A Complete Three-Moment Representation of Ice in the Predicted Particle Properties (P3) Microphysics Scheme

A new, complete three-moment bulk microphysics approach is proposed that includes the effects of all relevant microphysical processes on the evolution of ice particle size distribution (PSD) width. This extends the three-moment approach that was originally implemented in the Predicted Particle Properties (P3) scheme that assumed sedimentation and advection dominate and neglected the effects of most microphysical processes on PSD width. The new approach (FULL) is tested in idealized one-dimensional kinematic updraft and three-dimensional supercell simulations and compared to results using the original approach (ORIG). Although tendencies of the gamma PSD width parameter (μ) from several microphysical processes using FULL are large in magnitude relative to the sedimentation and advection tendencies, they have only minor impacts on the overall spatiotemporal patterns of μ; PSDs are narrower using FULL in regions with relatively narrow PSDs using ORIG and slightly wider in regions with relatively wide PSDs. The processes driving these impacts using FULL are ice-rain collection near convective cores and sublimation in the far forward flank, both leading to PSD narrowing, and broadening from aggregation in the near forward flank. A general theoretical expression is derived to explain whether a process broadens or narrows PSDs based in part on the ice particle mass-size relationship. However, the effects on bulk cloud and precipitation properties are limited, with only a 7%–8% decrease in mean surface precipitation using FULL compared to ORIG. Although overall impacts are modest in the tests conducted, the full approach improves physical realism with a negligible increase in computational cost.