A new unified mixed-phase particle fall speed in bulk microphysics parameterizations

Bulk microphysical parameterizations that represent mixed-phase processes divide particles into categories such as snow and graupel or hail. The distinction between these is in the primary habit of the particles, where snow is assumed to be unrimed ice crystals or aggregates of crystals that have predominantly grown by deposition of ice, while graupel and hail have grown mostly by collection of water droplets, and are therefore denser and rounder than the snow category. This distinction is important in numerical models because the density of the particles affects process rates, and particularly the fall speed of the particles that in turn affects cloud structure and precipitation intensity. In reality, of course, there is no such clear distinction between snow and graupel in clouds, and there are many degrees of riming between these extremes. By labeling particles as either snow or graupel, bulk schemes are unable to represent this continuum of habits, and, due to the assumptions about these habits there is a partitioning in behavior that is not likely to be so well defined in nature. Probably the most extreme difference would be due to the factor or two or three in the assumed fall speeds for a given mass content as graupel has a much greater density (further enhanced for hail). This difference in turn leads to a separation of trajectories of snow and graupel within the same modeled cloud, even when they have the same origin. In this paper, we introduce a simple method to alleviate the problem of species separation by revising the paradigm that a particle is either graupel or snow, particularly in the treatment of its fall speed, and hence trajectory, thus preventing a false separation and enhanced accretion due to their relative sedimentation rates. 2. Modifications to the WSM6 scheme