Description

The NSSL 2/3-moment bulk microphysical parameterization scheme that describes form and phase changes among a range of liquid and ice hydrometeors, as described in Mansell et al. (2010) [131], Mansell and Ziegler (2013) [130], and Mansell et al. (2020) [132]. The microphysical parameterization predicts the mass mixing ratio and number concentration of cloud droplets, raindrops, cloud ice crystals (columns), snow particles (including large crystals and aggregates), graupel, and (optionally) hail. Optionally, a third moment (reflectivity or 6th moment) of rain, graupel, and hail can be activated.

The graupel and hail particle densities are also calculated by predicting the total particle volume. The graupel category therefore emulates a range of characteristics from high-density frozen drops (includes small hail) to low-density graupel (from rimed ice crystals/snow) in its size and density spectrum. The hail category is designed to simulate larger hail sizes. Hail is only produced from higher-density large graupel.

Hydrometeor size distributions are assumed to follow a gamma functional form. Microphysical processes include cloud droplet and cloud ice nucleation, condensation, deposition, evaporation, sublimation, collection–coalescence, variable-density riming, shedding, ice multiplication, cloud ice aggregation, freezing and melting, and conversions between hydrometeor categories.

Cloud concentration nuclei (CCN) concentration is predicted as in Mansell et al. (2010) [131] with a bulk activation spectrum approximating small aerosols. The model tracks the number of unactivated CCN, and the local CCN concentration is depleted as droplets are activated, either at cloud base or in cloud. The CCN are subjected to advection and subgrid turbulent mixing but have no other interactions with hydrometeors; for example, scavenging by raindrops is omitted. CCN are restored by droplet evaporation and by a gradual regeneration when no hydrometeors are present. Aerosol sensitivity is enhanced by explicitly treating droplet condensation instead of using a saturation adjustment. Supersaturation (within reason) is allowed to persist in updraft with low droplet concentration.

Excessive size sorting (common in 2-moment schemes) is effectively controlled by an adaptive breakup method that prevents reflectivity growth by sedimentation (Mansell 2010 [133]). Activating the 3-moment scheme provides a natural sedimentation feedback that narrows the size spectrum as size-sorting procedes without the the artificial breakup induced by the 2-moment scheme.

The NSSL scheme is designed with deep (severe) convection in mind at grid spacings of up to 4 km, but can also be run at larger grid spacing as needed for nesting etc. It is also able to capture non-severe and winter weather.

Intraphysics Communication

Argument Table

General Algorithm

NSSL MP General Algorithm: interface to driver