Numerical Simulation of the Life History of a Hailstorm

Abstract

A two-dimensional, time-dependent cloud model has been used to simulate the evolution of hail cells and hailstorms. The model has been under development for several years tested on a few days of real data, and gives reasonable predictions of the convective characteristics and essential precipitation processes for both severe and nonsevere convection days. Hydrodynamic equations for. deep convection are integrated over a 20 km square grid with 200 m spacing between grid points. Cloud formation and precipitation processes employing bulk water techniques are simulated in the model. Autoconversion and accretion are used to transform cloud water to rain. Precipitating ice (hail) is formed by the freezing of rain and through an approximation to the Bergeron-Findeisen process to transform cloud liquid to precipitating ice. Accretion of cloud water by rain, and accretion of cloud water, cloud ice and rain by hail are modeled. Wet and dry growth of hail and the shedding of rain from hail are simulated. Cloud water freezes to cloud ice isobarically at a preselected temperature. Evaporation of all forms of cloud particles can occur, and melting of frozen particles is simulated. The simulation shows a sloping updraft and gust front formation and movement. The upshear slope of the updraft allows the precipitation to fall into the lower atmosphere with only minor recycling of the rain from below. Recycling of hail from above creates a radar overhang. The movement of the gust front is toward moist air, which provides a means to sustain the storm for long periods. Accelerating updrafts associated with warm, buoyant bubbles in the atmosphere lead to cellular patterns and a general picture in which the larger storm circulation has superimposed on it smaller perturbations which travel up the main sloping updraft and intensify the storm periodically.