Precipitation Uncertainty Due to Variations in Precipitation Particle Parameters within a Simple Microphysics Scheme

Abstract

This work reports on the sensitivity of accumulated precipitation to the microphysical parameterization in simulations of deep convective storms using a three-dimensional, nonhydrostatic cloud model with a simple liquid?ice microphysics scheme. Various intercept parameters from an assumed Marshall?Palmer exponential size distribution are tested along with two particle densities for the hail/graupel (qh) category. These variations allow testing of unique qh distributions that have been observed and documented in previous literature. Tests are conducted for a single thermodynamic profile and three idealized wind shear profiles. The amount of accumulated precipitation at the ground is very sensitive to the way the qh category is parameterized. Distributions characterized by larger intercepts and/or smaller particle density have a smaller mass-weighted mean terminal fall velocity and produce smaller qh mixing ratios spread over a larger area. For example, for a qh category weighted toward graupel, only a fourth as much precipitation accumulates on the ground over 2 h (and none is hail) compared to a qh category weighted toward large hail (with baseball-sized stones common). The inherent uncertainty within the qh distribution for this simple cloud-scale three-class ice microphysics scheme suggests limited usefulness in the forecasting of ground-accumulated precipitation and damaging hail.