A Complete 3-Moment Treatment of Ice Categories in the P3 Microphysics Scheme

Traditional bulk (and bin) microphysics schemes assume fixed ice particle properties (e.g., density or mass-size relations) corresponding to specific types of ice, such as cloud ice, snow, and graupel. In contrast, the P3 scheme predicts the continuous evolution of several properties in time and space for a user-specified number (one or more) of “free” ice-phase categories. Each of these categories can evolve and represent any type of ice-phase hydrometeor. Since its inception about 10 years ago, P3 has undergone several major developments, including prediction of 1) the sixth moment of the ice particle size distribution to predict the spectral width (3-moment approach) and 2) the liquid fraction of mixed-phase particles. In the original 3-moment ice version of P3, simplifying assumptions were made regarding some of the process rate formulations for the sixth moment. In particular, for some processes it was assumed that relative spectral dispersion was unchanged due to the process, meaning that in effect those processes were treated by a two-moment approach.

In this study, a complete three-moment treatment for all ice-phase microphysical processes is implemented into P3. The sixth moment tendency for every process is now obtained independently from the other moment tendencies, with few assumptions and in a manner consistent with the physical process and size distribution. The sixth moment tendencies for all processes are computed by numerical integration across the size distribution and stored in a lookup table, similar to the original P3 approach for the number and mass mixing ratio tendencies. Impacts of the new three-moment approach are examined in 1D kinematic (specified flow) simulations and 3D simulations of a hail storm using the Cloud Model 1 (CM1). The analysis focuses on how the improved process treatment affects various cloud and precipitation properties, especially hail at the surface.