and the computationally efficient but less robust bulk scheme is used to treat the poorly understood physics of the ice phase. In the bulk part, the prognostic variables are the mixing ratios of cloud ice, snow, and graupel and the number density of cloud ice particles. The bulk component is consistent with MSSG-Bulk, which is a conventional bulk model implemented in MSSG. One-dimensional kinetic simulations and three-dimensional cloud simulations have confirmed the reliability of MSSG-Bin for warm clouds, free from the approximations made in bulk parameterizations, and its applicability to cold clouds, without the significant additional costs required for a bin treatment of the ice phase. Compared with MSSG-Bulk, MSSGBin with 33 bins requires 8.3 times more floating-point operations for a one-dimensional shallow convection case, and 4.9 times more for a three-dimensional shallow convection case. Present results have shown the feasibility of using this model for a 25-m-resolution simulation of shallow cumulus on a 512 × 512 × 200 grid.