Development of a new bulk microphysical parameterization with varying riming intensity and its evaluation within WRF

Rather than treating snow and graupel/hail as two separate categories as in most bulk microphysical parameterizations (BMPS), this presentation describes a new BMP that considers partially-rimed ice particles using a diagnosed riming degree parameter. As a result, there is a gradual transition from pristine ice to partially-rimed particles and graupel, which is more realistic and it significantly simplifies the BMP scheme. In this scheme the ice particle mass-dimension and area-dimension relationships vary with the diagnosed riming intensity and environmental temperature to account for both the habit and riming effect. A fall speed-dimension relationship is derived theoretically using the area- and mass-diameter relationships. Varying snow capacitance is used in the snow depositional and sublimation parameterizations. A generalized Gamma size distribution with the slope parameter diagnosed from temperature and riming intensity to consider three distinct snow growth regimes (depositional, aggregation, and riming growth) is derived theoretically and verified with observational datasets. In addition, recent theoretical studies of autoconversion considering the dispersion effect has been implemented for both cloud water and cloud ice.

Derivation of the riming degree and its impact on the particle properties (fall velocity, mass) is compared with cloud observations collected at South Great Plain from the Atmospheric Radiation Measurements (ARM) program over Oklahoma and IMPROVE-2 project over the Pacific Northwest. The new scheme has been implemented within the WRF-ARW, and it has been compared with other BMPs in WRF using surface precipitation data and observed microphysics aloft from ARM and IMRPOVE. Modeled fall velocities are also compared with a vertically-pointing radar. It will be shown that the new scheme performs better than many other schemes in WRF, and at least as well as the more sophisticated Thompson scheme.