Journal Article

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

Milbrandt, Jason A., Cholette, Mélissa, Morrison, Hugh

2025

Overview

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. Plain Language Summary In atmospheric models, cloud and precipitation processes are represented by a microphysics scheme. In this study, we improve the Predicted Particle Properties (P3) microphysics scheme. The previous version of P3 made simplifying assumptions about how the ice particle size/mass distribution in a model grid volume changes from microphysical processes such as riming (collection of liquid drop by ice), sublimation, and aggregation. Specifically, it assumed that the relative spread of the distributions did not change from these processes. In the new version of P3, we explicitly calculate how the spread of particle sizes is affected by all relevant processes. In idealized simulations of a type of thunderstorm called a supercell, we show that the new approach produces narrower ice particle size distributions in parts of the storm where the distributions are relatively narrow, and slightly wider distributions where they are relatively wide, compared to the original approach. Despite these changes to the particle size distributions, the impacts on overall cloud and precipitation properties are modest. For example, the new approach produces 7%–8% less surface precipitation than the original version. Although bulk impacts are modest, the new scheme improves physical realism with little increase in computational cost. Key Points A new, complete 3‐moment approach to represent ice particles is proposed and implemented in the P3 bulk microphysics scheme Compared to the original 3‐moment closure in P3, size distribution width is altered by all microphysical processes in the new approach The new approach improves physical realism but impacts on cloud and precipitation properties are modest for a simulated supercell storm

• Publisher: John Wiley & Sons, Inc
• Subject: Advection / Aggregation / bulk microphysics / cloud modeling / Evolution / Ice / Microphysics / P3 scheme / Particle size distribution / Precipitation / predicted particle properties / Sedimentation / Sedimentation & deposition / Size distribution / Sublimation / three‐moment / Updraft