Numerical Modeling of Hailstorms and Hailstone Growth. Part II: The Role of Low-Density Riming Growth in Hag Production

Abstract

The past several years have seen a renewed interest in the importance of low-density riming growth to the development of hailstones. This paper reports on the results of a study that incorporates the physical factors controlling the density of the rime deposit in a two-dimensional, time-dependent numerical cloud model with discretized treatment of the graupel/hail size distribution. Comparisons are made between cases in which the mass-diameter relationship is fixed based on a priori assumed particle densities and cases in which the mass-diameter relationship is allowed to change in accordance with the variable particle density diagnosed from the riming density relationship and past growth history. Compared to the fixed particle density treatment common to earlier work, ice particles of lower density have enhanced surface and cross-sectional area for particles of equal mass, which in turn, increases the effective ventilation experienced by the particles and their capture volumes, thus allowing enhanced diffusional and accretional growth. The lower density particles also experience reduced sedimentation effects due to reduced fallspeeds. This leads to increased residence time in favorable growth environments for both the low density embryos in the region of the embryo curtain and for medium density particles growing to hail in the more active regions of the cloud.