Diagnosing the Shape Parameters of the Gamma Particle Size Distribution in a Two-Moment Microphysics Scheme and Improvements to Explicit Hail Prediction

Abstract:

Three-moment bulk microphysics schemes are attractive since they can directly predict the shape parameter of the gamma distribution, but they are more computationally expensive compared with two-moment schemes. In this study, new diagnostic relations between shape parameter and the mean-mass diameter are developed for rain, graupel and hail hydrometeor categories based on the simulations for a severe hailstorm over eastern China using the Milbrandt and Yau (MY) three-moment scheme.

The set of relations are introduced into the MY two-moment scheme, and applied to the hailstorm simulation using the Advanced Regional Prediction System at 1-km grid resolution. Different configurations of MY schemes, including the MY one-moment, two-moment with fixed-shape parameter and the original shape parameter diagnostic relations, and three-moment schemes, are also used to simulate the hailstorm. The new diagnostic-shape parameter two-moment scheme is found to improve the simulation of the general storm structure compared with the other two-moment schemes, and better reproduce the maximum estimated size of hail simulated by the three-moment scheme while still retaining the computational efficiency. Explicit prediction of hail by the new diagnostic-shape parameter two-moment scheme, including the surface accumulated hail mass, number and distribution, is the most consistent with that by the three-moment scheme. Detailed microphysical budget analyses indicate that the new diagnostic relations of shape parameter yield substantial improvements in the hail growth and melting processes, in particular for the processes within lower levels.